EFFECTS OF LEUCINE ON WHOLE-BODY LEUCINE, VALINE, AND THREONINE METABOLISM IN HUMANS

We tested whether expansion of the plasma leucine pool distorts leucin e or valine tracer kinetics, causing errors in the derived values of w hole body proteolysis. Seven normal adults received a 10-h primed-cont inuous tracer infusion of L-[5,5,5-H-2(3)]leucine, L-[1-C-13]valine, a nd L-[1-C-13]threonine, during the final 7 h of which L-leucine was in fused at a rate that more than tripled the plasma leucine concentratio n. Leucine, valine, and threonine rates of appearance were converted t o a common value of whole body proteolysis on the basis of their conce ntrations in body proteins. The conversion of labeled leucine and vali ne to their corresponding branched-chain alpha-keto and alpha-hydroxy acids was also monitored. Before the unlabeled leucine infusion, posta bsorptive whole body proteolysis was estimated similarly by the three tracers (similar to 160 mg protein.kg(-1).h(-1)). The leucine infusion reduced proteolysis by an average of 21% (P < 0.006), as estimated by use of valine or threonine kinetics, and by 10% by use of leucine kin etics (P < 0.02). No delay in the conversion of valine to alpha-ketois ovalerate occurred during the leucine infusion. Thus all three tracers indicated similar postabsorptive rates of whole body proteolysis and a reduction of proteolysis during leucine administration, although the magnitude of the effect was underestimated with use of the leucine tr acer.