Effects of Equimolar Doses of L-Methionine, D-Methionine and L-Methionine-dl-Sulfoxide on Plasma and Urinary Amino Acid Levels in Normal Adult Humans

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Abstract
Plasma and urinary amino acid levels were measured in four normal adult subjects administered equimolar quantities (0.0605 mmol/kg body wt) of L-methionine, D-methionine and L-methionine-dl-sulfoxide in a randomized crossover design. Plasma total methionine concentrations increased significantly (P < 0.05) over base line (3.7 ± 1.2 µmol/dl; mean ±SD) after loading with each compound. Mean peak plasma methionine levels were 9.8 ± 1.1, 14.4 ± 2.3 and 5.2 ± 1.0 µmol/dl after loading with L-methionine, D-methionine and L-methionine sulfoxide, respectively. D-Methionine accounted for the increased plasma levels seen after D-methionine loading. None of the three compounds affected plasma cystine, cysteine or taurine concentrations. Plasma methionine sulfoxide concentrations were not affected by loading with D- or L-methionine but increased significantly after ingestion of L-methionine sulfoxide. Urinary methionine excretion was 20 times higher after ingestion of D-methionine than after ingestion of L-methionine or L-methionine sulfoxide, with the increase due to D-methionine excretion. Urinary excretion of methionine sulfoxide and its N-acetyl derivatives was not significantly higher after loading with methionine sulfoxide. The data indicate that adult humans do not utilize D-methionine efficiently as a methionine source but probably do utilize L-methionine-dl-sulfoxide.