British Journal of Anaesthesia 85 (4): 599-610 (2000)
`
`BJA
`
`REVIEW ARTICLE
`
`The role of albumin in critical illness
`
`J. P. Nicholson, M. R. Wolmarans and G. R. Park*
`
`John Farman Intensive Care Unit, Addenbrooke's Hospital, Hill's Road, Cambridge CB2 2QQ, UK
`*Corresponding author
`
`BrJAnaesth 2000; 85: 599-610
`
`Keywords: plasma proteins, albumin; literature review
`
`The last 25 yr have seen major advances in our understand(cid:173)
`ing of albumin. We now know the amino acid sequences of
`bovine and human albumin, the complete gene sequence of
`human albumin, and the location of mutations in the gene
`sequence. During the 1990s,
`the heart-shaped crystalline
`structure of albumin has been described and a new protein,
`termed a-albumin (afamin), has been added to the albumin
`superfamily, which otherwise consists of serum albumin,
`vitamin D-binding protein and u-fetoprotein.i"
`The function of circulating albumin in critical illness is
`not fully understood. It may differ significantly from that in
`healthy subjects. A low serum albumin concentration in
`critical illness is associated with a poor outcome.r II 66
`Despite theoretical advantages for using human albumin
`solution as a plasma substitute, studies have shown that
`correcting hypoalbuminaemia has no impact on outcome in
`the criticall y ill. 28 96 97
`This review will examine the role of serum albumin in
`health and critical illness. It will also review aspects of the
`physiology of this protein that may be expected to lead to
`significant dysfunction in critical illness. Finally, the case
`for and against
`the use of exogenous albumin in the
`management of critically ill patients will be discussed.
`
`Structure of albumin
`In humans, albumin is the most abundant plasma protein,
`accounting for 55-60% of the measured serum protein.i" It
`consists of a single polypeptide chain of 585 amino acids
`with a molecular weight of 66 500 Da. The chain is
`characterized by having no carbohydrate moiety, a scarcity
`of tryptophan and methionine residues, and an abundance of
`charged residues, such as lysine, arginine, glutamic acid and
`aspartic acid. 77 The mature, circulating molecule is ar(cid:173)
`ranged in a series of a-helices, folded and held by 17
`disulphide bridges. The folding creates subdomains of three
`contiguous a-helices
`in parallel
`(Fig. 1). A pair of
`subdomains face each other to form domains. These can
`
`be seen as cylindrical structures with polar outer walls and a
`hydrophobic central core. 23
`The tertiary structure of human albumin crystal has been
`isolated by x-ray crystallography. It is seen as a heart(cid:173)
`shaped molecule 80 X 30 A. 36 In solution, the shape is quite
`different. The three domains appear to be arranged in an
`ellipsoid pattern, giving the molecule low viscosity (Fig. 2).
`The molecule is very flexible and changes shape readily
`with variations
`in environmental conditions and with
`binding of ligands." Despite this, albumin has a resilient
`structure and will
`regain shape easily, owing to the
`disulphide bridges, which provide strength, especially in
`physiological conditions. 14 After their rupture, the molecule
`can re-establish these bridges and regain its structure.
`Denaturation occurs only with dramatic and non-physio(cid:173)
`logical changes
`in temperature, pH and the ionic or
`chemical environment.
`
`Albumin metabolism
`The serum albumin concentration is a function of its rates of
`synthesis and degradation and its distribution between the
`intravascular and extravascular compartments. The total
`body albumin pool measures about 3.5-5.0 g kg-I body
`weight (250-300 g for a healthy 70 kg adult). The plasma
`compartment holds about 42% of this pool, the rest being in
`extravascular compartments. Some of this is tissue-bound
`and is therefore unavailable to the circulation. Each day,
`120-145 g of albumin is lost into the extravascular space.
