Inhibition of the catalytic subunit of phosphorylase phosphatase by oxalyl thioesters and its possible relevance to the mechanism of insulin action

Counter modulation of adipocyte mitochondrial processes by insulin and S-oxalylglutathione

Oxalyl thiolesters, a group of putative intracellular regulators, have been shown to be in vitro inhibitors of some cytosolic enzymes which are stimulated by insulin. In this study, the effects of insulin and oxalyl thiolesters on pyruvate dehydrogenase, beta-oxidation, and acyl-CoA hydrolase activities in mitochondria from rat epididymal adipocytes are compared. Using glutathione, CoASH, cysteine, and cysteamine as thiol sources, oxalyl thioesters were synthesized, purified, and quantitated. Mitochondria were isolated from rat epididymal adipocytes, some of which were incubated with or without insulin. Mitochondrial activities were determined by radioisotopic assay subsequent to control, insulin, or oxalyl thiolester incubation. Under the conditions used in this study, pyruvate dehydrogenase activity was increased 28% subsequent to 10-min incubation of adipocytes with 400 microU/ml insulin; in contrast, preincubation of adipocyte mitochondria with S-oxalylglutathione resulted in a dose-dependent 11-19% inhibition of pyruvate dehydrogenase. S-oxalylglutathione also attenuated the spermine-induced activation of pyruvate dehydrogenase. Insulin treatment resulted in a small but significant increase in beta-oxidation of palmitic acid while 100 microM S-oxalylglutathione mediated a 40% decrease in palmitate oxidation. Palmitoyl-CoA hydrolase activity was decreased 14% by insulin treatment; however, S-oxalylglutathione caused a 14-50% increase in hydrolase activity. The other oxalyl thiolesters were not as effective or as consistent as S-oxalylglutathione in modulation of the mitochondrial activities; free thiols and oxalic acid did not modulate the activities. In summary, pyruvate dehydrogenase, palmitate beta-oxidation, and palmitoyl-CoA hydrolase activities in adipocyte mitochondria were modulated in approximately equal but opposite directions by insulin and S-oxalylglutathione. These findings support the suggestion that oxalyl thiolesters may function as an intracellular signal recruited to return insulin to normal levels.

The structure, role, and regulation of type 1 protein phosphatases

Type 1 protein phosphatases (PP-1) comprise a group of widely distributed enzymes that specifically dephosphorylate serine and threonine residues of certain phosphoproteins. They all contain an isoform of the same catalytic subunit, which has an extremely conserved primary structure. One of the properties of PP-1 that allows one to distinguish them from other serine/threonine protein phosphatases is their sensitivity to inhibition by two proteins, termed inhibitor 1 and inhibitor 2, or modulator. The latter protein can also form a 1:1 complex with the catalytic subunit that slowly inactivates upon incubation. This complex is reactivated in vitro by incubation with MgATP and protein kinase FA/GSK-3. In the cell the type 1 catalytic subunit is associated with noncatalytic subunits that determine the activity, the substrate specificity, and the subcellular location of the phosphatase. PP-1 plays an essential role in glycogen metabolism, calcium transport, muscle contraction, intracellular transport, protein synthesis, and cell division. The activity of PP-1 is regulated by hormones like insulin, glucagon, alpha- and beta-adrenergic agonists, glucocorticoids, and thyroid hormones.

Determination of oxalyl thiolesters, N-oxalylcysteine and N-oxalylcysteamine in biological materials

A convenient method is described for the quantitative analysis of oxalyl thiolesters (OTEs), a newly discovered class of mammalian metabolites, in biological samples. By this particular technique the total concentration of all OTEs in the sample is determined. The method involves first reacting the biological material with cysteamine (2-aminoethanethiol) or cysteine under conditions that convert OTEs quantitatively to N-oxalylcysteamine (or N-oxalylcysteine), followed by reaction with monobromobimane to give a highly fluorescent derivative that is analyzed by reversed-phase ion-pair chromatography, with tetrabutylammonium ion as the counterion and N-(2-mercaptopropionyl)glycine as an internal standard. The method is capable of detecting as little as 0.6 pmol of the bimane derivative of the N-oxalyl compound in a single HPLC injection. The application of this method has led to the discovery that not only OTEs but also N-oxalylcysteine and N-oxalylcysteamine are normal mammalian metabolites. In various rat tissues the OTE concentration ranges up to 65 nmol/g (wet wt), the N-oxalylcysteine concentration is approximately 10 nmol/g, and the N-oxalylcysteamine concentration is 0-3 nmol/g.

S-oxalylglutathione is a substrate for gamma-glutamyltransferase (GGT). Implications for the role of GGT in cell proliferation

With glycylglycine or water as acceptor, bovine kidney gamma-glutamyltransferase catalyzes reactions of the known mammalian metabolite, S-oxalylglutathione, at rates comparable to those of L-gamma-glutamyl-p-nitroanilide, a known good substrate. N-Oxalyl-cysteinylglycine is the eventual product of the former reaction. Since oxalyl thiolesters are implicated as important cell proliferation inhibitors, it is proposed that this reaction plays a major role in controlling cell proliferation.

Oxalyl thiolesters and N-oxalylcysteine are normal mammalian metabolites

A method for the quantitative analysis of N-oxalylcysteine and oxalyl thiolesters (RSCOCOO-) in biological samples is described. These compounds were found in all rat tissues examined (kidney, liver, brain, heart, muscle and fat), with the amount of N-oxalylcysteine ranging up to 18 nmoles/g wet weight and that of oxalyl thiolesters up to 65 nmoles/g wet weight. The identification of such compounds in animal tissues adds further credence to the hypothesis that they may be important metabolic effectors.