The effect of (aminooxy)acetate, an inhibitor of aminotransferases, on the sulfate formation fromL-cysteine andL-cysteinesulfinate in rat liver mitochondria was studied. Incubation of 10 mML-cysteine with rat liver mitochondria at 37°C in the presence of 10 mM 2-oxoglutarate and 10 mM glutathione resulted in the formation of 4.60 and 1.52µmol of sulfate and thiosulfate, respectively, per 60 min per mitochondria obtained from 1 g of liver. Under the same conditions sulfate formation fromL-cysteinesulfinate was 24.96µmol, but thiosulfate was not formed. The addition of (aminooxy)acetate at 2 mM or more completely inhibited the sulfate and thiosulfate formation fromL-cysteine and the sulfate formation fromL-cysteinesulfinate. These findings support our previous conclusion that cysteine transamination and 3-mercaptopyruvate pathway (MP pathway) are involved in the sulfate formation fromL-cysteine in rat liver mitochondria (Ubuka et al., 1992).