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 o xalyl thiolesters on pyruvate dehydrogenase, beta-oxidation, and acyl- CoA hydrolase activities in mitochondria from rat epididymal adipocyte s are compared. Using glutathione, CoASH, cysteine, and cysteamine as thiol sources, oxalyl thiolesters were synthesized, purified, and quan titated. Mitochondria were isolated from rat epididymal adipocytes, so me of which were incubated with or without insulin. Mitochondrial acti vities were determined by radioisotopic assay subsequent to control, i nsulin, or oxalyl thiolester incubation. Under the conditions used in this study, pyruvate dehydrogenase activity was increased 28% subseque nt to 10-min incubation of adipocytes with 400 mu U/ml insulin; in con trast, preincubation of adipocyte mitochondria with S-oxalylglutathion e resulted in a dose-dependent 11-19% inhibition of pyruvate dehydroge nase. S-oxalylglutathione also attenuated the spermine-induced activat ion of pyruvate dehydrogenase. Insulin treatment resulted in a small b ut significant increase in beta-oxidation of palmitic acid while 100 m u M S-oxalylglutathione mediated a 40% decrease in palmitate oxidation . Palmitoyl-CoA hydrolase activity was decreased 14% by insulin treatm ent; however, S-oxalylglutathione caused a 14-50% increase in hydrolas e activity. The other oxalyl thiolesters were not as effective or as c onsistent as S-oxalylglutathione in modulation of the mitochondrial ac tivities; free thiols and oxalic acid did not modulate the activities. In summary, pyruvate dehydrogenase, palmitate beta-oxidation, and pal mitoyl-CoA hydrolase activities in adipocyte mitochondria were modulat ed in approximately equal but opposite directions by insulin and S-oxa lylglutathione. These findings support the suggestion that oxalyl thio lesters may function as an intracellular signal recruited to return in sulin to normal levels.