EVIDENCE FOR HEME-MEDIATED REDOX REGULATION OF HUMAN CYSTATHIONINE BETA-SYNTHASE ACTIVITY

Human cystathionine beta-synthase catalyzes the first step in the cata bolic removal of the toxic metabolite, homocysteine. It is unique in b eing dependent on both pyridoxal phosphate (PLP) and heme for activity . The reaction involves condensation of serine and homocysteine to giv e cystathionine. Although the role of PLP can be rationalized in analo gy with other PLP dependent enzymes that catalyze beta-replacement rea ctions, the role of the heme is unknown. In this study, we have purifi ed and characterized the recombinant human enzyme and have examined th e effect of heme oxidation state on enzyme activity. We find that unde r reducing conditions, generated by addition of titanium citrate, the enzyme exhibits a 1.7-fold lower activity than under oxidizing conditi ons. Reoxidation of the ferrous enzyme with ferricyanide results in al leviation of inhibition. This redox-linked change in enzyme activity c orrelates with changes in heme oxidation state monitored by UV-visible spectroscopy. Dithiothreitol, which does not reduce the enzyme-bound heme, does not perturb enzyme activity. These studies provide the firs t evidence for redox-linked regulation of cystathionine beta-synthase which is heme-dependent.