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The Activities of Rat Hepatic Cysteine Dioxygenase and Cysteinesulfinate Decarboxylase Are Regulated in a Reciprocal Manner in Response to Dietary Casein Level^1,2,3,

Division of Nutritional Sciences, Cornell University, Ithaca, NY 14853

The catabolism of cysteine and cysteinesulfinate, the activities of key enzymes in cysteine catabolic pathways, and the effects of inhibitors of specific enzymes on cysteine catabolism were investigated in hepatocytes isolated from rats fed low (100 g casein/kg diet), moderate (300 g casein/kg diet) or high (600 g casein/kg diet) levels of dietary protein. Cysteine was catabolized predominantly by cysteinesulfinate-dependent pathways. Cysteine dioxygenase activity increased with increases in dietary casein level, and the higher enzyme activity was paralleled by a greater total catabolite production (taurine + hypotaurine + sulfate) from cysteine. However, taurine production did not closely follow cysteine dioxygenase activity. Taurine production doubled with an increase in dietary casein from 100 to 300 g/kg but did not increase with a further increase in dietary casein to 600 g/kg. Taurine production as a percentage of total catabolism decreased progressively with the increases in dietary casein and closely paralleled observed decreases in cysteinesulfinate decarboxylase activity. Thus, taurine production was limited at high protein levels by the decrease in cysteinesulfinate decarboxylase activity such that sulfate production from cysteinesulfinate was favored. D-Cysteinesulfinate inhibited cysteinesulfinate-dependent catabolism of cysteine, but inhibition of cysteinesulfinate decarboxylase was not specific.

KEY WORDS: • cysteine • hepatocytes • rats • protein • taurine

¹ Supported by NRICGP/USDA Competitive Research Grant no. 92-37200-7583 and by USDA/CSRS grant no. 89-34115-4498.

² Presented in part at the 1992 FASEB Meeting, April 5–9, 1992, Anaheim, CA [Bagley, P. J. & Stipanuk, M. H. (1992) Cysteine dioxygenase and cysteinesulfinate decarboxylase activities regulate hepatic taurine production in response to dietary protein. FASEB J. 6: A1380 (abs.)].

³ The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

⁴ To whom correspondence should be addressed.

Manuscript received 18 January 1994. Revision accepted 3 June 1994.

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