Plasma C-peptide and diagnosis of factitious hyperinsulinism. Study of an insulin-dependent diabetic patient with "spontaneous" hypoglycemia

Biosynthesis of insulin and glucagon: a view of the current state of the art

It is now well established that insulin biosynthesis proceeds through a precursor molecule, proinsulin. This single polypeptide chain form has been identified as a ribosomal product in the microsomal fraction from islet tissues. The newly synthesized peptide chain, after folding and thiol oxidation, is transferred to the Golgi apparatus where it begins to undergo proteolytic processing to insulin and packaging into secretory granules. The secretion from the cells of significant amounts of newly synthesized material by exocytosis begins only one hour or more after biosynthesis and this process is regulated by several factors, including glucose. Foci of current attention discussed in this paper include (1) the possible existence of larger precursor forms than proinsulin, especially short-lived biosynthetic transients with extended NH2-termini analogous to the recently described immunoglobulin L chain and proparathyroid hormone precursors; (2) the large-scale production of insulin by chemical or genetic engineering approaches; (3) isolation of beta-cell plasma membranes; (4) regulatory mechanisms for the biosynthesis and secretion of insulin, the possible role of mRNA modification in this process, and effects of somatostatin on insulin biosynthesis and secretion; (5) studies on the secretion, metabolism and clinical usefulness of the proinsulin C-peptide; (6) finally, the biosynthesis of glucagon and other peptide hormones and the general significance of precursor forms.

Hypoglycemia: factitious and felonious

Factitious diseases are characterized by physical or psychologic symptoms that are voluntarily self-induced. These diseases are as old as mankind. Once called "malingerers," these patients must be distinguished from hysterics in whom symptoms are produced unconsciously. In factitious diseases, illness is produced by deliberate acts by the patient who when seeking medical help omits to mention them and may continue strenuously to deny them even when confronted with the evidence. Factitious diseases occur in patients who simulate or exaggerate symptoms or disability to obtain some kind of discernible personal gain or avoid an unpleasant situation; however, such actions may only produce disadvantages by exposing the patient to the risk of death or permanent injury. This has been described as Munchausen syndrome, which is probably a manifestation of severe psychiatric disease. The use of medicines or poisons to induce illness in others also produces a type of factitial disease and presents similar or greater difficulties in diagnosis. In both situations, the clinical history, ordinarily the most important clue to the correct diagnosis, is not only incomplete but often misleading. Sometimes referred to as Munchausen by proxy, this form of factitial disease may be impossible to distinguish from attempted murder or grievous bodily harm. The subtle differences between these disorders, if any, have not been discussed herein.

Case report: factitious hypoglycemia in diabetic patients

Factitious hypoglycemia (FH) in a diabetic patient represents a difficult diagnostic and costly management problem. An adolescent diabetic with FH is reported. A literature search revealed 10 adolescent and 45 adult diabetic patients with FH. Tests currently available for diagnosis are evaluated. The role of psychiatric therapy in relation to overall management and prognosis is stressed.

Factitious hypoglycemia. Clues to identifying an elusive disorder

Factitious hypoglycemia is similar in presentation to insulinoma and occurs most commonly in persons with ready access to insulin. Diagnosis previously was based on circumstantial evidence but now can be confirmed by demonstration of high insulin levels and low C-peptide levels in the presence of hypoglycemia. Treatment is primarily psychiatric, and success so far is limited.

Circulating C peptide: measurement and clinical application

Pancreatic beta-cell function is usually assessed by the measurement of plasma insulin concentration in various clinical situations. However, the advent of an assay for the measurement of connecting-peptide (C-peptide) concentration in plasma has provided a further method for the assessment of the secretory capacity of the pancreatic beta cell in clinical disorders, particularly in the investigation of hypoglycaemia. The metabolism and immunoassay methodology of C-peptide are reviewed, and its application in clinical practice is outlined.

Hypoglycemia secondary to factitious hyperinsulinism

The case is reported of a young woman with spontaneous hypoglycemia due to fractions hyperinsulinism. Hyperinsulinism was established as the cause by the fact that the level of immunoreactive insulin was significantly elevated and inappropriate for the hypoglycemia. Exogenous insulin injection was implicated on the basis of the finding that serum C-peptide was suppressed. The patient later admitted injecting insulin and was discharged under psychiatric care.

Insulinoma (apud cell carcinoma) in a diabetic

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Proinsulin and C-peptide in diabetes

The availability of C-peptide measurement continues to provide new and useful information about the state of beta cell secretory function and the natural history of diabetes. Measurement of proinsulin is of value in the diagnosis of insulin-secreting tumors.

Factitious hypoglycemia. Diagnosis by measurement of serum C-peptide immunoreactivity and insulin-binding antibodies

In seven patients with factitious hypoglycemia due to the surreptitious injection of insulin, we made the diagnosis by measurements of plasma insulin and C-peptide immunoreactivity (in seven patients), facilitated by the finding of circulating insulin-binding antibodies (in two patients). The simultaneous demonstration of low plasma glucose, high immunoreactive insulin and suppressed C-peptide immunoreactivity represents a triad of results pathognomonic of exogenous insulin administration. Determination of plasma free C-peptide and free insulin permitted patients with high titers of insulin antibodies, including those with a history of insulin-treated diabetes, to be studied and diagnosed in a way similar to that in subjects who had no circulating insulin antibodies.

