Insulin association with zinc

Many of the enzymes of intermediary metabolism contain zinc, and deficiency affects all macronutrients. Protein synthesis and DNA and RNA synthesis require zinc. Insulin is secreted from the pancreas and circulates in association with zinc. This secretion is diminished under conditions of zinc deficiency, leading to impaired glucose metabolism. Lipid metabolism is also affected, with zinc deficiency being associated with reductions in circulating high-density lipoproteins. 

Alloxan (1003) has been observed in the mucus associated with dysentery and it was the very first pyrimidine made synthetically when Brugnatelli oxidized uric acid in 1818. Alloxan has an interesting diabetogenic action which appears to be associated with removal of essential zinc from insulin by chelation. Such permanent diabetes may be induced in fish, dogs, cats, sheep, some birds, monkeys and other creatures, but not in man, owls or guinea-pigs certain pyrimidines related to alloxan show some such activity. 

Hormones. Zinc is thought to have a role in the synthesis and actions of many hormones, via zinc transcription factors. Zinc depletion is associated with low circulating concentrations of testosterone, free T4, insulin-like growth factor (IGF)-l, and thymulin. Both plasma IGE-1 and growth velocity increase in zinc-supplemented children. ... 

Because of the lipase deficiency, fat-soluble vitamin (A, D, E, and K) deficiencies may occur. Whether lipase activity or bile acids (e.g., in micelle formation) are involved in fat-soluble vitamin absorption with steatorrhea is unclear. Vitamin and zinc deficiencies also may occur as aresult of pancreatic enzyme deficiency. Although pancreatic involvement is predominantly exocrine in nature, insulin deficiency with glucose intolerance also occurs in CF patients, especially as they advance in age. Carbohydrate intolerance is characterized by low insulin concentrations and enhanced peripheral sensitivity to insulin but not by the presence of islet cell or anti-insulin antibodies. Carbohydrate intolerance in CF is not usually associated with the ketosis as commonly occurs in type 1 diabetes. This complication involves an increase in the number of insulin receptors with decreased affinity for insulin. Despite a concomitant increase in tissue affinity for insulin, 8% of CF children over 12 years of age require insulin therapy. 

Anaphylaxis also has been associated with the use of protamine. Although development of protamine anaphylaxis is not limited to diabetics, those patients with diabetes that have used protamine-containing insulin (NPH or protamine zinc) do have a slightly increased risk of anaphylaxis. Some... 

There also are chemical instability issues associated with insulin. For 40 years, the only rapid-acting form of insulin was a solution of zinc insulin, with pH 2 to 3. If this insulin is stored at 4°C, deamidation of the asparagine at A21 occurs at a rate of 1 to 2% per month. The C-terminal Asn, under acidic conditions, undergoes cyclization to the anhydride, which in turn can react with water, leading to deamidation. The anhydride also can react with the N-terminal Phe of another chain to yield a cross-linked molecule. If stored at 25°C, the inactive deamidated derivative constitutes 90% of the total protein after 6 months (Fig. 32.2) (34). 

Figure 5. Correlation between time to 50% disappearance from the injection site in pig studies and the mean association state of various insulins at lmmol/1. The association state was deduced from osmometry, and, in the case of hexameric insulins, the tendency to dissociation upon dilution was assessed by size-exclusion chromatography (SEC). Note that T-50% > 100 is log scale. Each figure and letter in the diagram represents the mean results for one insulin or insulin analog H, Human 2Zn-, B, bovine 2Zn-, and P, porcine 2Zn-insulin C, cobalt(III) human insulin Z, human, zinc-free insulin. For other relevant codes, see Table VII. (From Brange ei a/., 1990, with permission.)...

Fig. 13. Association of insulin molecules. At left two monomers are represented with their disulfide linkages. The histidyl residues can form a complex with zinc, thereby stabilizing the dimer. In the further associations the dimers are drawn schematically as columns.

In this study, we designed the sustained release tystem, which provides basal line insulin release for duration of over several weeks by one injectioa Hmnan insulin was entrapped in the hydrogel in order to sustain its release as a subcutaneous insulin delivery system. We tried to modify the association states of insulin by zinc in order to inhibit the initial bmst effect and obtain constant release rate. At otherwise equivalent conditions, insulin associated from monomer and dimer to hexamer with inaeasing zinc concentration [22]. Insulin samples with different zinc contents exhibited different release profiles due to association-state differences within the hydrogel. 

Generally, insulin forms a hexamer with zinc. The insulin association resulted from the zinc content in the insulin [22]. However, without zinc, insulin formed various association states such as monomer, dimer, hexamer, and aggregate. It is thought that insulin without zinc formed the aggregation state inside the gel. The aggregated insulin may not diffuse fast from the ReGel formulation, which presented a slower release (60% after 15 days). Insulin with 0.2 wt% zinc formed the hexameric state. The release profile of the insulin with... 

Only the insulin monomer is able to interaot with insulin receptors, and native insulin exists as a monomer at low, physiologioal oonoentrations (<0.1 pM). Insulin dimerizes at the higher oonoentrations (0.6 mM) found in pharmaceutioal preparations, and at neutral pH in the presence of zinc ions, hexamers form (34). These zino-associated hexamers also are the storage form of insulin in p cells. At concentrations greater than 0.2 mM, hexamers form even in the absence of zino ions. 

Figure 3. The primary structure of human insulin. The sites and types of mutation in the different analogs with reduced tendency to self-association are indicated. Also shown arc the amino acid residues involved in the association of two insulin molecules into a dimer (black residues) and in the assembly of three dimers and two zinc ion into a Zn-insulin hezamer (shaded residues). The putative sites interacting with the receptor are indicated by arrows. (From Brange cr aJ., 1990, with permission.)...

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