Insulin degradation

Nonspecific reaction of a thiol: protein disulfide oxidoreductase with the disulfide bonds of insulin

A thiol: protein disulfide oxidoreductase from bovine liver was isolated after separation from protein disulfide isomerase. The enzyme, after activation (reduction) with glutathione, was reacted with stoichiometric amounts of insulin and the sulfhydryl groups of the partially reduced hormone were labeled with iodo (l-14C)acetamide. After separation of the insulin chains, the radioactivity was found in both the peptides, with a ratio A-chain/B-chain equal to 2/1.

Characterization and partial purification of a cytoplasmic glutathione: disulfide oxidoreductase (thioltransferase) from adenohypophysis

A glutathione-dependent thioltransferase (thiol : disulfide oxidoreductase) has been partially purified (70-fold) from anterior pituitary cytosol, and characterized. Purification was effected by differential centrifugation, precipitation between 30 and 60% (NH4)2SO4, and sequential chromatography on Sepharose 6B, DEAE-cellulose, and CM-cellulose. Enzyme activity, monitored by the disappearance of NADPH, was associated with a protein of molecular weight 170 000 both by gel filtration and by polyacrylamide gel electrophoresis in SDS. There was apparent charge heterogeneity after the gel filtration step, and only the major DEAE-cellulose peak was further purified on CM-cellulose. When SDS-polyacrylamide gel electrophoresis was carried out in the presence of mercaptoethanol, the two predominant bands seen in its absence were converted to five major bands, all of different apparent molecular weights from the originals. Isoelectric focusing yielded two major peaks of enzyme activity, at pI 7.0 and pI 4.5-5.0. These peaks were shown to be interconvertible upon reelectrofocusing. Both low- and high-molecular weight disulfides could be reduced. The pH optimum was sharp, at pH 8.2. The Km values for glutathione and cystine (the standard assay disulfide) were 0.57 and 0.062 mM, respectively, each in the presence of saturating concentrations of the other substrate. N-Ethylmaleimide at 0.1 and 1.0 mM inhibited enzyme activity non-competitively, suggesting a non-catalytic role of enzyme thiol(s) for maintenance of optimal activity.

Insulin degradation by insulin target cells

Recent findings illustrate the complexities associated with the interaction between insulin and its target cells. These results suggest that the processes involved in insulin action and those involved in insulin degradation may have certain steps in common. Both apparently begin when insulin binds to the insulin receptor. The next step is unknown but it ultimately leads to the internalization of the hormone before insulin dissociates from the cell surface. Furthermore, internalization appears to be a requirement for efficient degradation of insulin since the vast majority (perhaps all in certain cells) of the degrading activity is intracellular. Internalization may not be required to produce certain actions of the hormone, however, and the two processes may diverge at the point. It is not clear how insulin enters the target cell other than the process appears to be receptor-mediated. Also, further work is needed to more fully characterize the vesicles that contain internalized insulin. Finally, the actual location of insulin degradation and the enzyme(s) involved need further study, especially to clarify the relative contributions of lysosomes, cytosolic protease, and GIT to physiological insulin destruction. An understanding of the overall process of insulin degradation is required for a complete description of the physiologic disposition of the hormone at the target cell. Moreover, this system has subtle control mechanisms that may have important implications for the management of diabetes and other endocrine and metabolic disorders.

Resolution of thiol-containing proteins by sequential-elution covalent chromatography

Elution of complex protein mixtures on a matrix containing reactive disulphide bonds (Thiopropyl-Sepharose 6B, Pharmacia) results in immobilisation of thiol-containing molecules. Specific protein fractions can be displaced from the gel using different low-molecular-weight reducing agents. Thus a single sequential elution can separate and resolve thiol-containing proteins in a rapid and convenient step. The method is illustrated with reference to beef liver thiol : disulphide oxidoreductases.

Interaction of glutathione-insulin transhydrogenase (disulfide interchange enzyme) with phospholipids

The effect of the addition of several phospholipids (lysophosphatidylcholine, alpha-lecithin, phosphatidylserine, phosphatidylethanolamine, lysophosphatidyl-ethanolamine, sphingomyelin, and disphosphatidylglycerol and phosphatidic acid) and related compounds (glycerophosphocholine, alpha- and beta-glycerophosphate, choline, serine, glycerol, dipalmitoylglycerol, and stearic acid) on the ability of purified (from beef pancreas) and microsomal (rat liver) glutathione-insulin transhydrogenase (Glutathione:protein-disulphide oxidoreductase, EC 1.8.4.2) to degrade insulin has been examined. With purified enzyme, except for phosphatidic acid and phosphatidylserine, all other phospholipids tested caused a slight activation at low concentration with phosphatidylethanolamine causing the highest activation. Lysophosphatidylcholine and phosphatidic acid are the only agents which cause inhibition of activity. The reaction rate as a function of concentration of inhibitor is hyperbolic for phosphatidic acid ([I]0.5 = 25 muM) and biphasic for lysophosphatidylcholine ([I]0.5 = 270 muM). Kinetic studies show that the two phospholipids are noncompetitive versus both substrates (insulin and GSH). Further, the structures of the phospholipids are quite different from the substrates and products of the reaction catalyzed by the enzyme. These dats, together with the data obtained with microsomes (see below), support the possibility that phospholipids, in particular lysolecithin and phosphatidic acid, might function by an interaction at an allosteric site or sites to bring about a conformational change in the enzyme. With a microsomal fraction, four phospholipids (lysophosphatidylcholine, lysophosphatidylethanolamine, phosphatidylethanolamine, and phosphatidic acid) caused an increase in GSH-insulin transhydrogenase activity. At low concentration the addition of each of these phospholipids led to a 2.5-fold increase in GSH-insulin transhydrogenase activity. At higher concentration, lysophosphatidylcholine almost totally inhibited the microsomal GSH-insulin transhydrogenase activity, as it did with purified enzyme, while phosphatidic acid showed only a slight inhibition, in contrast to its effect on purified enzyme. With the microsomal fraction in which GSH-insulin transhydrogenase activity had been previously unmasked by Triton X-100 treatment, the addition of small amounts of lysophosphatidylcholine and phosphatidic acid produced, as expected, only slight increase in the transhydrogenase activity for both phospholipids; again, only lysophosphatidylcholine but not phosphatidic acid caused inhibition when higher levels were used. It is concluded that the four phospholipids and Triton X-100 increase the GSH-insulin transhydrogenase activity in the microsomes by unmasking the catalytic site without fully unmasking the allosteric site, the point of reaction with the phosphatidic acid.