[Studies on the question of a possible autoregulation of insulin secretion (author's transl)]

In eight normal weight healthy volunteers it was proved by C-peptide assay whether high concentrations of insulin in serum would suppress the secretion of insulin in an autoregulative manner. Insulin combined with glucose, therefore was given intravenously as a single bolus injection as well as an infusion over a period of two hours. Moreover it was proved whether high levels of insulin would suppress the reactivity of the B-cell to sulphonylurea administration. It was demonstrated that the secretion of the B-cell is regulated only by the concentration of the blood sugar, but not by the level of serum insulin. Likewise the stimulation of the B-cell by sulphonyl-urea administration is not suppressed by high concentrations of insulin.

Proinsulin and C-peptide: a review

The recent work on proinsulin and C-peptide has been reviewed with major emphasis on the most significant findings since 1972. Proinsulin has now been established as the biosynthetic precursor of insulin in all species examined, including man, with a preproinsulin as a possible precursor of the prohormone. The conversion of proinsulin which appears to occur exclusively in the pancreas leads to equimolar production of insulin and C-peptide. Although proinsulin has a direct biologic effect which is one-tenth as much as that of insulin, C-peptide has no biologic activity on homologous or heterologous tissue and no ability to modify the action of insulin and/or proinsulin. Previous work on proinsulin immunoassay suggested that this prohormone, but not C-peptide, cross-reacts with insulin antiserum. On the other hand, in the C-peptide immunoassay, proinsulin but not insulin cross-reacts with the antiserum. Up to this time, therefore, it has not been possible to immunoassay human proinsulin or C-peptide specifically. The very recent work from the laboratory of Heding, however, has brought about major advances in this area in which human C-peptide and proinsulin can be separated in the plasma by the use of Sepharose particles. With this recent major advancement, it is now possible to measure human C-peptide specifically. This measurement has been shown to be a useful tool for the assessment of beta-cell function in diabetic patients treated with insulin and in insulinoma patients in whom endogenous C-peptide secretion is not suppressed with exogenous insulin-induced hypoglycemia. With the use of a specific enzyme which degrades insulin but not proinsulin, postprandial plasma proinsulin values have been measured in a large number of subjects under a variety of physiologic and pathologic conditions. These results, which are comparable to those obtained by the more laborious column chromatography, could be summarized as follows: (1) proinsulin values in lean, young normal subjects do not vary greatly in response to insulin secretagogues; (2) proinsulin secretion in response to glucose results in a greater percentage of proinsulin in the older age group than in the younger group; (3) in lean adult and juvenile diabetic patients, the percentage of proinsulin is not excessive, whereas obese diabetics and pregnant diabetics appear to secrete relatively greater proinsulin than their diabetic controls; and (4) whereas most hyperinsulinemic states (Cusing's syndrome, adult-onset diabetics, acromegaly, and glucocorticoid therapy) are not associated with an increase in percentage of proinsulin, hyperinsulinemia of insulinoma, selected cases of functional hypoglycemia, and genetic hyperproinsulinemia are associated with a greater percentage of proinsulin. Identification of a possible new proinsulin intermediate(s) in these conditions deserves further investigation...

Human C-peptide. Part II: Clinical studies

Human C-peptide is determined by radioimmunoassay. On gel filtration of serum from a healthy subject and from a patient with islet cell carcinoma, C-peptide (MW 3025) appears ahead of insulin (MW 5808) and shows much higher molar concentrations than the hormone. Human proinsulin cross-reacts with our antiserum to synthetic human C-peptide. On direct determination of immunomeasurable C-peptide (IMCP) in fasting serum of 25 healthy subjects we find an average of 1.8 (+/- 0.4) ng/ml, corresponding to 60.4 X 10(-11) Mol/l. The molar concentration is about five-fold as compared to IMI (immuno-measurable insulin). IMCP and IMI patterns are not identical on stimulation of beta-cell secretion in healthy subjects by i.v. glucose or glucose-glibenclamide. This is probably due to differences in peripheral metabolism of both compounds. We conclude from our results that C-peptide determined in peripheral venous serum is a better indicator of beta-cell secretion than is insulin. Among 26 insulin-treated juvenile diabetics 15 show not measurable and 11 subnormal IMCP levels in fasting serum. No rise in IMCP is found 1-2 h following breakfast. Four juvenile patients receiving no insulin in a phase of total diabetes remission have normal or raised fasting IMCP concentrations. Only 2 out of 24 adult diabetics (16 treated with insulin and 8 with tablets) show non-measurable fasting IMCP concentrations, in another 4 patients values are below and in the remaining 18 cases above 1 ng/ml serum. Stimulation of beta-cell secretion through glucose-glibenclamide is more or less impaired in all adult diabetics compared to the healthy subjects.