Albumin

Human serum albumin (HSA) is an abundant multifunctional non-glycosylated, negatively charged plasma protein, with ascribed ligand-binding and transport properties, antioxidant functions, and enzymatic activities.1 It is synthesized primarily in the liver and is thought to be a negative acute-phase protein. Physiologically, albumin is responsible for maintaining colloid osmotic pressure and may influence microvascular integrity and aspects of the inflammatory pathway, including neutrophil adhesion and the activity of cell signaling moieties. Clinically, albumin has been employed as a plasma expander in many patient populations, although the evidence from meta analyses2, 3 and the recently published SAFE investigation4 suggests it does not afford a survival benefit over crystalloid solutions when administered to the critically ill. However, studies of albumin usage as a volume expander and albumin dialysis therapy in patients with liver disease have led to some encouraging results. This review aims to highlight current thinking regarding albumin therapy in the critical care and hepatological setting and also discusses other potential therapeutic applications for its use based around the complex biochemistry of this multifunctional plasma protein. Potential contraindications are also discussed. HSA, human serum albumin; NO, nitric oxide; ROS, reactive oxygen species; RNS, reactive nitrogen species; COP, colloid oncotic pressure. Albumin normally accounts for over 50% of total plasma protein content, being present at concentrations of approximately 0.6 mmol/L. HSA is a small (66 kd) globular protein composed of 585 amino acids, with few tryptophan or methionine residues but an abundance of charged residues such as lysine, and aspartic acids and no prosthetic groups or carbohydrate. X-ray crystallography has shown albumin to possess a heart-shaped tertiary structure, but in solution HSA is ellipsoid. Some 67% of the tertiary structure of HSA is composed of α-helices. Indeed, the protein is composed of 3 homologous domains (I-III), each containing two sub-domains (A and B) composed of 4 and 6 α-helices respectively. The sub-domains move relative to one another by means of flexible loops provided by proline residues, which helps accommodate the binding of an array of substances, as does the flexibility provided by domain-linking disulfide bridges. Figure 1 depicts the tertiary structure with bound fatty acids. HSA contains 35 cysteine residues, most of which form disulfide bridges (17 in all), contributing to overall tertiary structure. However, it also contains 1 free cysteine-derived, redox active, thiol (-SH) group (Cys-34), which accounts for 80% (500 μmol/L) thiols in plasma. The thiol moiety of Cys-34 is reactive and capable of thiolation (HSA-S-R) and nitrosylation (HSA-S-NO), processes that are thought to contribute to several in vivo functions. Tertiary structure of albumin showing the binding of seven archidonic acid ligands is depicted. Illustration obtained from the RCSB protein data bank PDB ID:1gnj by David S Goodsell Scripps Research Institute. Primary reference source: Petitpas I, Gruene T, Bhattacharya AA, Curry S. Crystal structures of human serum albumin complexed with monounsaturated and polyunsaturated fatty acids. J Mol Biol 2001;314:955. Physiologically, HSA exists predominantly in a reduced form (that is, with a free thiol, HSA-SH) and is known as mercaptoalbumin. However, a small but significant proportion of the albumin pool exists as mixed disulfides (HSA-S-S-R); where (R) represents low-molecular-weight, thiol-containing substances in plasma, chiefly cysteine and glutathione. Mixed disulfide formation also increases as part of the aging process (reviewed by Droge5) and during disease processes characterized by oxidative stress. Dimer formation is also theoretically possible (HSA-S-S-ASH), but in practice is unlikely to occur in vivo because of stearic interference. However, this process is known to occur ex vivo on purification and storage and therefore may have implications related to certain aspects of the therapeutic use of albumin. HSA binds many endogenous and exogenous compounds, including fatty acids, metal ions, pharmaceuticals, and metabolites, with implications for drug delivery and efficacy, detoxification, and antioxidant protection. Several low- and high-affinity ligand-binding sites have been identified on HSA, the first of which to be identified (termed site I and II) are responsible for the binding of most pharmaceuticals that interact with the protein. Sites I and II are located in different domains and exhibit differential, but not always exclusive, ligand-binding affinities. Site I tends to bind relatively large heterocyclic compounds or dicarboxylic acids. A diverse array of unrelated compounds bind with high affinity to various locations within this site, indicating adaptability. Moreover, this site is large and able to bind bulky endogenous substances, including bilirubin and porphyrins. By