Studies of whole body protein turnover within three hours in man using 15 N-glycine as tracer and urinary ammonia as end-product - abstract

ABSTRACT

We have been investigating ways in which to simplfy the method for measuring proteing turnover, by shortening the time required for reliable measurements. It has been possible, using 15N-Glycine and urinary ammonia as the end-product, to measure turnover in two to three hours in normal man. Turnover was calculated following either oral or continuous infusion of 15N-Glycine (0.005 15N/kg/h) in 8 adult men. Plataeu was reached within 120 minutes and was maintained for at least 4 to 6 hours. The results were as follows AMMONIA ENRICHMENT Oral dose - turnover (mgN/kg/D) 960ñ128 (SD), synthesis (gPr/kg/D) 4.7ñ0.7, breakdown (gPr/kg/D) 4.9ñ0.8; AMMONIA ENRICHMENT I.V. dose turnover - (mgN/kg/D) 608ñ128, synthesis (gPr/kg/D) 3.1ñ0.8, breakdown (gPr/kg/D) 2.8ñ0.8. There was a significant linear relationship between ammonia enrichment (oral) and ammonia enrichment (I.V.) (R=0.82; p<0.05). In the same group, turnover measured from ammonia enrichment over 18 hours was 450 ñ 30mg N/kg/D. Turnover measured over 3 hours thus gave higher values than measurements over 18 hours; this is probably because more proteins with shorter half-lives are included in the 3 hour measurements. This would imply that previously reported measurements which used the longer measurement period underestimated turnover by at least 50 percent, and hence significantly underestimated the contribution of protein turnover to basal energy expenditure. The (3 hr) oral measurement appears to measure flux through a pool not included in the measurement intravenously. As this is of a similar magnitude to the pool into which hydrolysed urea nitrogen passess (Jackson et al, Hum. Nutr. Clin. Nutr., 38C, 339, 1984) it may represent a distinct enterohepatic pool of metabolic nitrogen (AU)