The role of the kidney in the biosynthesis of carnitine in the rat.

Intravenous administration of L-[methyl-3H]-labeled trimethyllysine to rats results in a very rapid accumulation of radioactivity by the kidneys, while the incorporation of the label into the liver occurs at approximately 1% of this rate when calculated per g of wet tissue. The kidneys convert a substantial portion of the trimethyllysine taken up to butyrobetaine and to beta-hydroxytrimethyllysine, a precursor of butyrobetaine, but fail to synthesize carnitine. Significant amounts of radioactivity are recovered in both carnitine and butyrobetaine of hepatic tissue after longer time periods, while the level of labeled trimethyllysine in this organ remains very low. Bilateral nephrectomy results in a marked decrease in the incorporation of label into the liver. These results indicate that in rats, the initial conversion of trimethyllysine to butyrobetaine occurs predominantly in kidney and that the liver capacity for this transformation is considerably smaller than its capacity to synthesize carnitine from butyrobetaine.