contrast, site II (also known as the indole-benzodiazepine site) is smaller and less flexible in nature, because binding is more stereo-specific. Importantly, additional high-affinity binding areas are present within HSA for some drugs and compounds that do not conform to either site I or II. Furthermore, the binding domains of some substances such as digitoxin and the bile acids remains to be elucidated; for a concise review on ligand-binding, see Kragh-Hansen et al.6 Cysteine-34 binds drugs including cisplatin, D-penicillamine, and N-acetyl-cysteine.6 Covalent interactions (thiolations) also occur with endogenous, low-molecular-weight, thiol-containing substances via disulphide bridge formation. Higher oxidation states of cys-34 can also occur, resulting in the formation of sulfenic, sulfinic, or sulfonic acid residues (Fig. 2), although levels seen in normal plasma (bovine) are low.7 Both endogenous and exogenous nitric oxide (NO) are known to interact with cys-34 via electrophilic addition of the nitrosonium ion (NO+). Indeed, until recently NO was thought to circulate in plasma primarily as an S-nitroso HSA adduct and to possess vasodilatory properties, augmented by NO transfer to low-molecular-weight thiols.8 However, recent in vitro and in vivo studies indicate that levels of s-nitroso-albumin that form under biologically relevant conditions in normal plasma are in the low nanomolar range (<10 nmol/L) and that several other reaction products of NO contribute to the NO plasma sink.9 It is less clear to what extent HSA contributes to NO binding in vivo under pathological conditions, or to what extent the availability of catalysts and or other NO-derived species impacts on s-nitrosolation. Furthermore, recent studies using a targeted s-nitrosoglutathione reductase murine model have demonstrated the importance of nitroso-thiol turnover in endotoxic shock.10 Further studies are required to determine HSA's role under such circumstances. Key reactions are summarized in Fig. 3. Scheme gives an overview of the steps involved (highlighted in blue) for the nitrosylation of Cys-34 of human serum albumin (HSA). Nitric oxide (NO) requires an electron accepting catalyst (reactive transition metal ion, or metal-containing proteins) to favor such reactions. Steps leading to Cys-34 oxidation and thiolation are highlighted in red. Formation of higher oxidation states of HSA are also shown. RSH, glutathione or free cysteine; Alb, albumin. Scheme depicts antioxidant (highlighted in blue) and the pro-oxidant potential (highlighted in red) of human serum albumin (HSA). The potential of iron and copper ions to catalyze the formation of the extremely aggressive and damaging hydroxyl radical · OH (the Fenton reaction) is shown. The potential ability of HSA to redox cycle these metal catalysts exacerbates this pro-oxidant response when these metals have access to Cys-34, in other words, when they are not bound at protected sites on this protein or elsewhere. As shown, such metal salts also can propagate membrane lipid peroxidation directly if stable lipid peroxides are already present. Nitric oxide and bilirubin binding may provide an indirect (supportive) antioxidant response attributable to albumin, as both compounds have reported lipid-phase antioxidant function. The N-terminal portion of HSA (N-Asp-Ala-His-Lys-) binds Cu, Ni, and Co ions with high affinity, whereas Au, Ag, and Hg ions bind to cysteine-34 (reviewed in Kragh-Hansen et al.6) HSA is also the major Zn binding protein in plasma, although there is some debate as to the nature of and location of its binding site.11 HSA has also been reported to possess a relatively weak, nonspecific, latent iron-binding capacity.12 This is, however, unlikely to be of significance under normal circumstances in plasma, because the specific, high-affinity, iron-binding protein transferrin binds all low-molecular-mass ferric iron. Aerobic metabolism is energy efficient. However, whereas oxygen-containing end products of these processes are relatively innocuous, many intermediates thereby formed are potentially, or directly, extremely reactive in nature. Such reactive oxygen species (ROS) can inflict damage on molecules, leading to the accumulation of toxic end products and cellular dysfunction or damage. Normally, the body uses protective (i.e., antioxidant) and reparative systems that limit the effects of oxidative stress. An antioxidant is any substance that when present at low levels significantly diminishes or prevents the oxidation of an oxidizable substrate, and may be dietary, constitutive, or inducible in origin. Primary antioxidants prevent ROS formation and include the iron-binding antioxidant transferrin. Secondary antioxidants scavenge pre-formed ROS. Examples include ascorbate and superoxide dismutase. For the definitive text on ROS in biology, see Gutteridge and Halliwell.13 Reactive nitrogen species (RNS) are nitrogen-centered species analogous to ROS. Evidence indicates that such species are formed