Dietary Control of Cysteine Dioxygenase in Rat Liver

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Abstract
A 20-fold increase in cysteine dioxygenase activity in the liver was found in intact rats fed on a 40% protein diet compared with rats fed on a 2.1% protein diet. The hepatic enzyme activity of rats fed on a 20% protein diet were only one-tenth of that of rats fed on the 40% protein diet. This suggests that cysteine dioxygenase in the liver is a regulatory enzyme in the cysteine degradative pathway under normal nutritional conditions. The hepatic cysteine contents exhibited no significant change in rats fed on 2–20% protein diets but those in rats fed on a 30% protein diet were 2.5-fold those in rats fed on the 20% protein diet. The hepatic taurine contents were not changed by feeding 2–30% protein diets but were increased to 3 times those of rats fed on the 20% protein diet by feeding the 40% protein diet. The hepatic contents of both cysteine and taurine were not further increased by feeding diets of higher protein contents. On the other hand, urinary taurine excretion increased proportionally to the dietary protein levels. The hepatic cysteine dioxygenase activities of adrenalectomized rats did not respond to the increase of dietary protein. The cysteine levels in the livers of adrenalectomized rats fed on a 75% protein diet were nevertheless increased to 0.32 μmol/g liver weight, whereas those of intact rats fed on the same diets were 0.19 μmol/g liver weight. The inclusion of only cysteine or methionine of the amino acids so far tested in the 2% protein diet caused the hepatic cysteine dioxygenase activity to increase to the level of rats fed on the 75% protein diet. The hepatic level of cysteine desulfhydrase, another cysteine-degrading enzyme, was much lower than that of cysteine dioxygenase and did not significantly respond to the dietary protein contents. In conclusion, cysteine dioxygenase plays an important role in the regulation of intra-cellular levels of both methionine and cysteine, and also glutathione, in rat liver.