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

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 cross-over 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.