Metabolism of Human Gamma Macroglobulins*

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Abstract
The metabolic behavior of I131-labeled normal human gamma macroglobulin is studied in 7 normal subjects and in 26 patients with a variety of diseases. The relative roles of synthesis and catabolism in determining serum [gamma]1-macroglobulin levels are assessed. The [gamma]1-macroglobulins remain predominantly in the intravascular compartment with an average value of 82% for the whole group. The biological half time (T1/2) of [gamma]1-ma-croglobulin averages 5.1 days and the turnover (synthetic) rate averages 6.9 mg/kg/day in the control subjects. Low serum [gamma]1-macro-globulin levels are found in agammaglobulinemia, multiple myeloma, and protein-losing enteropathy. In agammaglobulinemia and multiple myeloma, deficient [gamma]1-macroglobulin synthesis causes the low serum levels. In protein-losing enteropathy, increased catabolism (T1/2 averaged 1.5 days) causes the low serum [gamma]1-macroglobulin levels. In macroglobulinemia, markedly increased synthetic rates are present. Observations are made in patients with serum [gamma]1-macroglobulin levels ranging from 5 to 5000% of normal. These indicate that the serum [gamma]1- macroglobulin level is directly related to the rate of [gamma]1- macroglobulin synthesis (in the absence of abnormal gastrointestinal protein loss) and that there is no relationship between the fractional catabolic rate and the serum concentration.