`Most of this is recovered back into the circulation by
`lymphatic drainage. Albumin is also lost into the intestinal
`tract (about 1 g each day), where digestion releases amino
`acids and peptides, which are reabsorbed. There is minimal
`urinary loss of albumin in healthy subjects. Of the 70 kg of
`albumin that passes through the kidneys each day, only a
`few grams pass through the glomerular membrane. Nearly
`all of this is reabsorbed, and urinary loss is usually no more
`than 10-20 mg day".
`
`© The Board of Management and Trustees of the British Journal of Anaesthesia 2000
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`Nicholson et al.
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`I40A
`1
`
`- - - - - - - - - 1 4 0 A - - - - - - - -.....
`
`Fig 2 The ellipsoid structure of albumin in solution. l 4
`
`6
`
`,.
`
`EE0
`
`..
`"'0
`
`2
`
`3
`
`4
`
`5
`Days after
`
`6
`
`7
`
`8
`
`9
`
`10
`
`Fig 3 Decay pattern of labelled albumin versus time after i.v. injection of
`a tracer dose of l25I -labelled human serum albumin (thick line). Slope I
`(SI) is the transcapillary escape rate, which equals about 4.5% h -1.
`Slope 2 (S2) is the fractional degradation rate, which is about 3.7% per
`hour. EV is the calculated increase in extravascular labelled albumin
`concentration. Note that the activity of extravascular albumin is greater
`than that of intravascular albumin from about day 3 onwards. This
`suggests that degradation occurs directly from the vascular compartment.
`Reproduced with permission from Peters, 1996.76
`
`IA
`
`IB
`
`IIA
`
`liB
`
`lilA
`
`III B I
`
`Fig 1 Two-dimensional representation of the albumin molecule reflecting
`the heart-shaped structure (see Fig. 2). The regions of the molecule that
`are normally in the a-helix configuration are shown in dark grey. The
`seventeen disulphide bridges are depicted in light grey. The three
`domains, separated into A and B subdomains, are shown along the
`bottom axis. Reproduced with permission from Carter and Ho, 1994.14
`
`The distribution of albumin between body compartments
`can be examined by injecting radiolabelled albumin into the
`venous circulation. A typical biexponential plot of log
`plasma concentration versus
`time shows a first-order
`process
`(Fig. 3). A two-compartment model
`can be
`constructed. There is a rapid phase of disappearance from
`the plasma over
`the first 2 days. This represents the
`transcapillary exchange rate of 4.5% h- I
`, giving a distribu(cid:173)
`tion half-time of about 15 h. Then there is a slower
`exponential decay, representing the fractional degradation
`rate (FDR), of about 3.7% day" with an elimination half
`time of about 19 days. The FDR closely parallels the rate of
`synthesis in steady state (3.8% day-I).
`
`The extravascular pool is divided into exchangeable and
`remote components (Fig. 4). Significant
`locations of this
`large extravascular pool are listed in Table 1.
`The mechanism of
`the escape of albumin into the
`extravascular compartment has come under review recently.
`Albumin must cross capillaries. Most organs in the body
`have continuous capillaries, but in some there are wide-open
`sinusoids (liver, bone marrow) or fenestrated capillaries
`(small intestine, pancreas, adrenal glands). Starling's theory
`holds that the rate of escape depends on the permeability of
`the wall and hydrostatic and oncotic pressures on either side
`of the wal1. 29 Half of the escaping albumin does so through
`the continuous capillaries, and there appears to be an active
`transport mechanism to facilitate this. 76 Albumin binds to a
`surface receptor called albondin, which is widely distributed
`in many capillary beds, except in the brain.f" 91 Bound
`albumin enters vesicles within the endothelial cell and is
`discharged on the interstitial side within 15 s. The rate of
`transfer is increased with the addition of long-chain fatty
`acids (LCFAs) to albumin, and with the cationization and
`glycosylation of the molecule.i"
`
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`Albumin in critical illness
`
`Table 1 Distribution of extravascular albumin in the body '"
`
`Organ
`
`Fraction of
`body weight
`(%)
`
`Fraction of total
`extravascular albumin
`(%)
`
`41
`40
`
`7 3 9
`
`Skin
`Muscle
`Gut
`Liver
`Subcutaneous, etc.
