Effect of allylisopropylacetamide on glutathione metabolism in the rat liver. The possible role of glutathione in the induction of 5-aminolaevulinate synthase.

Administration of allylisopropylacetamide to rats caused a marked decline in the concentrations of reduced and oxidized glutathione in the liver. However, this decrease occurred in the presence of uninhibited activities of gamma-glutamylcysteine synthase and glutathione reductase, and unaltered activities of glutathione transferases A, B and C. The administration of cysteine, the rate-limiting precursor of glutathione formation, to rats treated with allylisopropylacetamide potentiated the inductive effects of the agent on 5-aminolaevulinate synthase, and markedly decreased the extent of decrease in glutathione concentrations by the agent. Conversely, the administration of diethyl maleate, which depletes the hepatic glutathione concentrations, to allylisopropylacetamide-pretreated rats (1h) diminished the extent of 5-aminolaevulinate synthase induction and the production of porphyrins by nearly 50%, when measured at 16h. This treatment did not alter the extent of non-enzymic degradation of liver haem by allylisopropylacetamide. When diethyl maleate was administered to the animals possessing high 5-aminolaevulinate synthase activity (at 3, 7 and 15h after allylisopropylacetamide), in 1h the enzyme activity was markedly decreased. Diethyl maleate had no effect on induction of 5-aminolaevulinate synthase by 3,5-diethoxycarbonyl-1,4-dihydrocollidine, also a potent porphyrinogenic agent. Diethyl maleate alone neither inhibited 5-aminolaevulinate synthase activity nor decreased the cellular content of porphyrins and haem. The data suggest that the decreases observed in the glutathione concentrations after allylisopropylacetamide administration are not the result of decreased production of the tripeptide. Rather, they most likely reflect the increased utilization of glutathione. The findings further suggest that the inhibition by diethyl maleate of allylisopropylacetamide-stimulated 5-aminolaevulinate synthase involves the inhibition of induction processes.