Excess Dietary Methionine Decreases Indices of Copper Status in the Rat

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Abstract
Two groups (n = 5) of male weanling Wistar rats were housed individually and fed copper (Cu)-deficient (0.5 mg Cu/kg) diets either with or without methionine supplementation (18 g/kg) for 49 days. Plasma caeruloplasmin (EC 1.16.3.1) and erythrocyte superoxide dismutase (EC 1.15.1.1, CuSOD) activities were measured in blood. Tissue Cu levels and the activities of cytochrome c oxidase (EC 1.9.3.1, CCO) and CuSOD were measured in the heart and liver. Hepatic activities of the sulfhydryl-sensitive enzymes, creatine kinase (EC 2.7.3.2), fumarase (EC 4.2.1.2) glutathione S-transferase (EC 2.5.1.18) and lipoamide dehydrogenase (EC 1.6.4.3) were also measured. Apart from cardiac CCO activity all of the measured indices of Cu status were found to be significantly (p < 0.05) decreased in the methionine supplemented rats. Although fumarase activity was significantly (p < 0.05) decreased in the methionine-supplemented animals compared with controls, the activities of the other sulfhydryl-sensitive enzymes were not significantly decreased. These results suggest that some of the toxic effects of excess dietary methionine may be mediated through interference with copper metabolism rather than through the previously postulated inhibition of sulfhydryl-sensitive enzymes by metabolites of methionine.