`
`18.0
`45.5
`2.8
`4.1
`8
`
`Table 2 Factors that modify albumin metabolism (see text for details)
`
`Intravascular
`118 9
`
`Synthesis
`
`13.6 9 day"
`(3.8% day')
`
`TCER
`
`4.5%
`day'
`
`Extravascular
`242 9
`
`Exchangeable .Remote
`
`177 9
`
`65 9
`
`Loss 13.3 9 day'
`(3.7% day")
`
`1Loss
`
`0.3 9 dar'
`
`Fig 4 Typical albumin distribution in a healthy 70 kg adult. Reproduced
`with permission from Peters, 1996.76
`
`Reduced albumin synthesis
`
`Decreased gene transcription
`
`Trauma, sepsis (cytokines)
`Hepatic disease
`Diabetes
`Decreased growth hormone
`Decreased corticosteroids (in vitro)
`Fasting, especially protein depletion
`
`Ribosome disaggregation
`
`Hydrocortisone and dexamethasone both increase gene
`transcription in vitro,65 67 but the overall in vivo effects are
`complex. Steroids
`also increase
`albumin catabolism.
`Growth hormone has been shown to stimulate gene
`transcription in cultured hepatocytes."
`The rate of synthesis depends on nutritional intake, more
`so than for other hepatic proteins76 Fasting reduces albumin
`production, but specifically omitting protein from the diet
`causes a greater reduction in synthesis. Early in protein
`deprivation, there is rapid disaggregation of free and bound
`poly somes, which can be reversed rapidly by refeeding the
`subject with amino acids.r" Two amino acids are particu(cid:173)
`larly effective, tryptophan and ornithine. 83 Ornithine, unlike
`tryptophan, is not incorporated into albumin. It is a product
`of the urea cycle and acts as a precursor of the polyamine
`spermine. The increases in polysome aggregation and
`albumin synthesis with ornithine refeeding suggest
`that
`the urea cycle plays more of a role in protein metabolism
`than mere waste disposal.P" Protein deprivation for a longer
`to a 50-60% decrease in the activity and
`time leads
`concentration of the mRNA, presumably through increased
`breakdown, as gene transcription is not slowed in rats on a
`0-4% protein diet. 86
`Calories are important, however. There is a reduction in
`synthesis in starved rats, and poly somes will reaggregate
`with glucose feeding alone.i" Energy rather than the amino
`acid supply may be more important
`in determining
`polysome
`aggregation under normal
`circumstances.r"
`Table 2 summarizes
`factors known to alter albumin
`synthesis.
`
`Degradation
`Total daily albumin degradation in a 70 kg adult is around
`14 g day-lor 5% of daily whole-body protein turnover.
`Albumin is broken down in most organs of the body. Muscle
`
`Synthesis
`In humans, albumin synthesis takes place only in the
`liver. 56 60 Albumin is not stored by the liver but is secreted
`into the portal circulation as soon as it is manufactured. In
`healthy
`young
`adults,
`the
`rate
`of
`synthesis
`is
`194 (SD 37) mg kg-1 day-I, or about 12-25 g of albumin
`per day.76 The rate of synthesis rate varies with nutritional
`and disease states. The liver can increase albumin synthesis
`to only 2-2.7 times normal because most of the liver's
`synthetic machinery is already devoted to albumin at rest. 76
`The synthetic pathway is common to eukaryotes and is also
`used for synthesis of other proteins.Y
`Albumin will be synthesized only in a suitable nutritional,
`hormonal and osmotic environment. The colloid osmotic
`the
`interstitial
`fluid bathing the
`(COP) of
`pressure
`hepatocyte is the most
`important regulator of albumin
`synthesis.Y 85 107 Synthesis requires:
`
`• mRNA for translation;
`• an adequate supply of amino acids, activated by binding
`to tRNA;
`• ribosomal machinery for assembly;
`• energy in the form of ATP and/or GTP.
