Metabolism of Albumin and Immunoglobulins in the Nephrotic Syndrome

Abstract

The nephrotic syndrome is characterized by the increased urinary excretion of albumin and of other serum proteins of intermediate molecular weight accompanied by a decrease in their serum concentration. Albumin synthesis is increased at the level of mRNA synthesis in response to decreased serum oncotic pressure. However, the magnitude of these responses is dependent upon dietary protein and is insufficient to normalize serum albumin. The absolute rate of albumin catabolism is decreased, serving to normalize serum albumin stores, but in contrast to what occurs in other hypoalbuminemic states, the fractional rate of albumin catabolism is increased. This observation is consistent with a hypothesis that increased renal catabolism contributes to total albumin catabolism in nephrosis. Like albumin, IgG is lost in the urine, its serum concentration is decreased, and the fractional rate of its catabolism is increased, suggesting that the kidney contributes to IgG catabolism in the presence of proteinuria. IgG synthesis responds in a variable fashion in the nephrotic syndrome, and may be decreased, thus contributing to its reduced serum concentration. In contrast, the serum concentration of the high-molecular-weight immunoglobulin IgM is increased, as is the serum concentration of a variety of high-molecular-weight liver-derived proteins. It is unknown by what mechanism serum IgM concentration is increased, but this response serves to defend serum protein concentration and oncotic pressure.