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

Background: Results of tracer studies indicate that skeletal muscle contrib utes to approximate to 70% of overall glutamine production in healthy adult s; the contribution of de novo synthesis being estimated at approximate to 60%. However, measurement of the de novo synthesis rate in muscle tissue re quires knowledge of the appearance rate of glutamine in plasma and the quan tity of glutamine derived from intracellular proteolysis. Thus, the content of glutamine in muscle protein is a prerequisite for an accurate calculati on. Objective: The objective of the study was to measure glutamine in muscle pr otein. 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 a ssessed via prehydrolysis derivatization, rapid microwave-enhanced acid hyd rolysis, and 5-dimethylamino-naphthalene-1-sulfonyl chloride (dansyl chlori de) 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 mu/mg ASP; the differences were not species related. The combined means (+/-SDs) for the 4 species were 43.6 +/- 4.9 mu g/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 nov o synthesis to the endogenous production of glutamine in selected age group s of healthy humans; these contributions remained remarkably constant at ap proximate to 13% and approximate to 87%, respectively.