`
`action on
`The mRNA concentration available for
`ribosomes is an important factor controlling the rate of
`albumin synthesis. Trauma and disease processes will affect
`the mRNA content.F' 64 A reduction in albumin mRNA
`concentration, caused by a decrease in gene transcription, is
`seen in the acute-phase reaction mediated by cytokines,
`mainly interleukin-6 (IL-6) and tumour necrosis factor a
`(TNF-a).12 15 72 A decrease in gene transcription is also
`seen in hepatoma cells and in hepatocytes damaged with
`carbon tetrachloride.i"
`the mRNA
`The hormonal
`environment can affect
`concentration.
`Insulin is required for adequate albumin
`synthesis. Diabetic subjects have a decreased synthetic rate,
`which improves with insulin infusion.2o Perfused livers of
`diabetic rats have a 50% decrease in gene transcription.55
`Corticosteroids have complex effects on albumin synthesis.
`There is increased albumin synthesis with combinations of
`steroids and insulin, and of steroids with amino acids.?" 76
`
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`Nicholson et al.
`
`and skin break down 40-60% of a dose of labelled
`albumin. 108 The liver, despite its high rate of protein
`metabolism, degrades 15% or less of the total. The kidneys
`are responsible for about 10%, while another 10% leaks
`through the stomach wall into the gastrointestinal tract.
`The mechanism of breakdown involves uptake into
`endocytotic vesicles, which fuse with
`lysosomes
`in
`endothelial cells. This may involve binding to endothelial
`surface membrane scavenger receptors, called gp 18 and
`gp30, which are widespread in the body tissues.f" They bind
`altered or denatured albumin, and it is likely that chemical
`modification of the circulating albumin is a signal
`for
`receptor-linked lysosomal degradation. It is also possible
`that modification prevents degradation. Binding of LCFAs
`to albumin seems to protect the molecule from breakdown.
`In analbuminaemia, the LCFAlalbumin ratio is increased
`and degradation is suppressed.i'' The final breakdown
`products are free amino acids that add to the pools of
`amino acids within cells and in the plasma.
`
`Albumin and critical illness
`Critical illness alters the distribution of albumin between the
`intravascular and extravascular compartments. There are
`also changes in the rates of synthesis and degradation of the
`protein. The serum albumin concentration will decrease,
`often dramatically, from early in the course of a critical
`illness. It will not increase again until the recovery phase of
`the illness. The kinetics of albumin given i.v. will differ
`greatly between critically ill patients and healthy subjects.
`The implication of this, given the important
`functions
`albumin has in health, is that using exogenous albumin to
`increase the intravascular albumin concentration during
`critical illness is beneficial. But studies have failed to show
`any benefit of albumin over other colloidal therapies in
`adults.
`The altered distribution in critical illness is related to an
`increase in capillary leakage.r" This occurs in sepsis'" and
`after major surgical stressr'" 99 It involves dysfunction of the
`endothelial barrier, resulting in capillary leakage and loss of
`protein, inflammatory cells and large volumes of fluid into
`the interstitial space. The precise mediators of this capillary
`leakage are still being discovered and currently include:
`
`• endotoxin from Gram-negative bacteriar' 71
`• cytokines-TNF-a and IL-6;12 15
`• arachidonic acid metabolites-leukotrienes and prosta-
`glandins; 10 31
`• complement components C3a and C5a;31
`• other vasoactive peptides-bradykinin, histamine;"
`• chemokines-macrophage inflammatory protein 1a.95
`
`for albumin
`transcapillary escape rate
`The normal
`increases by up to 300% in patients with septic shock, and
`by 100% after cardiac surgery.r" In septic patients,
`the
`transcapillary exchange rate may well improve with appro(cid:173)
`priate treatment. With increased flow of albumin across
`
`should be an increase in
`there
`capillary membranes,
`lymphatic return to the intravascular compartment. Studies
`of albumin kinetics during major surgery have shown a
`reduction in the flow rate of lymph and the albumin
`concentration in lymph.i" It is not known ifthis extends into
`the postoperative period. Measurement of total circulating
`and total
`exchangeable albumin pools
`shows
`a 30%
`reduction with major surgery,38 consistent with sequestra(cid:173)
`tion of albumin into non-exchangeable sites,
`such as
`wounds, the intestine and extra-abdominal sites. 65
`The rate of albumin synthesis may be significantly altered
`in the critically il1.27 In the acute-phase response to trauma,
`inflammation or sepsis,
`there is an increase in the gene
`transcription rate for the positive acute-phase proteins such
`as C-reactive protein, and decreases in the rate of transcrip(cid:173)
`tion of albumin mRNA and the synthesis of albumin. 64 IL-6
`to reduce gene transcription.l'' 15
`and TNF-a both act
`Induced inflammation in rats decreased the concentration of
`albumin mRNA and the rate of albumin synthesis, which
`reached a minimum by about 36 h and then began to rise
`again.i" 92 A sustained inflammatory response in critical
`illness may lead to prolonged inhibition of albumin
`synthesis.
