Effects of leucine on whole body leucine, valine, and threonine metabolism in humans.
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.
Hoffer
L J
LJ
McGill Nutrition and Food Science Centre, McGill Medical Research Council of Canada, McGill University, Montreal, Quebec, Canada.
Taveroff
A
A
Robitaille
L
L
Hamadeh
M J
MJ
Mamer
O A
OA
D000328:Adult; D000596:Amino Acids / Q000097:blood; D001711:Biotransformation; D002247:Carbon Isotopes; D003903:Deuterium; D005260:Female; D045782:Hemiterpenes; D006801:Humans; D007651:Keto Acids / Q000097:blood; D007700:Kinetics; D007930:Leucine / Q000378:metabolism* / Q000494:pharmacology*; D008297:Male; D011865:Radioisotope Dilution Technique; D013912:Threonine / Q000378:metabolism*; D013997:Time Factors; D014633:Valine / Q000378:metabolism*
D000596:Amino Acids; D002247:Carbon Isotopes; D045782:Hemiterpenes; D007651:Keto Acids; D013912:Threonine; C001505:alpha-ketoisovalerate; C013082:alpha-ketoisocaproic acid; D003903:Deuterium; D007930:Leucine; D014633:Valine