title: Diurnal pattern of the interrelationships among leucine oxidation, urea production, and hydrolysis in humans.
abstract: We investigated in six healthy adult men, who received an adequate intake of protein (1 g.kg-1.day-1), the relationship among urea production, excretion, and hydrolysis. At the end of a 6-day diet-adjustment period, subjects were studied using a 24-h continuous intravenous [1-13C]leucine and [15N,15N]urea tracer protocol (A. E. El-Khoury, N. K. Fukagawa, M. Sánchez, R. H. Tsay, R. E. Gleason, T. E. Chapman, and V. R. Young. Am. J. Clin. Nutr. 59: 1000-1011, 1994) to determine rates of irreversible protein nitrogen loss and urea kinetics. By combining leucine and urea kinetic data, we found a significant degree of urea hydrolysis over the 24-h period but no evidence to support the thesis that there is a net retention or "salvage" of the urea nitrogen liberated. Our measurements revealed little or no urea hydrolysis during the fed 12-h period of the 24-h tracer protocol but substantial hydrolysis during the 12-h fasting phase. Furthermore, a mass balance model and calculations (APPENDIX) indicated that nitrogen 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, although 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.
authors.affiliation: Laboratory of Human Nutrition, School of Science and Clinical Research Center, Massachusetts Institute of Technology, Cambridge 02139, USA. antoine@crc.mit.edu
authors.forename: A E
authors.initials: AE
authors.lastname: el-Khoury
authors.forename: A M
authors.initials: AM
authors.lastname: Ajami
authors.forename: N K
authors.initials: NK
authors.lastname: Fukagawa
authors.forename: T E
authors.initials: TE
authors.lastname: Chapman
authors.forename: V R
authors.initials: VR
authors.lastname: Young
mesh_terms: D000328:Adult; D002940:Circadian Rhythm; D006801:Humans; D006868:Hydrolysis; D007930:Leucine / Q000378:metabolism*; D008297:Male; D008954:Models, Biological; D017672:Nitrogen Compounds / Q000378:metabolism*; D010084:Oxidation-Reduction; D014508:Urea / Q000378:metabolism*
chemical_list: D017672:Nitrogen Compounds; D014508:Urea; D007930:Leucine