Free and
peptide-linked
trimethyllysine were measured in fed and 5-d starved rats. The
trimethyllysine content of liver and kidney was significantly increased on d 5 of
starvation to two to three times the levels found in fed animals. Skeletal muscle of fed rats contained over six times as much
trimethyllysine (19.3 nmol/g) as that found in liver (3.2 nmol/g) or kidney (2.7 nmol/g). Plasma free
trimethyllysine significantly increased from 1 nmol/ml in fed rats to 2.2 nmol/ml in 5-d starved rats. During this same period, daily total
trimethyllysine excretions averaged approximately 400 nmol/d. Urinary free
trimethyllysine was significantly depressed during
starvation. Assuming that
trimethyllysine in plasma does not exist in a
protein-bound form, clearance calculations based on concentrations of plasma and urinary
trimethyllysine indicated that this compound is readily reabsorbed by the kidney. As previous studies have indicated that
trimethyllysine is not readily absorbed by other tissues, this indicates that the kidney may be the primary regulatory site for the disposition free of free
trimethyllysine in plasma and urine. Estimates of the efficiency of entry of
trimethyllysine into the
carnitine biosynthetic pathway were calculated and ranged from 41% in 3-d starved rats to 80% in fed rats. We conclude that
carnitine biosynthesis is limited by the availability of
trimethyllysine, which, in the starved rat, is limited by the rate of
protein turnover.