Cystathionine b-synthase (CBS) catalyzes the pyridoxal phosphate-dependent condensation of serine and homocysteine into cystathionine and participates in the transsulfuration pathway which converts methionine into cysteine. CBS is unique in having a heme cofactor of unknown function with axial cysteine and histidine ligands. Since the heme does not participate in the enzymatic mechanism, it could be involved in the redox regulation of the enzyme. In this study, we have assessed the redox behavior of CBS by direct electrochemistry and we have estimated its reduction potential by cyclic voltammetry. Human dimeric CBS, a variant lacking the C-terminal domain, showed a quasi-reversible response on a platinum electrode, with anodic and cathodic peaks at -0.325 V and -0.454 V vs NHE, respectively, yielding an estimated formal reduction potential of -0.158 V vs NHE at pH 6.46. Similar electrochemical profiles were obtained with CBS that had been treated with N-ethylmaleimide to block the thiols. The results obtained are interpreted in terms of the known structure of CBS and suggest that the heme could be in the reduced state in vivo.