Methionine toxicity in the rat in relation to hepatic accumulation of S-adenosylmethionine: prevention by dietary stimulation of the hepatic transsulfuration pathway.

Rats were fed toxic levels of methionine with or without simultaneous dietary supplements of glycine and serine. Feed intake, growth rate, and metabolite concentrations in intestine, plasma, liver, skeletal muscle, and kidneys were monitored. Both toxic amounts of methionine and supplemental glycine and serine affected the tissue distribution of several amino acids resulting in similar, opposite, and diet-specific effects on the parameters studied. These changes were considered to be normal responses of amino acid metabolism to diet and to reflect metabolite flows between tissues. The feeding of toxic levels of methionine resulted in the accumulation of methionine, taurine, and glutathione in all tissues measured, but caused marked accumulation of S-adenosylmethionine and its catabolites only in liver. Hepatic accumulation of S-adenosylmethionine was accompanied by 40% stimulation of methionine adenosyltransferase and 40% repression of spermine synthase over a 2-week period. Simultaneous dietary supplements of glycine and serine combined with toxic levels of methionine markedly stimulated hepatic methionine catabolism. As a result, tissue distribution of methionine and glutathione returned close to normal in all tissues measured and accumulation of hepatic S-adenosylmethionine and its catabolites was prevented. Concentrations of taurine in liver, blood, and kidneys were further elevated, suggesting increased conversion of methionine to taurine followed by urinary excretion. These changes were accompanied by normalization of the above enzyme activities and the absence of symptoms of methionine toxicity. It was concluded that methionine toxicity is likely to be linked to hepatic accumulation of S-adenosylmethionine, resulting in liver dysfunction probably due to nonenzymatic methylation of liver macromolecules. Accumulation of tissue glutathione may also contribute to toxicity.

Adaptation of adenosylmethionine metabolism and methionine recycling to variations in dietary methionine in the rat.

Weanling rats were fed a casein-based diet supplemented to give dietary methionine (Met) concentrations of 0.41, 0.61, and 1.50%. After 2 weeks of feeding, the rats received intraperitoneally 800 nCi of 2-14C-labeled and/or methyl-3H-labeled L-Met. The animals were killed 20 min, 1 hr, or 2 hr after the isotope injection and the specific radioactivity of adenosylmethionine (AdoMet) as well as the total acid-soluble radioactivity was analyzed in the liver and skeletal muscle. Met concentrations of the liver and skeletal muscle were increased 20-fold by the diet containing 1.50% of Met. In the liver, but not in skeletal muscle, accumulation of AdoMet closely followed changes in Met concentration. Within 2 hr after intraperitoneal injection, the rate of disappearance of 3H label from the acid-soluble fraction was slow in both tissues; increasing in the liver and decreasing in skeletal muscle with increasing dietary Met concentration. At the same time, disappearance of 14C label was slow in both tissues in the rats fed the toxic Met diet, and also in the liver of the rats fed the Met-deficient diet. Decline of the specific radioactivity of the AdoMet pool with respect to 3H label was similar to that of 14C label in the skeletal muscle at all dietary Met concentrations. In the liver, the rate of disappearance of 14C label from the AdoMet pool was markedly increased and that of the 3H label slightly decreased with increasing dietary Met supply. Met deprivation resulted in rapid disappearance of 3H label from the hepatic AdoMet pool, whereas the disappearance of the 14C label was very slow. The results indicate that hepatic Met recycling is very effective with deficient or adequate dietary Met concentrations. In skeletal muscle, the capacity to catabolize extra Met is very limited and continuous flow of Met to liver takes place. Unlike in the liver, in skeletal muscle the transsulfuration route is not adaptable to changes in Met supply and plays a minor role in Met catabolism. The approach used to determine the efficacy and adaptation of methionine salvage pathways by following simultaneously the decline of the specific radioactivities of the methyl group and the methionyl carbon chain of AdoMet following intraperitoneal injection of double-labeled Met has several advantages over that used in literature reports. It offers a reliable means of observing these metabolic pathways in whole animals without disruption of metabolite fluxes.

Synthesis and analysis of selenomethionine metabolites.

This paper describes the enzymatic synthesis of selenomethionine metabolites of the transmethylation and polyamine synthesis pathways: adenosylselenomethionine, adenosylselenohomocysteine, decarboxylated adenosylselenomethionine, and methylselenoadenosine. These compounds and the corresponding methionine metabolites were simultaneously separated by a single HPLC run. The sensitivity of the HPLC method is about 20 pmol per compound. The method may be used for direct analysis of the metabolite levels in tissues or cells treated with selenomethionine and it provides an assay method for the pulse-chase type of analysis of relative flows for both selenium- and sulfur-containing compounds in transmethylation and polyamine pathways.

Effects of dietary polyamine precursors on the metabolism and tissue concentrations of amino acids in the rat.

Experiments were conducted to determine the effects of dietary supplements of amino acid precursors of polyamines on amino acid metabolism in the rat in order to better understand comparative aspects of polyamine metabolism. Rats were fed isonitrogenous combinations of methionine, ornithine, arginine and 2-difluoromethylornithine in casein-based diets. It was observed that hepatic concentrations of methionine, 5'-deoxy-5'-methylthioadenosine and decarboxylated S-adenosylmethionine were more easily influenced by diet than were arginine and metabolites. It was concluded that rats may be more refractory to exogenous polyamine precursor amino acids than are chicks because of the presence of a functional urea cycle.

Effect of dietary methionine, arginine and ornithine on the metabolism and accumulation of polyamines, S-adenosylmethionine and macromolecules in rat liver and skeletal muscle.

The interrelationship and possible causality of polyamine synthesis and the transmethylation pathway in the growth-retarding effects of inadequate or excess dietary methionine was studied in young male rats. Feeding the rats for 2 weeks diets containing toxic concentrations of methionine had no effect on polyamine and S-adenosylmethionine metabolism in skeletal muscle, but resulted in markedly elevated concentrations of S-adenosylmethionine and S-adenosylhomocysteine and slightly decreased accumulation of spermine and RNA in the liver. These changes were accompanied by liver-specific stimulation of methionine adenosyltransferase and reduction of spermine synthase activities. Inadequate arginine feeding or supplementation of the diets with ornithine or excess arginine resulted in no apparent changes in tissue methionine or polyamine metabolism and did not alleviate the effects of varied dietary methionine supply. Inhibition of putrescine synthesis by supplementing the diets with 2-difluoromethylornithine did not modify the effects of toxic concentrations of dietary methionine. It is suggested that although hepatic spermine synthase is sensitive to excess methionine feeding, methionine toxicity is not mediated by defective polyamine metabolism.