Mitochondrial and cytosolic rhodanese from liver of DAB-treated mice. III. Inhibition kinetic studies.

Rhodanese (thiosulphate:cyanide sulphurtransferase) shows distinctive mitochondrial and cytoplasmic activities in several models of tumorigenesis. To investigate the basis for these differences, the enzyme was purified from mitochondrial and cytosolic liver fractions of mice treated with the carcinogen p-dimethyl-aminoazobenzene (DAB) and some inhibition kinetic studies were carried out. When both substrates were assayed at inhibitory levels, non-competitive inhibition was observed for the second substrate at variable concentrations, the reversible connection between both substrates was attained by the instability of the second enzyme form. It is suggested that the enzyme might be changing from an unstable ES form to a more stable sulphur substituted intermediate as a consequence of DAB treatment. Sulphite was a competitive inhibitor vs thiosulphate for rhodanese isolated from normal liver and a hyperbolic activator for the enzyme isolated from liver of DAB-treated animals.