Albumin. A protein with a multitude of functions

Transcription

1 Albumin A protein with a multitude of functions

2 Overview The albumin molecule Albumin distribution, metabolism and binding Albumin in fluid homeostasis Other functions of albumin

3 The albumin molecule

4 The albumin molecule Single polypeptide chain amino acids 66 kda molecular weight Negatively charged 2 Non-glycosylated 2 Serum albumin. Reproduced with permission from David Goodsell & RCSB Protein Data Bank, Gatta et al. Intern Emerg Med 2012; 7 Suppl 3: S193 9; 2. Fanali et al. Mol Aspects Med 2012; 33:

5 Albumin distribution, metabolism and binding

6 Albumin distribution ~350 g total 1 ~40% ~60% intravascular 2 extravascular 2 1. Hankins. J Infus Nurs 2006; 29: 260 5; 2. van der Vusse. Drug Metab Pharmacokinet 2009; 24:

7 Albumin metabolism Up to 14 g of albumin is synthesized daily 1 Site of synthesis: 1 Liver Regulators of synthesis: 1,2 Colloid osmotic pressure (COP) Cortisol and thyroid hormone ~4% of the albumin pool is degraded daily 1 COP, colloid osmotic pressure 1. Gatta et al. Intern Emerg Med 2012; 7 Suppl 3: S193 9; 2. van der Vusse. Drug Metab Pharmacokinet 2009; 24:

8 Albumin binding Many albumin functions are mediated by ligand binding Albumin is endowed with multiple specific binding sites Fatty acids Bilirubin Haem ROS Calcium Copper Iron Nitric oxide Tryptophan Glucocorticoids Thyroxine Vitamins Amyloid-β Endotoxin Drugs Contrast agents Albumin bound to 7 arachidonic acid molecules. Reproduced with permission from David Goodsell & RCSB Protein Data Bank, ROS, reactive oxygen species Gatta et al. Intern Emerg Med 2012; 7 Suppl 3: S193 9; Fanali et al. Mol Aspects Med 2012; 33:

9 Therapeutic drugs bound by albumin NSAIDs Cardiovascular Antidepressants Diuretics Ibuprofen Digitoxin Amitryptaline Furosemide Naproxen Hydralazine Chlorpromazine Thiazides Indomethacin Phenyl butazone Salicylates Propranolol Quinidine Verapamil Imipramine Chemotherapy Cisplatin Carbonic anhydrase inhibitors Anti-epileptics Antimicrobials Cephalosporins Anxiolytics Diazepam Midazolam Paclitaxel Tamoxifen Valproate Phenytoin Penicillins Sulfonamides Tetracyclins Vancomycin Lorezepam Phenobarbitol Anaesthetics Propofol Halothane Thiopentone Other Warfarin Clofibrate Glipizide NSAIDs, non-steroidal anti-inflammatory drugs Gatta et al. Intern Emerg Med 2012; 7 Suppl 3: S193 9; Fanali et al. Mol Aspects Med 2012; 33:

10 Albumin in fluid homeostasis

11 Albumin in fluid homeostasis Net filtration pressure (mm Hg) Arteriolar 15 Venular -3 Albumin supplies 80% of total plasma COP Albumin retards: Fluid efflux from plasma Oedema formation 2 37 Arteriolar hydrostatic pressure gradient (mm Hg) 3 20 COP, colloid osmotic pressure Hankins. J Infus Nurs 2006; 29: mm Hg interstitial COP Venular hydrostatic pressure gradient (mm Hg)

12 Other functions of albumin

13 Other functions of albumin Albumin is involved in numerous other functions, many due to binding, including: Transport/delivery Detoxification Reservoir (i.e. provides storage) for signalling molecules and nitric oxide Acid base balance Apoptosis Cell proliferation Oxidation reduction Immunomodulation Gatta et al. Intern Emerg Med 2012; 7 Suppl 3: S193 9; Fanali et al. Mol Aspects Med 2012; 33:

14 Fatty acid transport/delivery Fatty acids (FA) are essential for: Energy metabolism Synthesis of membrane phospholipids and regulatory mediators Albumin-bound FA Albumin: Binds and solubilizes FA Is the main protein binding FA in extracellular fluids Plasma albumin delivers FA to vascular endothelium Interstitial albumin transports FA to target tissues Serum albumin. Reproduced with permission from David Goodsell & RCSB Protein Data Bank, FA, fatty acids van der Vusse. Drug Metab Pharmacokinet 2009; 24:

15 Detoxification of bilirubin The majority of circulating unconjugated bilirubin is bound to albumin Albumin solubilizes bilirubin and neutralizes its toxic effects Albumin transports bilirubin to the liver In the liver, bilirubin: Dissociates at the sinusoidal surface of hepatocytes Undergoes conjugation and excretion in bile Albumin-bound bilirubin Serum albumin. Reproduced with permission from David Goodsell & RCSB Protein Data Bank, Wang et al. Curr Paediatr 2006; 16: 70-74

