Effect of cadmium on purified hepatic flavokinase: involvement of reactive -SH group(s) in the inactivation of flavokinase by cadmium

The effect of cadmium (Cd2+) was studied in vitro on the flavokinase (ATP : riboflavin 5'-phosphotransferase, EC 2.7.1.26) activity purified from rat liver. Cadmium inhibited flavokinase activity in a concentration-dependent manner and the effect was completely reversed by increasing concentration of zinc (Zn2+), indicating a competition between Zn2+ and Cd2+ for binding with the enzyme. Further, a competition between riboflavin and Cd2+ hints at the possibility that Zn2+ and Cd2+ probably compete for the same site on the enzyme where riboflavin binds. Our studies further reveal that hepatic flavokinase contains essential, accessible and functional thiol group(s) as evidenced by a concentration-dependent inhibition by sulfhydryl reagents and protection by thiol protectors like glutathione or dithiothreitol. Furthermore, the enzyme could also be protected from the inhibitory effect of Cd2+ and Hg2+ by glutathione and dithiothreitol suggesting that Cd2+ probably interacts with reactive thiol group at or near the active site of the enzyme to cause inhibition.