Metabolism of cysteine, cysteinesulfinate and cysteinesulfonate in rats fed adequate and excess levels of sulfur-containing amino acids.

The oxidation of cysteine, cysteinesulfinate and cysteinesulfonate labeled with 14C in the 1- and 3-positions was studied in rats that had been fed diets with adequate or excess cysteine. Consumption of excess cysteine for 5 or 10 days resulted in an increase in hepatic cysteine dioxygenase activity and a decrease in hepatic cysteinesulfinate decarboxylase activity but had no effect on the oxidation of the C-1 or C-3 of cysteine, cysteinesulfinate or cysteinesulfonate. When the labeled compounds were administered by intraperitoneal injection, 41% of cysteine, 100% of cysteinesulfinate and 37% of cysteinesulfonate were oxidized over an 8-hour period. The percentage of the oxidized cysteine, cysteinesulfinate and cysteinesulfonate that was converted to taurine was calculated to be 83, 70 and 100%, respectively. When these same compounds were administered intragastrically, the relative flux to taurine was lower for all compounds; 41% of the oxidized cysteine, none of the cysteinesulfinate and 11% of the oxidized cysteinesulfonate appeared to be converted to taurine. Metabolism of intragastrically administered cysteine may be more indicative of what happens to dietary cysteine, whereas metabolism of intraperitoneally administered cysteine and cysteinesulfinate may be more indicative of liver metabolism and of the metabolism of endogenous cysteine and cysteinesulfinate.