AJP - Endocrinology and Metabolism, Vol 272, Issue 6 E1037-E1042, Copyright © 1997 by American Physiological Society
Effects of leucine on whole body leucine, valine, and threonine metabolism in humans
L. J. Hoffer, A. Taveroff, L. Robitaille, M. J. Hamadeh and O. A. Mamer
McGill Nutrition and Food Science Centre, McGill Medical Research Council of Canada, McGill University, Montreal, Quebec, Canada.
We tested whether expansion of the plasma leucine pool distorts leucine or
valine tracer kinetics, causing errors in the derived values of whole body
proteolysis. Seven normal adults received a 10-h primed-continuous tracer
infusion of L-[5,5,5-2H3]leucine, L-[(1-13)C]valine, and
L-[(1-13)C]threonine, during the final 7 h of which L-leucine was infused
at a rate that more than tripled the plasma leucine concentration. Leucine,
valine, and threonine rates of appearance were converted to a common value
of whole body proteolysis on the basis of their concentrations in body
proteins. The conversion of labeled leucine and valine to their
corresponding branched-chain alpha-keto and alpha-hydroxy acids was also
monitored. Before the unlabeled leucine infusion, postabsorptive whole body
proteolysis was estimated similarly by the three tracers (approximately 180
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 kinetics (P < 0.02). No delay in the
conversion of valine to alpha-ketoisovalerate 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 tracer.