`Catabolism of albumin may also be altered. The FDR is
`mass-dependent. That is, as the serum albumin concentra(cid:173)
`tion decreases, so does the FDR. Studies have shown a
`significantly shorter plasma half-life in hypoalbuminaemic
`patients on total parenteral nutrition (9 days), but with a
`catabolic rate similar to normal.?" However, in situations of
`increased transcapillary albumin flux, an increase in the
`FDR has been observed" It is possible that the vascular
`endothelium has an important role in the degradation of
`albumin. In animal experiments, the tissues most actively
`involved in albumin catabolism are those with fenestrated or
`discontinuous capillaries.l'" It may be that a high rate of
`tissue exposure in situations of increased capillary perme(cid:173)
`studies of
`ability may increase catabolism. However,
`albumin extravasation in myxoedema found that, while
`there was an increase in the extravascular pool of albumin,
`there was a decrease in the catabolic rate, implying tissue
`exposure and trapping of albumin, protecting it
`from
`degradation. 84
`
`Functions of albumin
`Albumin has
`extensively studied and well-established
`physiological functions in health. There are, however, few
`studies on the function of albumin in the critically ill.
`
`Oncotic pressure
`In healthy subjects, the role of albumin in the maintenance
`of normal COP is well recognized, but there appears to be
`little correlation between albumin and COP in the critically
`ill?5 In health, albumin contributes up to 80% of the normal
`COP of about 25 mmHg. 34 101 106 This is because of its high
`
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`Albumin in critical illness
`
`molecular weight and concentration in plasma. Albumin is
`present at a higher concentration than other plasma proteins,
`and though its molecular weight of 66.5 kDa is less than the
`average for serum globulins (about 147 kDa), it still has the
`greatest osmotic significance. This direct osmotic effect
`provides 60% of the oncotic pressure of albumin. The
`remaining 40% is a result of its negative charge, providing
`an attractive force for
`the intravascular
`retention of
`positively charged solute particles (the Gibbs-Donnan
`effect). Due to the large extravascular pool of albumin, its
`water-solubility and its negative charge, albumin also plays
`a significant role in the regulation oftissue fluid distribution.
`Critically ill patients have a lowered serum COP. A
`sequential series of 200 critically ill patients had an
`mean COP of 19.1 mmHg. 106 A lowered COP is associated
`with increased morbidity and mortality in critically ill
`patients.P" 100106 A serum COP of 15 mmHg was associated
`with a survival rate of 50%.100 Proponents of albumin
`supplementation argue that giving albumin will increase the
`COP and avoid potentially fatal complications such as
`pulmonary oedema,
`though the association with fatal
`progression of respiratory failure 63 has not been substanti(cid:173)
`ated by other studies. The pulmonary lymphatic system is
`capable of a sevenfold increase in flow rate in response to
`isobaric reduction in COP, to a level sufficient to induce
`massive peripheral oedema and ascites in baboons. 109 There
`is evidence to suggest that the pulmonary dysfunction in
`critically ill, septic patients is independent of COP.52
`
`Binding of substances to albumin
`The structure of the albumin molecule is such that it can
`incorporate many different substances.