16 Extracorporeal detoxification Accumulation of albumin-bound metabolites occurs in liver failure Albumin-bound substances in plasma are decreased by extracorporeal albumin dialysis including: Bilirubin Bile acids Tryptophan Middle- and short-chain FA Aromatic amino acids TNF-α IL-6 Copper Diazepam FA, fatty acids; TNF, tumour necrosis factor; IL, interleukin Mitzner et al. J Am Soc Nephrol 2001; 12: S75 S82

17 Nitric oxide reservoir Nitric oxide (NO) has diverse actions, including: Vasodilation Platelet aggregation Neutrophil adhesion Superoxide production and removal Plasma NO is mostly covalently bound to albumin S-Nitrosylated adduct (SNO-albumin) forms at cysteine 34 SNO-albumin is more stable than low-molecular weight S-Nitrosothiols NO is released from the SNO-albumin reservoir as needed by: Transfer to low-molecular weight thiols SNO-albumin binding to albumin cell surface receptor NO, nitric oxide; SNO-albumin, S-nitrosylated adduct Ishima et al. Drug Metab Pharmacokinet 2009; 24:

18 Mean arterial pressure (mm Hg) Phosphocreatinine (nmol-mg/protein) SNO-albumin in cardiopulmonary bypass In the ischaemia/reperfusion model, SNO-albumin increased: Mean arterial pressure Cardiac energetic reserve 60 P< P< SNO-Albumin Control SNO-Albumin 0 SNO-Albumin Control SNO-Albumin SNO-albumin, S-nitrosylated adduct Hallström et al. Cardiovasc Res 2008; 77:

19 Acid base balance Stewart model of acid base balance 3 independent variables govern acid base balance: pco 2 Total concentration of weak acids (A TOT ) Strong ion difference (SID) Albumin is the main determinant of A TOT A TOT, total concentration of weak acids; SID, strong ion difference Stewart. Can J Physiol Pharmacol 1983; 61:

20 Apoptosis Albumin prevents apoptosis of proximal renal tubular cells Effective at physiological albumin concentrations This activity is independent of both bound lipid and COP Albumin Dextran 7-day culture Xanthine oxidase + albumin Xanthine oxidase 24-hour culture Survival (%) Iglesias et al. Am J Physiol 1999; 277: F711 22; Bolitho et al. J Vasc Res 2007; 44:

21 Cell proliferation Albumin stimulates proliferation of proximal renal tubular cells Effective at physiological albumin concentrations Maintains proximal tubular integrity/function Ca 2+ Albumin EGF receptor Protein kinase C Mitogen-activated protein kinases NF-κB DNA synthesis Lee et al. Am J Physiol Renal Physiol 2008; 294: F

22 Oxidation reduction Diverse oxidation reduction reactions are affected by albumin Cysteine 34 of the albumin polypeptide chain Participates reversibly in oxidation reduction reactions Provides 80% of plasma thiols Major contributor to total plasma antioxidant capacity Binding to albumin can influence oxidation reduction Albumin binds and inactivates pro-oxidant Cu 2+ Unsaturated FA are protected from oxidation by binding to albumin Albumin-bound bilirubin is endowed with antioxidant activity Quinlan et al, Clin Sci (Lond) 1998; 95: ; Quinlan et al, Crit Care Med 2004; 32: ; Bourdon et al. Free Radic Res 2005; 39: 15 20

23 LDL oxidation t ½ (min) 50% haemolysis time (min) Antioxidant activity 200 p< p<0.001 Blockade by albumin of: Cu 2+ -mediated LDL oxidation Free-radicalmediated haemolysis Albumin Control Albumin Control LDL, low-density lipoprotein Bourdon et al. Free Radic Res 2005; 39: 15 20

24 Immunomodulation An immune response is central for: Host response to infection Damage repair But an overexuberant response can be harmful, for example: SIRS after cardiac surgery Excessive capillary permeability during shock Albumin: Is free of the pro-inflammatory effects exhibited by other resuscitation fluids Displays immunomodulatory activity SIRS, Systemic Inflammatory Response Syndrome Wheeler et al. Adv Pharmacol Sci 2011; Dubois et al. Crit Care Med 2006; 34:

25 Pro-inflammatory effects of resuscitation fluids * p<0.05 Phosphate-buffered saline Normal saline Ringer s lactate Dextran-40 6% HES 5% albumin 25% albumin * * * * * Neutrophil oxidative burst activity Whole blood from volunteers diluted 75% in vitro with test fluids Crystalloids and artificial colloids showed significant pro-inflammatory effects No significant effect of 5% or 25% albumin Rhee et al, Crit Care Med 2000; 28: 74 78

