Glutamine Concentration May Limit Glutathione Synthesis in the Presence of α-Adrenoceptor Agonists and Glucagon

Abstract

ABSTRACTGlutathione (GSH) is an ubiquits tripeptide which is composed of glutamine-cysteine-glycine. In general, methionine is the preffered precursor of cysteine in cell culture. It has been indicated that glucagon, adrenaline and phenylephrine inhibit hepatocellular glutathione synthesis in the presence of methionine. Therefore, cysteine is considered rate-limiting for GSH synthesis. To examine whether changes in glutamine concentration in the medium are also rate-limiting for GSH synthesis, short term cultured hepatocytes were treated with different concentrations of glutamine in the presence of the other two precursors. The inhibitory effects of glucagon were observed when lower concentrations of glutamine (0.2 or 0.5mM) were used. However, this effect was attenuated when hepatocytes were incubated with 2mM concentrations of glutamine. The effect of phenylephrine on GSH synthesis was measured in the presence of different concentrations of methionine (2, 0.5, 0.2 and 0.1mM), given as a precursor of cysteine, with 2mM glutamine and 2mM glycine. The inhibitory effect of phenylephrine was not apparent at any concentration of methionine. Our results suggest that, under physiological conditions, the availability of glutamine in the plasma, and possible changes in L-glutamate concentration, appear to be important for maintaining GSH synthesis in the presence of glucagon and the α-adrenergic agents, adrenaline and phenylephrine.