Measurement of insulin secretion in type 2 diabetes: problems and pitfalls

Analysis of proinsulin and its conversion products by reversed-phase high-performance liquid chromatography

Proinsulin is synthesized in the beta-cells of the endocrine pancreas, one of the four cell types found in the islets of Langerhans. Specific enzymatic cleavage of proinsulin results in the formation of equimolar amounts of insulin and C-peptide, via several intermediate split-proinsulin forms. Most mammals produce a single insulin, but in rodents two non-allelic insulin genes are expressed. There is an inverse ratio between the two insulins in rats and mice, the reason for this being unknown. It has been suggested that differences in transcription, translation (biosynthesis) and/or posttranslational processes (enzymatic conversion, intracellular degradation) could be possible explanations. Elevated amounts of proinsulin-immunoreactive material (PIM) have been described to occur in various conditions/diseases, suggesting alterations in beta-cell function, but the composition of the secreted PIM (intact proinsulin or its intermediates) has been incompletely determined. Studies of the biosynthesis of proinsulins and their conversion with the purpose of revealing some of these points depend on accessible reversed-phase high-performance liquid chromatographic (RP-HPLC) analyses capable of separating all the relevant, closely related polypeptides involved. This review will deal with the optimization of the RP-HPLC separations as well as sample preparation and recovery. Applications of the selected methods in the study of proinsulin biosynthesis and its conversion will also be presented.

Insulin deficiency rather than hyperinsulinaemia in newly diagnosed type 2 diabetes mellitus

This paper reports beta cell function as assessed during OGTT using specific IRMAs for insulin, intact and 32/33 split proinsulin in subjects with newly diagnosed Type 2 diabetes matched to normal controls. The relationships between insulin and the proinsulins to risk factors for cardiovascular disease were also examined. Similar fasting insulin concentrations but lower 30-min post-glucose-load insulin concentrations were found in diabetic subjects (mean +/- SEM 143 +/- 12 pmol-1 vs 304 +/- 19 (p < 0.001). Subjects with diabetes had increased fasting intact (10.6 +/- 1.1 pmol-1 vs 3.3 +/- 0.2, p < 0.001) and 32/33 split proinsulin concentrations (8.1 +/- 0.9 pmol-1 vs 2.2 +/- 0.3, p < 0.0001). Beta cell dysfunction, as expressed by a reduction in the 30-min insulin to glucose ratio (9.4 +/- 1 vs 34.8 +/- 2.3, p < 0.0001) and an increase in the fasting percentage of total proinsulin-like to total insulin-like molecules (24.5 +/- 9% vs 11.6 +/- 5, p < 0.001), was present in subjects with diabetes. In diabetic subjects beta cell dysfunction and insulin deficiency increased relative to the degree of fasting hyperglycaemia. It seems clear that beta cell dysfunction and insulin deficiency are major features of Type 2 diabetes. Only the fasting concentration of 32/33 split proinsulin positively correlated with both the waist/hip ratio (r = 0.36, p < 0.001), diastolic blood pressure (r = 0.23, p < 0.01) in addition to plasma triglyceride concentration (r = 0.46, p < 0.001). It is questionable whether hyperinsulinaemia plays a pathogenic role in cardiovascular disease in subjects with glucose intolerance.

Non-insulin dependent diabetes mellitus: defects in insulin secretion

NIDDM is a heterogeneous disorder, characterized by defects in insulin secretion as well as in insulin action. Several pathophysiological mechanisms are involved in the development of disturbances in insulin secretion. One of the histological features of islets of NIDDM patients is the deposition of amyloid-like material. Accumulation of amyloid over many years can lead to slowly progressive disruption of islet architecture and possibly to some of the abnormalities in insulin secretion, as found in NIDDM patients. Loss of pulsatility is the earliest detectable abnormality of insulin secretion in the disease, either as a specific early defect or as a disturbance caused by minimally elevated blood glucose levels. Although it has been shown that maximum insulin release is decreased by 50% in NIDDM, the B-cell sensitivity to glucose appears to be normal. Coregulatory factors such as prostaglandins do not play a major role in the derangements of insulin secretion in NIDDM. An imbalance between stimulatory and inhibitory endorphins, or in sympathetic tone may be of more importance. Hyperglycaemia by itself has a deleterious effect on insulin release, and may perpetuate the disturbances of insulin secretion.