Effect of retinol toxicity on hepatic S-adenosylmethionine-dependent transmethylation in rats

Hepatic metabolism of the labile methyl group donor, S-adenosylmethionine (SAM), was investigated in rats fed toxic levels of retinol (1,000 IU/g of diet) since this treatment is known to decrease hepatic SAM concentration. The turnover rate of the hepatic SAM pool was not affected by the excess retinol, but the use of SAM as a labile methyl donor was restricted. Incorporation of the methyl group into phosphatidylcholine was reduced by 51% and oxidation of the methyl group to CO2 was decreased by 40%. In addition, the concentrations of cysteine and cystine, which are synthesized subsequent to demethylation of SAM, were reduced by 32% and 30%, respectively, in liver of high-retinol-fed rats, while methionine concentration was unchanged. The toxic level of dietary retinol may bring about a shift in the metabolism of SAM from transmethylation toward pathways that regenerate methionine via 5'-methylthioadenosine.