Lj. Hoffer et al., EFFECTS OF LEUCINE ON WHOLE-BODY LEUCINE, VALINE, AND THREONINE METABOLISM IN HUMANS, American journal of physiology: endocrinology and metabolism, 35(6), 1997, pp. 1037-1042
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.