Effect of dietary cystine on methionine metabolism in rat liver.

Cystine supplementation of adequate diets resulted in significantly higher hepatic levels of betaine-homocysteine methyltransferase. Other changes occurred but were a function of the basal diet. When the latter contained 0.25% methionine + 0.5% cystine, the additional cystine caused a markedly lower hepatic cystathionine synthase activity and lower levels of both adenosylmethionine and serine. The metabolic effect of these changes may be enhanced methionine retention and diminished transsulfuration.