L-Lysine (Lys) and L-arginine (Arg), but not L-homoarginine (hArg), are proteinogenic amino acids. In healthy humans, oral administration of hArg increased the plasma concentration of Lys, suggesting Lys as a metabolite of hArg. In humans and animals, hArg is biosynthesized from Arg and Lys by arginine:glycine amidinotransferase (AGAT). In vitro, recombinant human arginase and bovine liver arginase I hydrolyzed hArg to Lys, suggesting Lys as a metabolite of hArg. The aim of the present study was to investigate whether changes in blood concentrations of hArg and Lys in old rats fed for 4 months with varied controlled experimental diets could suggest interconversion of these amino acids. Blood samples (n = 253) were taken before (T0) and after 2 months (T2) and 4 months (T4) of the experiment. Plasma concentrations of Lys and hArg were determined by gas chromatography–mass spectrometry. The plasma hArg concentration markedly correlated with the plasma Lys concentration at all timepoints (r ≥ 0.7, P < 0.0001). Further analysis demonstrated that hArg and Lys are closely and specifically associated independently of experimental time/rat age and diet, suggesting that hArg and Lys are mutual metabolites in old rats. Based on the plasma concentration changes, the median yield of hArg from Lys was determined to be 0.17% at T0 and each 0.27% at T2 and T4. With a circulating concentration of about 3 µM, hArg a major metabolite of Lys in healthy humans. hArg supplementation is currently investigated as a cardioprotective means to improve impaired hArg synthesis. Present knowledge suggests that Lys rather than hArg supplementation may be even more favorable.

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Background Practical and non-cognitive skills are essential to medical professions; yet, success in medical studies is primarily assessed with cognitive criteria. We show that practical exams can benefit students who have only average high school final grades, but working experience in medical professions. Methods With a cross-sectional study, we compare the performance of undergraduate medical students with working experience in adjacent health-care professions (and below-average school leaving-grades) with students who entered medical school directly based on their excellent school records in an Objective Structured Clinical Examination (OSCE). For a sample of more than 1,200 students, we use information on OSCE scores in medical and practical skills, doctor-patient communication/interaction, performance in MC-exams, and core sociodemographic variables. Results Waiting list students outperformed their classmates in the demonstration of practical skills. Students admitted via their excellent school grades scored best overall. This difference vanishes once we control for school-leaving grade and age, the two main factors separating the analysed groups. Students from the waiting list have a significantly smaller overall chance to reach excellent grades in the first two years of study. Conclusions Students who gathered experiences in health-care professions before enrolling at medical school can benefit from an expanded role of practical elements in medical studies. Student selection instruments should take these different starting positions and qualities of applicants into account, for example with a quota for the professionally experienced.

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