Handbook of Diabetes. Rudy Bilous

for two non‐polar regions involved in the aggregation of the monomers into dimers and hexamers.

Figure 5.6 The primary structure (amino acid sequence) of human insulin. The highlighted residues are those that differ in porcine and bovine insulins, as shown in the inset.

In concentrated solution (such as in the insulin vial supplied by the pharmaceutical company for injection) and in crystals (such as in the insulin secretory granule), six monomers self‐associate with two zinc ions to form a hexamer (Figure 5.7). This is of therapeutic importance because the slow absorption of native insulin from the subcutaneous tissue partly results from the time taken for the hexameric insulin to dissociate into the smaller, more easily absorbed monomeric form.

Figure 5.7 The double zinc insulin hexamer composed of three insulin dimers in a threefold symmetrical pattern.

Insulin is synthesised in the β cells from a single amino acid precursor called proinsulin (Figure 5.8). Synthesis begins with the formation of an even larger precursor, preproinsulin, which is cleaved by protease activity to proinsulin. The gene for preproinsulin (and therefore the ‘gene for insulin’) is located on chromosome 11. Proinsulin is packaged into vesicles in the Golgi apparatus of the β cell; in the maturing secretory granules that bud off it, proinsulin is converted by enzymes into insulin and connecting peptide (C‐peptide).

Insulin and C‐peptide are released from the β cell when the secretory granules are transported (‘translocated’) to the cell surface and fuse with the plasma membrane (exocytosis) (Figure 5.9). Microtubules, formed of polymerised tubulin, probably provide the mechanical framework for granule transport, and microfilaments of actin, interacting with myosin and other motor proteins such as kinesin, may provide the mechanical force that propels the granules along the tubules. Although the actin cytoskeleton is a key mediator of biphasic insulin release, cyclic GTPases are involved in F‐actin reorganization in the islet β cell and play a crucial role in stimulus‐secretion coupling.

The ‘regulated pathway’, with almost complete cleavage of proinsulin to insulin, normally accounts for about 95% of the β cell insulin production. In certain conditions, however, e.g insulinoma and type 2 diabetes, an alternative ‘constitutive’ pathway operates, in which large amounts of unprocessed proinsulin and intermediate insulin precursors (‘split proinsulins’) are released directly from vesicles that originate in the endoplasmic reticulum (Figure 5.10).

Figure 5.8 Insulin biosynthesis and processing. Proinsulin is cleaved on the C‐terminal side of two dipeptides. The cleavage dipeptides are liberated, so yielding the ‘split’ proinsulin products and ultimately insulin and C‐peptide.

Figure 5.9 (a) Electron micrograph of insulin secretory granules in a pancreatic β cell and their secretion by exocytosis. Arrows show exocytosis occurring. Ca, capillary lumen; Is, interstitial space. (b) Freeze‐fracture views of β cells that reveal the secretory granules in the cytoplasm (asterisks) and the granule content released by exocytosis at the cell membrane (arrows). Magnification: ×52,000.

      From Orci. Diabetologia 1974; 10: 163–187.

Figure 5.10 The regulated (normal) and constitutive (active in type 2 diabetes) pathways of insulin processing.

Insulin secretion

Glucose is the main stimulator of insulin release from the β cells, and insulin secretion occurs in a characteristic biphasic pattern – an immediate ‘first phase’ response that lasts only a few minutes, followed by a more gradual sustained ‘second phase’ (Figure 5.11). The first phase of insulin release involves a small, readily releasable pool of granules fusing with the plasma membrane. Of particular importance is the observation that first‐phase insulin secretion is lost in patients with type 2 diabetes.

      Various types of fuels, hormones, and neurotransmitters regulate insulin secretion. Glucose is the most important regulator and glucose stimulates insulin secretion by mechanisms that depend upon the metabolism of glucose and other nutrients in the β cells. A triggering pathway involves closure of ATP‐sensitive potassium channels (K_ATP channels), cellular depolarisation, an influx of calcium through voltage‐dependent calcium channels and an increase in intracellular calcium concentration. Simultaneously, a metabolic amplifying pathway augments the stimulatory effect of calcium on the exocytosis of insulin‐containing granules. The second messenger cAMP is an important amplifier of insulin secretion triggered by Ca²⁺ elevation in the β cells.

Glucose enters the β cell by facilitated diffusion via GLUT‐2 transporters. It is then phosphorylated by the enzyme glucokinase, which acts as the ‘glucose sensor’ that couples insulin secretion to the prevailing glucose concentration (Figure 5.12). Glycolysis and mitochondrial metabolism of glucose produce adenosine triphosphate (ATP), which leads to the closure of the K_ATP channels. This in turn causes depolarisation of the β cell plasma membrane, leading to an influx of extracellular calcium through cell‐surface voltage‐gated channels. The increase in cytosolic calcium triggers translocation of insulin granules and exocytosis.

Figure 5.11 (a) The biphasic glucose‐stimulated release of insulin from pancreatic islets. (b) The glucose–insulin dose–response curve for islets of Langerhans.

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