Circulating C-peptide Immunoreactivity: Studies in Normals and Diabetic Patients

Abstract

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.

Prevalence of residual B cell function and its metabolic consequences in Type 1 (insulin-dependent) diabetes

“…In 1972 Block et al [15], using C-peptide measurements, supported the view that Type 1 diabetes is characterized by a total deficiency of endogenous insulin secretion. Cpeptide was undetectable in newly diagnosed untreated Type 1 patients and in those who had received insulin for more than 5 years.…”

C-peptide, insulin and proinsulinlike components in diabetic and nondiabetic human pancreas.

“…Recently the immunoassay system of human C-peptide of insulin has been developed by Melani et al (1970a), and Kaneko et al (1974), and many clinical results on the plasma or urine C-peptide immunoreactivity (CPR) have been reported in various diseases, especially in patients with [insulinoma or in insulin-treated diabetic subjects (Block et al, 1972(Block et al, , 1973Horwitz et al, 1975;Heding and Rasmussen, 1975;Kuzuya et al, 1977;Jaspan et al, 1977). The PLC contents of pancreas from diabetic and nondiabetic subjects were already studied (Rastogi et al, 1973;Hayashi et al, 1977), but the amount of C-peptide in human pancreas has not been reported.…”

Radioimmunoassay of Human Proinsulin C-peptide Using Synthetic Human Connecting Peptide

“…Melani et al (Melani et al, 1970) have developed the radioimmunoassay using antiserum to albumin-conjugated natural human C-peptide. Although Rubenstein and his coworkers have demonstrated the presence of proinsulin and C-peptide in human serum using this assay system (Block et al, 1972(Block et al, ), 1972, it is extremely difficult to obtain suffrom natural source for immunization.…”