Oxidation and reduction of copper ions in catalytic reactions of Rhus laccase.

1. It was demonstrated by colorimetric as well as EPR measurements that the native (aerobic, resting state) Rhus vernicifera laccase contains both Cu2+ and Cu+ (total Cu content was 4.0 gram atoms/mole). The ratio of Cu2+ to total Cu in laccase varied (42-90%) in samples of latex collected from various districts. The absorption maximum at 615 nm was proportional to the content of total Cu in the enzyme sample. Laccase activity was found to almost parallel the content of the Cu2+ form. The oxidized minus reduced difference absorbance of the enzyme at 330 nm shoulder was proportional to the amount of Cu2+. 2) Steady state level of oxidation of laccase copper during the laccase copper catalytic action, the rates of reduction by substrates and the oxidation by O2 were determined by following absorbance changes at 615 and 330 nm by the stopped flow method. 3) All the results from titrimetric and kinetic experiments were consistent with the laccase model previously proposed by Makino and Ogura in which a laccase molecule contains 1 Cu(615) and 3 Cu(330). Our expanded model states that a laccase sample originally contains active as well as inactive enzymes. In the active enzyme, Cu ions are reactive to O2 but in the inactive enzyme, Cu can be oxidized only by oxidizing agents such as H2O2 or ferricyanide, or by a slow intermolecular electron transfer from Cu(615) to the active enzyme. In both species of enzyme rapid reduction of Cu2+ ions by substrate takes place. In comparative studies of the reactivities of Cu ions in various copper proteins, we would like to suggest that oxidatic activity of a copper protein is due to the Cu+ form of the enzyme ions with O2.