THE IMMUNOGLOBULINS OF MICE : V. THE METABOLIC (CATABOLIC) PROPERTIES OF FIVE IMMUNOGLOBULIN CLASSES

The metabolic properties of immunoglobulin were investigated by comparing five classes of mouse immunoglobulin. Three forms of 7S immunoglobulin had different rates of catabolism. The fractional rates of catabolism were found to be about 13 per cent per day for 7S γ(2a)-globulin; 25 per cent for 7S γ(2b)-globulin; and 17 per cent for 7S γ(1)-globulin. Catabolism of the three classes of 7S γ-globulin (γ(2a), γ(2b), and γ(1)) were prolonged at low serum 7S γ-globulin levels and accelerated at high serum 7S γ-globulin levels. Each of the 7S γ-globulin components was influenced by the serum level of the other mouse 7S γ-globulin components and by exogenously administered human 7S γ-globulin. They were not appreciably altered, however, by the serum level of IgA (γ(1)A-, β(2)A-globulin). The progressively changing (longer) half-times observed in turnover studies of normal IgG (7S γ-globulin) may be caused by catabolic heterogeneity of normal 7S immunoglobulins which are immunochemically and catabolically related to γ(2a)-, γ(2b)-, and 7S γ(1)-myeloma proteins. These studies indicate that the 7S γ(2a)-, 7S γ(2b)-, and 7S γ(1)-globulins share a common catabolic control mechanism. This mechanism is influenced by the serum level of each of these components, but is independent of the serum level of IgA (γ(1)A-globulin) and probably is independent of IgM (γ(1)M-globulin). Catabolism of IgA (γ(1)A-, β(2)A-globulin) and IgM (γ(1)M-globulin) was much more rapid than the catabolism of the 7S γ-globulins. The halftimes of the IgA and IgM were approximately 1.2 and 0.5 days respectively. The fractional rate of catabolism of IgA and IgM seemed to be independent of their serum concentration. The rate of catabolism, as well as the rate of synthesis, was shown to play a major role in determining the serum level of each class of immunoglobulin.