Biosynthesis of carnitine and 4-N-trimethylaminobutyrate from lysine

Abstract

The conversion of l-[U-¹⁴C]lysine into carnitine was demonstrated in normal, choline-deficient and lysine-deficient rats. In other experiments in vivo radioactivity from l-[4,5-³H]lysine and dl-[6-¹⁴C]lysine was incorporated into carnitine; however, radioactivity from dl-[1-¹⁴C]lysine and dl-[2-¹⁴C]lysine was not incorporated. Administered l-[Me-¹⁴C]methionine labelled only the 4-N-methyl groups whereas lysine did not label these groups. Therefore lysine must be incorporated into the main carbon chain of carnitine. The methylation of lysine by a methionine source to form 6-N-trimethyl-lysine is postulated as an intermediate step in the biosynthesis of carnitine. Radioactive 4-N-trimethylaminobutyrate (butyrobetaine) was recovered from the urine of lysine-deficient rats injected with [U-¹⁴C]lysine. This lysine-derived label was incorporated only into the butyrate carbon chain. The specific radioactivity of the trimethylaminobutyrate was 12 times that of carnitine isolated from the urine or carcasses of the same animals. These data further support the idea that the last step in the formation of carnitine from lysine was the hydroxylation of trimethylaminobutyric acid, and are consistent with the following sequence: lysine+methionine → 6-N-trimethyl-lysine → → 4-N-trimethylaminobutyrate → carnitine.