TRITIUM AND C-14 ISOTOPE EFFECTS USING TRACERS OF LEUCINE AND ALPHA-KETOISOCAPROATE

To test if different leucine tracers behave in an indistinguishable ma nner and, by implication, that their metabolism is identical to that o f natural leucine, we measured whole body leucine turnover in dogs and humans and fibrinogen synthesis in dogs by simultaneously infusing ei ther [1-C-14]leucine or [4,5-H-3]leucine or [1-C-14]alpha-ketoisocapro ate (KIC) and [4,5-H-3]KIC. Whole body leucine fluxes calculated from the plasma specific activity of the transaminated product of the infus ed tracer (reciprocal pool model) were lower (dogs by 5.7%; humans by 6.4%, both P < 0.02) when the plasma H-3 specific activity compared to C-14 specific activity were used with leucine tracers and were also l ower (dogs by 4.4%, P < 0.02; humans by 8.6%, P < 0.06) using the KIC tracers. Using leucine or KIC tracers in dogs, the fractional rate of fibrinogen synthesis was 6.7% or 9.4% lower, respectively, (P < 0.02) using the H-3 versus the C-14 tracer. The apparently lower incorporati on of H-3 into protein was only in part accounted for by detritiation (2.1%, P = 0.05) of [H-3]leucine during acid hydrolysis of proteins. T hese results suggest that in vivo and/or in vitro differential isotope effects are small (approximately 5%), but should be considered when d ual isotopes infusions are employed to partition amino acid metabolism .