`is a flexible
`It
`molecule, and bound compounds can be buried within the
`structure. Some general trends have emerged from binding
`studies. Most strongly bound are medium-sized hydro(cid:173)
`phobic organic anions,
`including long-chain fatty acids,
`bilirubin and haematin. Less hydrophobic and smaller
`substances can be bound specifically but with lower affinity,
`such as ascorbate and tryptophan. The chirality of the
`compound may be important: L-tryptophan is bound more
`strongly than u-tryptophari" Monovalent cations do not
`bind, but divalent cations do, namely calcium and magne(cid:173)
`sium. Albumin has a strong negative charge, but there is
`little correlation between the charge of the compound and
`the degree of binding to albumin" Acidic drugs tend to
`bind to other plasma proteins such as o.l-acid glycoprotein
`whereas basic drugs tend to bind to albumin. There are
`exceptions, and drugs may bind to both.
`Other endogenous compounds that bind to albumin
`include bile acids, eicosanoids, copper, zinc, folate and
`aquacobalamin. Albumin is also a secondary or tertiary
`carrier for some substances
`that have specific binding
`proteins, for example, steroids, including derivatives such as
`vitamin D and thyroxine. This can be clinically significant.
`Steroids have a low binding affinity for albumin but there is
`
`a large capacity owing to the high concentration of
`albumin." Thus a significant amount may be carried by
`albumin, and the lower binding affinity means that there is
`easy off-loading at target sites.
`Drug-binding studies have traditionally been performed
`in vitro, measuring affinity and competition between
`ligands, at non-physiological
`temperature and with non(cid:173)
`human albumin species. It is difficult to draw conclusions
`about in vivo binding from these studies. In recent years
`techniques such as DNA sequencing, fluorescence emission
`of 'reporter' compounds, which respond to the presence of
`ligands, x-ray diffraction and the isolation of functional
`fragments of albumin have given insight into the functional
`sites of binding.F'
`Drug binding strongly affects the delivery of bound drug
`to tissue sites and the metabolism and elimination of the
`drug. The free serum concentration is the relevant factor in
`these processes. Highly bound drugs have only a small
`percentage of the total serum concentration in the free form.
`Other factors that are important
`in drug-albumin inter(cid:173)
`actions and may be responsible for the wide interindividual
`variation seen include age (binding may decrease at the
`extremes); temperature, pH and ionic strength, which can
`affect the number of binding sites in vitro; and competition
`between drugs for binding sites.51
`Displacement of drugs from their binding sites by other
`drugs or by endogenous substances occurs and may alter the
`distribution, pharmacological action, metabolism and excre(cid:173)
`tion of the displaced drug. There are a variety of binding
`sites on the albumin molecule. Sudlow et al.98 have
`classified drugs into two groups according to two broad
`binding sites, site I and site II. Site I appears to lie along the
`long loop of subdomain IIa, extending into the shorter
`loop?5 Many different drugs seem to bind here, including
`salicylates, warfarin, phenylbutazone,
`indometacin, digi(cid:173)
`toxin, furosemide, phenytoin, chlorpropamide and some
`penicillins.F' Dyes such as sulfobromophthalein, iophen(cid:173)
`oxate (a radio-opaque dye), methyl red, Evans blue and
`bromocresol green also bind here, as do endogenous
`compounds such as bilirubin.
`Site II is a hydrophobic pocket of residues located in
`subdomain IlIa. 14 It is responsible for binding compounds
`such as L-tryptophan, thyroxine (which may also bind at
`site I), medium-chain fatty acids and chloride. Drugs that
`bind here include diazepam and other 2,3-benzodiazepines,
`non-steroidal anti-inflammatory agents that have ionized
`carboxyl groups (such as ibuprofen and naproxen) and
`clofibrate. Many other substances bind to various different
`sites on the albumin molecule.
`There are many factors influencing drug-albumin inter(cid:173)
`actions that become relevant in critically ill patients. Renal
`failure provides
`a good example of
`the mechanisms
`involved. The serum albumin concentration may be directly
`altered, due to increased loss of albumin through damaged
`glomeruli. Renal failure may influence drug binding to
`albumin. 1Possible mechanisms involved include changes in
`
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`Nicholson et al.
