INSULIN DEGRADATION IN-VITRO AND IN-VIVO - A COMPARATIVE-STUDY IN MEN- EVIDENCE THAT IMMUNOPRECIPITABLE, PARTIALLY REBINDABLE DEGRADATION PRODUCTS ARE RELEASED FROM CELLS AND CIRCULATE IN BLOOD

The products of insulin metabolism generated in vitro and in vivo were compared in this study. Monocytes from 10 control subjects were incub ated with (125)IA(14)-labeled insulin, acid washed, and solubilized or reincubated in insulin-free binding buffer to study both intracellula r radioactivity or radioactivity released from cells to medium. To eva luate in vivo insulin metabolism, labeled insulin (100-120 mu Ci) was injected as a single intravenous bolus in 5 of the 10 subjects. Cellul ar and plasma radioactivity was characterized by high-performance liqu id chromatography (HPLC). The results of the study show the following: 1) Products with superimposable HPLC elution profiles are found withi n cells and in medium. Two new labeled products are observed in the la tter, suggesting that a membrane degradation process exists in monocyt es. 2) Intermediates found within monocytes, in medium from monocytes, and in plasma have identical elution profiles, supporting the possibi lity that insulin is metabolized in various cells by a common pathway. 3) Insulin metabolism produces intermediates that bind well to anti-i nsulin antibody. The presence in plasma of these products induces a si gnificant difference in the value of the metabolic clearance rate of i nsulin when HPLC or immunoprecipitation is used to detect intact insul in. 4) Immunoprecipitable products maintain, in part, the capacity to bind to insulin receptors and to be internalized into monocytes.