Both synthetic and catabolic processes determine the serum γ-globulin level. The rate of γ-globulin synthesis appears to be the primary factor determining the amount of serum γ-globulin. Increase of γ-globulin synthesis (as may occur following immunization or development of plasma cell tumor) elevates the serum γ-globulin level. This, in turn, accelerates the fractional rate of γ-globulin catabolism. The change in catabolic rate reduces the dimensions of the serum change from that which would occur if synthesis alone determined the serum γ-globulin level. The present studies indicate the existence of a homeostatic mechanism controlling the rate of γ-globulin catabolism.
The mechanisms of γ-globulin catabolism are specific and selective. Marked serum increase of other immunoglobulin components (ß2A-globulins and γ1-macroglobulins) do not accelerate γ-globulin catabolism. Similarly, serum albumin increases do not influence γ-globulin catabolism.
The site determining γ-globulin catabolism is restricted to a part of the γ-globulin molecule; i.e., on the F piece obtained by papain digestion and, by inference, on the H chains obtained by reduction and alkylation of γ-globulin molecules.