`
`pH 19 and the accumulation of compounds which compete
`with drugs for binding sites. Thus, there may be an increase
`in the free fraction of drugs in renal failure, resulting in an
`increased drug effect. We have compared cyclosporin (not
`bound to albumin) with tacrolimus (bound to albumin) in
`patients after liver transplantation who were given human
`albumin solution to maintain a serum albumin concentration
`of >20 g litre-lora gelatin solution. lo2 Patients given
`tacrolimus with the gelatin solution had a greater increase in
`their serum creatinine than all the other groups. The reason
`for this may be the greater free fraction of tacrolimus,
`causing nephrotoxicity. Reves and colleagues'f showed that
`the onset of sleep in patients given midazolam was faster in
`those with a lower serum albumin concentration. We have
`shown that the rate of metabolism of midazolam by human
`liver microsomes is inversely proportional to the albumin
`concentration in the incubate, although the reverse occurred
`when human hepatocytes in long-term primary culture were
`used (unpublished observation, Park GA, Miler L).
`Measurement of the free fraction of a drug such as
`midazolam in vivo is difficult. Patients with renal failure in
`an intensive care unit commonly require renal replacement
`therapy in the form of continuous venovenous haemofiltra(cid:173)
`tion. This uses a semipermeable membrane with a pore size
`of 45 000 Da. Substances of small molecular weight, such as
`midazolam, have a sieving coefficient of I; they are freely
`filtered. The concentration in the filtrate will be the same as
`the free plasma concentration. Simultaneous measurement
`of blood and filtrate concentrations might allow the bound
`and free midazolam to be calculated (boundetotal mid(cid:173)
`azolam-free midazolam).
`A thorough knowledge of the pharmacokinetic principles
`outlined above, and of possible drug interactions and
`the management of
`displacement reactions,
`is vital
`for
`critically ill patients.
`In many cases it
`is necessary to
`monitor the free serum concentrations of drugs to avoid
`toxicity.
`
`Metabolic function
`and
`in transporting drugs
`role
`Apart
`from its vital
`endogenous compounds, albumin is also involved in the
`inactivation of a small group of compounds.I? Disulfiram is
`inactivated by binding with albumin. Members ofthe penem
`group of antibiotics bind irreversibly to albumin, through
`acetylation of an s-lysine group close to the surface of the
`molecule in the region of Sudlow site 1.15 The resulting
`complex may be clinically significant. Penicillin allergy has
`been linked to irreversible coupling of penicilloyl groups to
`these lysine groups. Coupling causes 'bisalbuminaemia',
`seen as a more rapidly moving albumin on an electro(cid:173)
`phoretic strip. This is associated with the appearance of
`antibodies to the drug-albumin complex (antipenicilloyl
`antibodies) in patients treated with penicillin.53
`Albumin is also involved in the metabolism of endo(cid:173)
`genous substances such as lipids and eicosanoids, because
`
`of the avidity with which these compounds bind to albumin.
`For instance, lipoprotein lipase activity in adipose tissue can
`be stimulated by the avidity with which fatty acids, freed
`from lipids in fat stores, bind to available albumin. Albumin
`can stabilize some eicosanoids during metabolism, such as
`prostaglandin 12 and thromboxane A2; it can increase the
`release of arachidonate from macrophages; and it seems to
`favour lipo-oxygenase over cyclo-oxygenase activity.I"
`
`Acid-base function
`The presence of many charged residues on the albumin
`molecule and the relative abundance of albumin in plasma
`mean that
`it can act as an effective plasma buffer?3 At
`physiological pH, albumin has a net charge of negative 19.
