Abstract
The bioavailability of L-methionine sulfoxide was evaluated by measuring the restoration of the liver glutathione level following amino acid administration, the sulfur incorporation into liver glutathione molecules, and the urinary excretion of inorganic sulfate. The amino acid was administered by intraperitoneal injection (at a level of 150μmol/animal once a day) to rats fed a 10% casein diet for 3 weeks. Two or three injections of the amino acid led to a partial restoration of the decreased glutathione level in the liver. The magnitude of the increase was below the expectation from equivalent methionine administration. Similar effects were substantiated by experiments with [35S]-labeled amino acids, in which the sulfur of L-methionine sulfoxide was incorporated into the glutathione molecule with half the effectiveness of L-methionine. Inorganic sulfate predominated in the sulfur metabolites in the urine of rats given [35S] methionine, while sulfate excretion accounted for only a half of the sulfur metabolites in the urine of rats given [35S] methionine sulfoxide. Provided that methionine sulfoxide is converted to methionine prior to its incorporation into liver glutathione as well as its excretion as inorganic sulfate in the urine, the glutathione and sulfate measurements can be taken as reliable indices to evaluation of methionine sulfoxide bioavailability.
