Comparison of the catalytic oxidation of cysteine and o-dianisidine by cupric ion and ceruloplasmin

Several features of the catalytic oxidation of cysteine by ceruloplasmin and nonenzymic Cu(II) at pH 7 have been compared. The oxidation of cysteine by ceruloplasmin has several properties in common with the Cu(II) catalyzed oxidation of cysteine: pH maxima, thiol specificity, lack of inhibition by anions, and high sensitivity to inhibition by copper complexing reagents. These two catalysts differed in their molecular activity, in their ability to oxidize penicillamine and thioglycolate, and in that H₂O₂ was produced as a primary product only during Cu(II) oxidation. The oxidation of cysteine by ceruloplasmin was compared also with the ceruloplasmin catalyzed oxidation of o-dianisidine, a classical pH 5.5 substrate. The mechanism of the oxidation of cysteine by ceruloplasmin at pH 7 differed from that of o-dianisidine oxidation because the latter substrate was inhibited by anions but not by copper complexing agents. Spectral and other data suggest that during the ceruloplasmin reaction with cysteine there is a one electron transfer from cysteine to ceruloplasmin resulting in the specific reduction of type lb Cu(II).