The discovery of proinsulin and recognition of its intracellular conversion to insulin and C-peptide, which are stored and subsequently secreted together, have provided a simple method for monitoring β-cell function in insulinrequiring diabetics in whom insulin antibodies hinder measurement of immunoreactive insulin. Using an antiserum which reacts with antigenic determinants in human Cpeptide and this region in proinsulin and its intermediates we have measured C-peptide immunoreactivity (CPR) in unextracted serum and characterized its components.

During oral glucose tolerance tests (OGTT) in nine healthy subjects, fasting serum CPR was 1.3 ± 0.3 ng./ml. and peaked at 4.4 ± 0.8 ng./ml. (60 min.). These values were positively correlated with immunoreactive insulin levels (IRI). CPR was undetectable in five newly diagnosd, untreated diabetics with fasting blood sugars above 200 mg./100 ml. and unmeasurable IRI. Similarly, serum CPR could not be measured in five ketosis-prone juvenile diabetics who had received insulin for longer than five years.' In contrast, fasting CPR was 2.7 ± 0.7 ng./ml. in twelve insulin-treated adult onset diabetics and rose to 5.0 ± 1.1 ng./ml. two hours after glucose. Several of these sera were extracted in acid-ethanol, gel-filtered. and measured against human proinsulin and C-peptide standards. Fasting proinsulin levels were markedly elevated, while C-peptide levels were comparatively low. Both components increased after glucose administration. As these diabetics had circulating insulin antibodies it was postulated that the elevated proinsulin levels represented circulating proinsulininsulin antibody complexes. Evidence favoring this possibility was derived from in vitro experiments in which human I-125-proinsulin was incubated with diabetic sera and the samples analyzed by gel filtration, paper electrophoresis and immune precipitation. Similar experiments with 1-125-C-peptide showed no evidence for preferential binding to plasma proteins in diabetic sera.

Residual β-cell secretory capacity is thus present in many insulin-treated adult onset diabetics. The C-peptide immunoassay should facilitate the prospective study of β-cell function in these patients.

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