Down-regulation of rat liver beta-adrenergic receptors by cysteine

Down-regulation of hepatic beta-adrenergic receptors was indicated by a 56% decrease in the specific activity of 125I-iodocyanopindolol bound to rat liver membrane preparations from rats fed diets containing 15% of casein supplemented with cysteine, instead of methionine or unsupplemented. Down-regulation of hepatic beta-adrenergic receptors by cysteine appears to be mediated through an effect of cysteine on the tissue concentration of S-adenosyl methionine (SAM). The liver tissue concentration of SAM in rats fed cysteine-supplemented diets decreased 53% compared to those fed diets supplemented with methionine. The decrease in liver SAM in rats fed the diet supplemented with cysteine appears to reflect a non-competitive inhibition of methionine adenosyl-transferase by cysteine. Lineweaver-Burk plots demonstrated a dose-related Vmax response to cysteine but did not change the apparent Km at any concentration tested.