`It is responsible for about half of the normal anion gap. A
`reduction in plasma protein concentration causes metabolic
`alkalosis. A decrease in serum albumin of 1 g dl- I may
`increase standard bicarbonate by 3.4 mmol litre", produce a
`base excess of 3.7 mmollitre- I and reduce the anion gap by
`3 mmollitre- I .58
`
`Antioxidant function
`Under physiological conditions, albumin may have signifi(cid:173)
`cant antioxidant potential. It is involved in the scavenging of
`oxygen free radicals, which have been implicated in the
`pathogenesis of inflammatory diseases. Physiological solu(cid:173)
`tions of human serum albumin have been shown to inhibit
`the production of oxygen free radicals by polymorpho(cid:173)
`nuclear leukocytes.V This may be related to the abundance
`of sulfhydryl (-SH) groups on the albumin molecule. These
`such as
`are important scavengers of oxidizing agents,
`hypochlorous acid (HOCI)
`formed from the
`enzyme
`myeloperoxidase, which is released by activated neutro(cid:173)
`phils?8 103 Other plasma substances, such as uric acid and
`ascorbic acid, are less important scavengers, but may be
`more important
`in extracellular fluids that have a low
`albumin concentration.f" The implication of this is that
`hypoalbuminaemic patients have a reduced potential for
`oxygen radical scavenging. Serum from patients with
`rheumatoid arthritis shows decreased protection against
`105 The
`o.l-antiproteinase inactivation by HOCI and H202.
`situation in critically ill patients has not been investigated.
`
`Maintaining microvascular integrity
`It is possible that albumin has a role in limiting the leakage
`from capillary beds during stress-induced increases in
`capillary permeability.Y Endothelial cells seem to be able
`the permeability properties of the capillary
`to control
`membrane, possibly by altering the nature and distribution
`of glycoproteins in the vessel wall. Albumin plays a part in
`this action, though the exact mechanism is not clear. It may
`involve the strong negative charge on the albumin molecule
`repelling other negatively charged molecules in the mem-
`
`604
`
`MPI EXHIBIT 1098 PAGE 6
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`MPI EXHIBIT 1098 PAGE 6
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`

`Albumin in critical illness
`
`brane, or it may be a space-occupying function of the
`albumin molecule that reduces the size of channels. It is
`likely that only a small amount of albumin is necessary for
`this function. A direct protective function of albumin is
`suggested by the observation that
`albumin prevents
`apoptosis in cultured endothelial cells. ll D Peak protection
`was seen at physiological concentrations of albumin.
`is also likely that other colloids are effective in
`It
`preserving microvascular architecture.l'" Medium molecu(cid:173)
`lar weight starches have been shown to produce higher
`reflection coefficients and less transcapillary leakage in
`animal studies, when compared with crystalloid, albumin
`and starches of smaller molecular weight. Human studies
`are needed, especially in the light of encouraging evidence
`of the beneficial effects of hydroxyethyl starch (HES) over
`and
`albumin in terms of cardiorespiratory variables
`splanchnic perfusion," which suggest attenuation of endo(cid:173)
`thelial cell activation, and improved intravascular volume
`that is possibly caused by the plugging of capillary channels.
`Albumin is the most
`important source of sulfhydryl
`groups in the circulation. Nitric oxide (NO) binds to these
`sulfhydryl groups to form a stable S-nitrosothiol group, and
`is thus protected from rapid degradation. The effects of
`albumin on the vasodilatory properties of NO have been
`studied in vitro.46
`47 Albumin slowed the onset and reduced
`the maximal intensity of the vasodilatory response to NO. It
`is possible that albumin has a role in the modulation of
`vascular tone in different vascular beds, though there is no
`evidence for this.
`
`Anticoagulant effects
`Albumin has effects on blood coagulation. It seems to exert
`a heparin-like action, perhaps related to a similarity in the
`structures of the two molecules. Heparin has negatively
`charged sulphate groups that bind to positively charged
`groups on antithrombin III, thus exerting an anticoagulant
`effect. Serum albumin has many negatively charged groups.
`There is a negative correlation between albumin concen(cid:173)
`tration and the heparin requirement in patients undergoing
`haemodialysis.V These
`investigations have
`shown
`a
`heparin-like activity of albumin, through enhancement of
`the neutralization of factor Xa by antithrom