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

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.

[1]  W. F. Taylor,et al.  Studies of diabetic instability. II. Tests of insulinogenic reserve with infusions of arginine, glucagon, epinephrine, and saline. , 1971, Metabolism: clinical and experimental.

[2]  J. Roth,et al.  Proinsulin-like component of circulating insulin in the basal state and in patients and hamsters with islet cell tumors. , 1971, The Journal of clinical investigation.

[3]  F. Alford,et al.  Improvement in Both Insulin Sensitivity and Release Following Diabetic Coma , 1971, Diabetes.

[4]  D. Steiner,et al.  Studies on human proinsulin. Isolation and amino acid sequence of the human pancreatic C-peptide. , 1971, The Journal of biological chemistry.

[5]  L. Winterscheid,et al.  Hypoglycemic activity and immunological half-life of porcine insulin and proinsulin in baboons and swine. , 1971, Endocrinology.

[6]  K. Johansen.  Endogenous plasma insulin in a juvenile diabetic after ten years of insulin therapy. , 1971, Archives of internal medicine.

[7]  D. Steiner,et al.  Identification of proinsulin and C-peptide in human serum by a specific immunoassay. , 1970, Proceedings of the National Academy of Sciences of the United States of America.

[8]  J. Ensinck,et al.  Substances Immunologically Related to Proinsulin or Connecting Peptide in Swine Plasma , 1970, Hormone and metabolic research = Hormon- und Stoffwechselforschung = Hormones et metabolisme.

[9]  D. Steiner,et al.  Human serum proinsulin. , 1970, The Journal of clinical investigation.

[10]  D. Steiner,et al.  Secretion of Proinsulin C-Peptide by Pancreatic β Cells and its Circulation in Blood , 1969, Nature.

[11]  D. Steiner,et al.  Proinsulin. Secretion, metabolism, immunological and biological properties. , 1969, Postgraduate medical journal.

[12]  I. Pastan,et al.  "Big insulin": a new component of plasma insulin detected by immunoassay. , 1968, Proceedings of the National Academy of Sciences of the United States of America.

[13]  R. Chance,et al.  Porcine Proinsulin: Characterization and Amino Acid Sequence , 1968, Science.

[14]  D. Kipnis,et al.  Juvenile Diabetes Mellitus, a Deficiency in Insulin , 1968, Diabetes.

[15]  H. Metzger.  The chemistry of the immunoglobulins. , 1967, JAMA.

[16]  D. Steiner,et al.  The biosynthesis of insulin and a probable precursor of insulin by a human islet cell adenoma. , 1967, Proceedings of the National Academy of Sciences of the United States of America.

[17]  W. Gepts.  Pathologic Anatomy of the Pancreas in Juvenile Diabetes Mellitus , 1965, Diabetes.

[18]  G. Grodsky.  Production of Autoantibodies to Insulin in Man and Rabbits , 1965, Diabetes.

[19]  G. Fasman,et al.  Antibodies to Bradykinin and Angiotensin: A Use of Carbodiimides in Immunology , 1964, Science.

[20]  A. Lazarow,et al.  Immunoassay of Insulin: Two Antibody System: Plasma Insulin Levels of Normal, Subdiabetic and Diabetic Rats , 1963, Diabetes.

[21]  P. Davoren.  The isolation of insulin from a single cat pancreas. , 1962, Biochimica et biophysica acta.

[22]  S A BERSON,et al.  Immunoassay of Plasma Insulin in Man , 1961, Diabetes.

[23]  G. Grodsky,et al.  Effect of modification of insulin on specific binding in insulin-resistant sera. , 1959, Archives of biochemistry and biophysics.

[24]  S. Berson,et al.  Insulin-I131 metabolism in human subjects: demonstration of insulin binding globulin in the circulation of insulin treated subjects. , 1956, The Journal of clinical investigation.

[25]  W. Hoffman.  A RAPID PHOTOELECTRIC METHOD FOR THE DETERMINATION OF GLUCOSE IN BLOOD AND URINE , 1937 .

[26]  F. Bischoff,et al.  STUDIES ON INSULIN , 1927 .