Free C-peptide immunoreactivity in insulin-treated diabetics.

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Abstract
Serum samples of healthy and insulin-treated subjects were gel-filtrated before and after acid-alcohol extraction. The sera from insulin-treated diabetics contained a variable amount of proinsulin-like components (PLC) which were complexed with insulin antibodies and accumulated in the circulation. These PLC-insulin antibody complexes cross-reacted with C-peptide antiserum. There was serum C-peptide immunoreactivity (CPR) before and after the removal of PLC-insulin antibody complexes, expressed as total CPR and free CPR, respectively. The separation of PLC-insulin antibody complexes was performed by the addition of polyethylene glycol to the serum. The free and total CPR values in the subjects never on insulin (7 healthy subjects and 25 adult onset diabetics) correlated well with each other. The ratios free CPR:total CPR (F/T) were 1.02 .+-. 0.17 (fasting) and 1.00 .+-. 0.08 (60 min after glucose). F/T in insulin-treated diabetics (adult onset 29 cases) were apparently lower than that of the former group; 0.58 .+-. 0.23 (fasting), 0.67 .+-. 0.22 (60 min after glucose). The total CPR values were reflected to a considerable extent by the PLC-insulin antibody complexes. Some cases of insulin-treated diabetics revealed that over 50% of their serum CPR was attributed to PLC-insulin antibody complexes. This may be responsible for the higher CPR levels of these cases. Determination of free CPR may provide a more accurate index to evaluate the endogenous insulin responses to various stimuli in some of the insulin-treated diabetics.

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