Tracer priming in human protein turnover studies with [15N] glycine

Sixty-three studies in healthy normal volunteers (n = 29), malnourished cancer (n = 8) or non-cancer patients (n = 9), and postoperative radical cystectomy patients (n = 17) were conducted to evaluate the primed constant infusion labeling technique for the estimation of whole-body protein turnover under a variety of dietary conditions. [¹⁵N]Glycine was used as the tracer with a prime to infusion ratio of 1300 to 3300 min and a continuous-infusion rate of 0.11 to 0.33 μg ¹⁵N · kg⁻¹ · min⁻¹ for 24 to 36 hr. The isotopic steady-state enrichment was reached in all subjects both in urinary urea and ammonia between 10 and 26 hr (mean 18 ± 2). During protein calorie fasting the attainment of isotopic steady state is much quicker (10 to 18 hr) with a primed constant infusion than with a constant infusion alone (≈38 hr). A $\text{P}\text{I}$ ratio greater or less than 1800 (min) usually resulted in a delay of plateau attainment without affecting the protein turnover values. Reliable estimates of protein kinetics in humans can be made in clinical conditions with a 26-hr infusion of glycine at the rate of 0.28 μg ¹⁵N · kg⁻¹ · min⁻¹ with a $\text{P}\text{I}$ ratio of 1800 min, collecting six urine samples every 2 hr from 16 hr and analyzing for both urinary urea and ammonia enrichments.