Effect of starvation in the rat on trimethyllysine in peptide linkage

Trimethyllysine residues in peptide linkage, precursors of carnitine biosynthesis, were measured in fed and 3-day fasted rats. Whole-body peptide-linked trimethyllysine in fasted rats was significantly less than in fed rats when expressed per initial body weight (17.8 vs 24.8 mumol/100 g initial body weight). Skeletal muscle had the highest peptide-linked trimethyllysine content (2.5 nmol/mg protein), followed by heart (1.9 nmol/mg protein). The content in kidney, liver and small intestine were similar, but less than in heart. Of the eight tissues tested, the brain had the only significant increase with fasting. The hepatic peptide-linked trimethyllysine in fasted rats was significantly decreased when expressed per milligram DNA. The study shows a commensurate loss of peptide-linked trimethyllysine accompanying protein loss during fasting. The study also shows that muscle contains over 65% of the whole-body peptide-linked trimethyllysine, and as such is a major reservoir of precursor for carnitine biosynthesis.