抄録
The hepatic NAD decreased significantly in the rats fed a 10% or 20% casein diet supplemented with 5% leucine for 2 weeks. The effects of leucine and its related compounds on NAD biosynthesis from tryptophan or nicotinic acid were investigated in isolated rat hepatocytes. Leucine was ineffective, while α-ketoisocaproic acid, α-keto acid of leucine, and ketone bodies were inhibitory. α-Ketoisocaproic acid was quickly metabolized to ketone bodies by hepatocytes. Palmitic acid, which was not structurally related to leucine but quickly metabolized to ketone bodies, was also inhibitory. The hepatic ketone bodies of the rats fed a leucine-supplemented diet were raised to the level where inhibition of NAD biosynthesis could occur. Intestinal absorption and incorporation of tryptophan into liver and the biosynthesis of hepatic protein were not inhibited by excessive leucine when orally administered with an amino acid mixture. From the results, it was concluded that excessive ketone bodies derived from dietary supplemented leucine acted inhibitory on NAD biosynthesis.
