Branched Chain Amino Acids in Clinical Nutrition by Rajkumar Rajendram Victor R. Preedy & Vinood B. Patel

Author:Rajkumar Rajendram, Victor R. Preedy & Vinood B. Patel
Language: eng
Format: epub
Publisher: Springer New York, New York, NY

Regulation of Albumin Turnover

As compared with albumin synthesis, there is little research on albumin distribution and degradation. However, it is now understood that the albumin degradation process is controlled by a receptor-mediated mechanism. The major histocompatibility complex-related neonatal Fc receptor (FcRn) for IgG binds not only IgG but also albumin in a pH-dependent manner [29]. FcRn is widely expressed in several organs and tissues. Circulating IgG and albumin are pinocytosed nonspecifically and are transported to acidic endosomes where they bind FcRn. The binding of albumin and IgG to FcRn is observed at acidic pH but not at neutral pH. FcRn-ligand complexes then return to the cell surface membrane, where exposure to the neutral pH of the bloodstream triggers the release of the ligands [30]. The albumin and IgG that do not bind to FcRn progress to lysosomes for proteolytic degradation (Fig. 12.6). The half-life of albumin is shortened in FcRn-deficient mice, and the plasma albumin concentration of FcRn-deficient mice is less than half that of wild type mice [29]. These results indicate that FcRn protects albumin from degradation. Furthermore, it seems that the activity of albumin synthesis is upregulated in liver of FcRn-deficient mice, probably in response to a lower osmotic pressure caused by a lower plasma albumin concentration in the absence of FcRn. Thus, FcRn recycling is fundamentally important for albumin homeostasis. Whether FcRn plays a role in the distribution of albumin to the vascular and the extravascular spaces has not yet been determined.

Fig. 12.6Model of FcRn-mediated albumin recycling. Albumin and IgG in the bloodstream are continuously taken up by pinocytosis. The low pH in the vesicles allows binding of albumin and IgG to FcRn. FcRn-ligand complexes are exported to the cell surface, where exposure to the higher physiological pH of the bloodstream triggers release of albumin and IgG into the circulation. When the concentrations of albumin and IgG are high, FcRn is saturated and unbound ligands undergo lysosomal degradation

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