Determination of glutamine in muscle protein facilitates accurate assessment of proteolysis and de novo synthesis-derived endogenous glutamine production

Abstract

Background: Results of tracer studies indicate that skeletal muscle contributes to ≃70% of overall glutamine production in healthy adults; the contribution of de novo synthesis being estimated at ≃60%. However, measurement of the de novo synthesis rate in muscle tissue requires knowledge of the appearance rate of glutamine in plasma and the quantity of glutamine derived from intracellular proteolysis. Thus, the content of glutamine in muscle protein is a prerequisite for an accurate calculation. Objective: The objective of the study was to measure glutamine in muscle protein. Design: Muscle specimens (open biopsies) were obtained from humans (10 men and 4 women), rats (n = 4), cows (n = 4), and pigs (n = 4). Glutamine was assessed via prehydrolysis derivatization, rapid microwave-enhanced acid hydrolysis, and 5-dimethylaminonaphthalene-1-sulfonyl chloride (dansyl chloride) reversed-phase HPLC, and expressed per mg alkali-soluble protein (ASP) and DNA. Results: Glutamine concentrations in muscle cell protein of various species ranged from 41 to 49 μg/mg ASP; the differences were not species related. The combined means (±SDs) for the 4 species were 43.6 ± 4.9 μg/mg ASP and 11.9 ± 2.0 mg/mg DNA, respectively. In humans, there was no apparent influence of age, sex, or BMI. Conclusions: Direct and specific measurements of glutamine in intact muscle protein were 50% lower than assumed previously. We used data compiled from earlier studies to recalculate the contributions of proteolysis and de novo synthesis to the endogenous production of glutamine in selected age groups of healthy humans; these contributions remained remarkably constant at ≃13% and ≃87%, respectively.