in vivo; some, such as nitric oxide, contribute to various biological signaling responses. Others, however, are powerful oxidants and nitrating species capable of damaging biomolecules; antioxidant protection also limits the damage inflicted by RNS. Several such antioxidant functions have been ascribed to HSA. HSA in plasma, or bovine serum albumin in artificial systems, provides protection from lipid peroxidation propagated by inorganic ROS generated from xanthine oxidase/hypoxanthine.14 However, thiol oxidation occurs, indicating the cys-34 moiety to be the source of the antioxidant protection afforded. In more recent studies, hydrogen peroxide (H2O2) and the RNS peroxynitrite (ONOO−) have been shown to oxidize cys-34 to a sulfenic acid derivative (HSA-SOH).15 This is subsequently converted to a disulfide with the potential to be redox cycled to mercapto-albumin (HSA-SH), thereby restoring antioxidant function (Fig. 2). Increased ROS and RNS formation have been implicated as contributory factors in the onset and progression of critical illness.16 Albumin may provide effective extracellular scavenging antioxidant protection under such circumstances. Thus, albumin supplementation has been shown to replenish extracellular thiol status in patients with sepsis by means other than that which would be expected on purely stoichiometric grounds.17 Moreover, such supplementation was shown to improve thiol-dependent antioxidant protection in plasma obtained from patients with acute lung injury and to be associated with decreased levels of oxidative markers (protein carbonyls),18 although there was no difference in survival rates between groups. Persistent hypoalbuminemia is also associated with peroxidation of erythrocyte membranes in patients undergoing chronic hemodialysis, indicating that HSA protects against lipid oxidation.19 In vitro studies have shown that bovine serum albumin scavenges neutrophil-derived ROS, including hydrogen peroxide, superoxide, and hypochlorous acid.20 Inflammatory cell-derived oxidants contribute to oxidative stress during acute inflammation and the consequences thereof. HSA could potentially reduce such effects through scavenging antioxidant actions in humans, which may, also through modifying redox balance, regulate cell signaling moieties active in mediating pro-inflammatory forces (Fig. 3). In vitro, albumin has been shown to offer antioxidant protection against the oxidative effects of carbon tetrachloride and uremic toxins,21, 22 findings with implications for both hepatic and renal failure. HSA may provide a supportive antioxidant role in vivo, through its ability to bind and transport substances with known antioxidant function, specifically, bilirubin and NO, which are effective lipid phase antioxidants23, 24 (Fig. 3). Bilirubin may also protect albumin from oxidant-mediated damage.25 Heme is thought to possess pro-oxidant properties through the redox properties of iron. HSA is an effective heme-binding protein.26 Once bound to albumin, such pro-oxidant properties are decreased, indicating an antioxidant function,27 although under physiological circumstances the heme-binding plasma protein hemopexin provides most of this form of antioxidant protection.28 Free, or loosely bound, redox-active transition metal ions (low molecular mass) are potentially extremely pro-oxidant, having the ability to catalyze the formation of damaging and aggressive ROS from much more innocuous organic and inorganic species (Fig. 3). In strictly biological terms the 2 most important such metals are iron and copper. In specific circumstances (certain disease states and poisoning), these metal ions can become free of constraints, which normally limit and control their reactivity. By virtue of its high-affinity copper-binding site, HSA limits copper-catalyzed oxidative damage to other biomolecules by directing damage toward the albumin molecule itself in a sacrificial fashion.29 In similar fashion, HSA can limit damage caused by accidental biological contamination by redox active metal ions such as vanadium, cobalt, and nickel. Although HSA iron-binding is weak and nonspecific, it may offer antioxidant protection when other specific protective stratagems become overwhelmed, such as under conditions of iron overload or pronounced hemolysis (Fig. 3). Evidence indicates that accessible thiol groups can signal inflammatory cell regulatory changes dependent on their redox state.30 Thus, 25% albumin has been shown to modulate neutrophil/endothelial cell interactions after shock and resuscitation and to attenuate lung injury.31 Furthermore, HSA augments intracellular glutathione levels and influences activation of the ubiquitous transcription factor nuclear factor-kappa B using both in vitro and in vivo protocols.32 Moreover, several recent studies using a rat model of hemorrhagic shock have indicated that the type of resuscitation fluid administered greatly influences proinflammatory responses, including lung apoptosis and rates of neutrophil