Diethyl pyrocarbonate modification abolishes fast electron accepting ability of cytochrome b(561) from ascorbate but does not influence electron donation to monodehydroascorbate radical: Identification of the modification sites by mass spectrometric analysis

Cytochrome b(561) from bovine adrenal chromaffin vesicles contains two heme B prosthetic groups and transports electron equivalents across the vesicle membranes to convert intravesicular monodehydroascorbate radical to ascorb ate. To elucidate the mechanism of the transmembrane electron transfer, eff ects of the treatment of purified cytochrome b561 With diethyl pyrocarbonat e, a reagent specific for histidyl residues, were examined. We found that w hen ascorbate was added to the oxidized form of diethyl pyrocarbonate-treat ed cytochrome b(561), less than half of the heme iron was reduced but with a very slow rate. In contrast, radiolytically generated monodehydroascorbat e radical was oxidized rapidly by the reduced form of diethyl pyrocarbonate -modified cytochrome b(561), as observed for untreated cytochrome b(561). T hese results indicate that the heme center specific for the electron accept ance from ascorbate was perturbed by the modification of amino acid residue s nearby. We identified the major modification sites by mass spectrometry a s Lys85, His88, and His161, all of which are fully conserved and located on the extravesicular side of cytochrome b(561) in the membranes. We suggest that specific N-carbethoxylation of the histidyl ligands of the heme b at e xtravesicular side abolishes the electron-accepting ability from ascorbate.