THE ACTIVITIES OF RAT HEPATIC CYSTEINE DIOXYGENASE AND CYSTEINESULFINATE DECARBOXYLASE ARE REGULATED IN A RECIPROCAL MANNER IN RESPONSE TO DIETARY CASEIN LEVEL

The catabolism of cysteine and cysteinesulfinate, the activities of ke y enzymes in cysteine catabolic pathways, and the effects of inhibitor s of specific enzymes on cysteine catabolism were investigated in hepa tocytes isolated from rats fed low (100 g casein/kg diet), moderate (3 00 g casein/kg diet) or high (600 g casein/kg diet) levels of dietary protein. Cysteine was catabolized predominantly by cysteinesulfinate-d ependent pathways. Cysteine dioxygenase activity increased with increa ses in dietary casein level, and the higher enzyme activity was parall eled by a greater total catabolite production (taurine + hypotaurine sulfate) from cysteine. However, taurine production did not closely f ollow cysteine dioxygenase activity. Taurine production doubled with a n 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 produc tion as a percentage of total catabolism decreased progressively with the increases in dietary casein and closely paralleled observed decrea ses in cysteinesulfinate decarboxylase activity. Thus, taurine product ion was limited at high protein levels by the decrease in cysteinesulf inate decarboxylase activity such that sulfate production from cystein esulfinate was favored. D-Cysteinesulfinate inhibited cysteinesulfinat e-dependent catabolism of cysteine, but inhibition of cysteinesulfinat e decarboxylase was not specific.