activation. Plasma albumin was found to be the least proinflammatory of the fluids utilized.33 The formation of sulfenic acid residues by cys-34 oxidation15 also may be a key factor determining signaling responses, because recent evidence indicates that such groups impact on cellular signaling functions, reviewed in Poole et al.34 Paradoxically, and in common with other redox active antioxidant substances, albumin can display pro-oxidant properties, through its ability to redox cycle/recycle transition metal ions such as iron and copper from the less reactive (ferric/cupric) to more pro-oxidant (ferrous/cuprious) states (Fig. 3). Thus, a recent study has shown that copper/HSA could become pro-oxidant after fatty acid binding and subsequent cys-34 oxidation.35 Iron has much less binding affinity for HSA and is more likely to be recycled as a free agent able to catalyze damaging ROS formation at sites distant from HSA. Such an action is, therefore, potentially more deleterious. Indeed, HSA administration was reported recently to be adversely associated with a decline in iron-binding antioxidant protection in patients with acute lung injury,18 an effect thought to be related to the redox cycling of iron. Intravenous albumin therefore may be inadvisable in circumstances when pronounced extracellular iron mobilization or overload are complicating factors. In healthy adults, albumin synthesis occurs predominantly in polysomes of hepatocytes (10-15 g/day) and accounts for 10% of total liver protein synthesis. Relatively small amounts of albumin are hepatologically stored (<2 g), the majority being released into the vascular space. Approximately 30%-40% of albumin synthesized is maintained within the plasma compartment. The remaining pool is located within tissues such as muscle and skin. Studies of radiolabeled albumin catabolism in normal healthy young adult males indicate a variation of half-life of between 12.7 and 18.2 days (mean, 14.8 days),36 although a dynamic exchange between plasma and the interstitium occurs. Albumin leaks from plasma at a rate of 5%/hour and is returned to the vascular space at an equivalent rate through the lymphatic system. Synthesis is a constant process, regulated at both transcriptional and posttranscriptional levels by specific stimuli, but change in interstitial colloid oncotic pressure is thought to be the predominant regulatory influence.37 Albumin homeostasis is maintained by balanced catabolism that is not well characterized, occurring in all tissues. However, most albumin (40%-60%) is degraded in muscle, liver, and kidney. Plasma hyperalbuminemia is rare, whereas hypoalbuminemia is a feature of a variety of pathological processes, including liver disorders, cancer, and severe sepsis. Ascites formation is a common complication of cirrhosis and contributes to vascular hypoalbuminemia. Perturbations in hepatic vascular control are thought to be responsible for ascites formation, although the precise mechanisms remain a matter of debate (reviewed in Arroyo38). However, cirrhosis in the advanced stages is also characterized by protein wasting and hence albumin depletion. The reasons for such dysfunction remain unresolved, but nutritional, metabolic, and hormonal abnormalities and uncharacterized responses to the release of bioactive substances including chemokines may contribute to hyopalbuminemia (reviewed in Tessari39). The capillary bed is known to be hyperpermeable in patients with sepsis, thereby leaking albumin. However, the extent to which altered liver biosynthesis and rates of catabolism contribute to the plasma albumin deficiency seen in sepsis and critical illness remains uncertain. Indeed, the half-life of HSA in patients with hypoalbuminemia supported with total parenteral nutrition is only 9 days, although rates of catabolism are normal.40 Moreover, catabolism actually may be decreased while the extracellular pool is increased, suggesting that HSA may be spared or protected.41 Studies in healthy individuals given endotoxin, as well as in the critically ill, indicate that albumin synthesis increases under these circumstances, even though hypoalbuminemia is By contrast, in of sepsis and decreased rates of liver albumin synthesis at the of acute phase protein synthesis is of Further studies in are required to this HSA is a relatively small protein that accounts for some of protein in plasma in healthy of its to the plasma protein albumin is also responsible for approximately of plasma colloid oncotic pressure pressure osmotic pressure as the negative the protein remaining to is to the effect attributable to its overall negative Moreover, HSA may influence directly vascular integrity by binding in the interstitial and and the of these to large molecules, and through its scavenging HSA has been to vascular in as an solution for volume and as an solution for the of fluid between and the of the of HSA in the critically have been not least because it is with colloid have been regarding the of albumin