Rat Liver Cysteine Dioxygenase (Cysteine Oxidase)

Abstract

Rat liver cysteine dioxygenase has been purified to homogeneity. It is a single subunit protein It is a single subunit protein having a molecular weight of 22,500 ±1,000, with a pl of 5.5. The enzyme purified was catalytically inactive and activated by anaerobic incubation with either L-cysteine or its analogues such as carboxymethyl-L-cysteine, carboxyethyl-L-cysteine, S-methyl-L-cysteine, D-cysteine, cysteamine, N-acetyl-L-cysteine, and DL-homocysteine. The enzyme thus activated with i.-cysteine was rapidly inactivated under aerobic condition. This rapid inactivation was observed at 0°C where no formation of either the reaction product cysteine sulfinate or the autoxidation product of cysteine, cystine, was detected. Further analysis shows that the inactivation of the activated enzyme was due to oxygen but unrelated to either the presence of substrate, enzyme turnover or accumulation of inhibitor produced during assay. A distinct rat liver cytoplasmic protein, called protein-A, could completely prevented the enzyme from the aerobic inactivation. The loss of activity during assay in the absence of protein-A was shown to be a first order decay process. From the plots of log(Jproduct/min) versus time, the initial velocity (V_O) and the velocity at 7 min (V7) were obtained. The apparent K_m value for L-cysteine in the absence of protein-A was calculated from the initial velocity as 4.5 × 10⁻⁴ M. Protein-A did not alter the apparent K_m value for L-cysteine. The chelating agents such as 0-phenanthroline, α,α‘-dipyridyl, bathophenanthroline, 8-hydroxyquinoline, EGTA, and EDTA strongly inhibited the enzyme activity when these chelating agents were added before preactivation. The purified cysteine dioxygenase contains 1 atom of iron per mol of enzyme protein. By the activation procedure, the enzyme became less susceptible to the heat de-naturation, the inhibitory effects of chelating agents and the tryptic digestion.