Biosynthesis of carnitine and 4-N-trimethylaminobutyrate from 6-N-trimethyl-lysine

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Abstract
The conversion of 6-N-[Me-14C]trimethyl-lysine into carnitine and 4-N-trimethylaminobutyrate (butyrobetaine) was demonstrated in rats kept on a lysine-deficient diet. After the rats were given [14C]trimethyl-lysine for 4 days, a total of 17% of the injected label was recovered as carnitine from carcass and urine extracts. Another 8% of the trimethyl-lysine label was converted into 4-N-trimethylaminobutyrate, most of which was recovered from the urine. The conversion of trimethyl-lysine into the above two metabolites supports the pathway of carnitine biosynthesis as lysine+methionine → 6-N-trimethyl-lysine → 4-N-trimethylaminobutyrate → carnitine. In addition, three other metabolites representing 2% of the injected dose were recovered. Only an insignificant portion of the label was recovered as free trimethyl-lysine from the carcass, whereas 22% of the injected label was recovered in the urine. A relatively low specific radioactivity in carnitine was found when 5-N-[Me-14C]trimethylaminopentanoate and 6-N-[Me-14C]trimethylaminohexanoate were administered to rats in amounts similar to the [14C]trimethyl-lysine, suggesting that they were not free intermediates.