in states of altered capillary such as sepsis. However, have that albumin remains a volume expander with crystalloid solutions even under these However, a recent study that administration of HSA to patients with sepsis led to a significantly decline in plasma albumin with healthy of albumin Furthermore, in these states of altered capillary the formation of as is more by pressure than administration to patients with acute lung injury does not and may in reduce A variety of have regarding the and of albumin administration to critically the that albumin administration may the of whereas a published subsequently found no difference in have to potential reasons as to albumin would adversely but have to the However, in one of the in critically the SAFE study critically patients fluid resuscitation to either albumin or normal no between groups in of and in of in the care or albumin in a therapeutic in the critically ill, its range of potentially significant that remains for a of the range of patients with critical illness that have been to may specific in which HSA may be or Indeed, HSA is as a by its ability to significantly improve in patients with cirrhosis by This is supported by of the SAFE which has indicated that albumin may have effects in different patient groups. these patients with may actually be by albumin administration for whereas sepsis patients may benefit the use of albumin in all published to has on its The total administered may therefore be if properties such as to antioxidant or are the end Such properties for any (i.e., effects of HSA albumin may have properties when as an with other as in patients with acute lung injury and acute where the of HSA and therapy has been shown to improve fluid balance, and The mechanisms responsible for the in acute lung injury and acute are and may be specific to albumin with is no clear evidence that may be by one colloid more than The ability of HSA to bind many is a that may impact on Furthermore, binding of biologically active moieties such as NO levels of which become during critical may influence the ability of HSA to bind other albumin was in patients with cirrhosis for vascular volume because of its oncotic As regarding the nature of vascular control and ascites formation and with a of the use of and other the use of albumin for the of this However, it is that the properties of albumin, in with other therapeutic is of benefit to patients with thereby the renal that liver albumin has been not as a for but as a part of a in patients with hepatic the molecular for a review see et uses an that is by dialysis against solution subsequent to carbon and exchange to the of normally by the with by the has been to liver dysfunction and in more than patients over the 4 and has been shown to improve renal function and and to and hepatic (reviewed in et HSA binds including copper ions, and protein substances that in liver including and are also for the of liver disease and during The ability of to other and pro-inflammatory such as and lipid peroxidation and free may have implications for the inflammatory evidence exists of NO during both chronic and acute liver that may to such as renal and including NO is bound by HSA at may therefore also modulate NO levels during liver thereby against the of other that acute liver However, as the levels of s-nitroso-albumin found in healthy plasma to relevant physiological with NO are therefore be when of in this with the for the extent of to be under such pathological circumstances. may well be implications for although studies in this are However, the has been to a small of patients with contraindications to albumin therapy include a known to albumin and states in which fluid overload could be or severe of certain may be in specific patient populations, such as or in patients with significant or to patients with severe albumin has similar properties to these the effect is less although it could be pro-oxidant under circumstances with as HSA from different may in terms of the of metals bound to it and in levels of Albumin employed for use therefore may from endogenous HSA. Such may influence properties, and HSA can thereby in its ability to influence adhesion molecule from in Furthermore, and formation on storage may contribute to of reactions being and contamination of of HSA has been shown to adversely influence renal function in patients with 2 implications associated with albumin A more of possible effects from colloid administration can be found in a recent Human serum albumin (HSA) has many physiological and properties that it relevant to many aspects of the vascular and cellular functions that the critically by the inflammatory response severe sepsis, and The use of albumin as a volume agent in the care setting be in terms of a or over crystalloid However, albumin may be in specific circumstances, such as in patients with cirrhosis by and its potential to modulate the inflammatory response is, of The the The The and the David for their and the of for via