26 Prevention of haem-mediated delay in neutrophil apoptosis * p<0.05 Haemolysis causes a release of haem Frequent in cardiac surgery Haem is known to be proinflammatory and probiotic Haem-mediates a delay in neutrophil apoptosis This effect is blocked by albumin Therefore, the replacement of albumin in a bypass circuit may avert the pro-inflammatory and probiotic effects of haemolysis Control Albumin Haem Haem + albumin Relative neutrophil survival * Bundy et al, Am J Respir Crit Care Med 2010; 181: A2798

27 Host cell response to Gram-negative bacteria LPS Albumin forms a complex with: Lipopolysaccharide (LPS) Soluble CD14 (scd14) This complex acts on tolllike receptor 4 IL-8 is generated Albumin is an essential facilitator of the response to endotoxin scd14 Endothelial cell LPS-sCD14-Albumin Complex Toll-like receptor 4 Albumin LPS, liposaccharide; scd14, soluble CD14; IL-8, interleukin-8 Gioannini et al, J Biol Chem 2002; 277: Interleukin-8

28 Conclusions Albumin is a central mediator of physiological homeostasis Albumin maintains fluid balance by maintaining COP Albumin has numerous other functions, many due to binding, including: Transport/delivery Detoxification Reservoir Acid base balance Apoptosis Cell proliferation Oxidation reduction immunomodulation

29 References (A-L) Bolitho C, et al. The anti-apoptotic activity of albumin for endothelium is mediated by a partially cryptic protein domain and reduced by inhibitors of G-coupled protein and PI-3 kinase, but is independent of radical scavenging or bound lipid. J Vasc Res 2007; 44: Bourdon E et al. Differential effects of cysteine and methionine residues in the antioxidant activity of human serum albumin. Free Radic Res 2005; 39: Bundy et al. Albumin inhibits heme mediated delay of spontaneous neutrophil apoptosis. Am J Respir Crit Care Med 2010; 181: A2798 Dubois MJ et al. Albumin administration improves organ function in critically ill hypoalbuminemic patients: A prospective, randomized, controlled, pilot study. Crit Care Med 2006; 34: Fanali G et al. Human serum albumin: from bench to bedside. Mol Aspects Med 2012; 33: Gatta A et al. Hypoalbuminemia. Intern Emerg Med 2012; 7 Suppl 3: S193 9 Gioannini TL et al. An essential role for albumin in the interaction of endotoxin with lipopolysaccharide-binding protein and scd14 and resultant cell activation. J Biol Chem 2002; 277: Hallström S et al. S-nitroso human serum albumin reduces ischaemia/reperfusion injury in the pig heart after unprotected warm ischaemia. Cardiovasc Res 2008; 77: Hankins J. The role of albumin in fluid and electrolyte balance. J Infus Nurs 2006; 29: He X et al. Atomic structure and chemistry of human serum albumin. Nature 1992; 358: Iglesias J, et al. Albumin is a major serum survival factor for renal tubular cells and macrophages through scavenging of ROS. Am J Physiol 1999; 277: F Ishima Y et al. Albumin as a nitric oxide-traffic protein: characterization, biochemistry and possible future therapeutic applications. Drug Metab Pharmacokinet 2009; 24: Lee YJ et al. Albumin-stimulated DNA synthesis is mediated by Ca2+/PKC as well as EGF receptor-dependent p44/42 MAPK and NF-kappaB signal pathways in renal proximal tubule cells. Am J Physiol Renal Physiol 2008; 294: F534 41

30 References (M-W) Mitzner SR et al. Extracorporeal detoxification using the molecular adsorbent recirculating system for critically ill patients with liver failure. Am Soc Nephrol 2001; 12: S75 S82 Quinlan GJ et al. Administration of albumin to patients with sepsis syndrome: a possible beneficial role in plasma thiol repletion. Clin Sci (Lond) 1998; 95: Quinlan GJ et al. Albumin influences total plasma antioxidant capacity favorably in patients with acute lung injury. Crit Care Med 2004; 32: Rhee P et al. Human neutrophil activation and increased adhesion by various resuscitation fluids. Crit Care Med 2000; 28: 74 8 Stewart PA. Modern quantitative acid-base chemistry. Can J Physiol Pharmacol 1983; 61: van der Vusse GJ. Albumin as fatty acid transporter. Drug Metab Pharmacokinet 2009; 24: Wang X et al. Bilirubin metabolism: Applied physiology. Curr Paediatr 2006; 16: Wheeler DS et al. The immunomodulatory effects of albumin in vitro and in vivo. Adv Pharmacol Sci 2011; 2011: