Diurnal pattern of the interrelationships between leucine
oxidation, urea production and hydrolysis in humans.
El-Khoury, Antoine E., Alfred M. Ajami, Naomi K. Fukagawa, Thomas E.
Chapman, and Vernon R. Young.
Laboratory of Human Nutrition, School of Science and Clinical
Research Center, Massachusetts Institute of Technology, Cambridge, MA
02139, and MassTrace, Woburn, MA, 01801
APStracts 3:0067E, 1996.
We investigated in six healthy adult men, receiving an adequate intake
of protein (1g.kg- 1.day-1), the relationship between urea
production, excretion and hydrolysis. At the end of a 6-day diet
-adjustment period, subjects were then studied using a 24h continuous,
intravenous [1- 13C]leucine and [15N,15N]urea tracer protocol (El
-Khoury et al; Am J Clin Nutr 59: 1000-1011, 1994) in order to
determine rates of irreversible protein nitrogen loss (IPNL) and urea
kinetics. By combining leucine and urea kinetic data, we find a
significant degree of urea hydrolysis over the 24h period but no
evidence to support the thesis that there is a net retention or
"salvage" of the urea N liberated. Our measurements revealed
little or no urea hydrolysis during the fed 12h period of the 24h
tracer protocol but substantial hydrolysis during the 12h fasting
phase. Further, a mass balance model and calculations (APPENDIX)
indicated that N "salvage", if any, is quantitatively
indistinguishable from insensible nitrogen losses and aggregate
estimation errors, accounting for no more than 5% of the nitrogen
intake.We conclude that urea hydrolysis, via the intestinal
microflora, while representing a component of the overall cycles of
nitrogen flow within the body, does not contribute via a net
retention of amino nitrogen to the maintenance of body nitrogen
homeostasis in healthy adults consuming an adequate diet.
Received 30 October 1995; accepted in final form 12 March 1996.
APS Manuscript Number E516-5.
Article publication pending Am. J. Physiol. (Endocrinol. Metab.).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 27 March 96