Electrochemical behaviour of human adrenodoxin on a pyrolytic graphite electrode

Adrenodoxin (Adx) functions as a redox protein in the delivery of electrons to all mitochondrial cytochromes P450. In order to further characterize the human form of this protein, direct electrochemistry of human adrenodoxin (Hadx) has been observed for the first time on a pyrolytic graphite electrode (PGE) modified with poly-L-lysine. A single well-defined redox wave was observed with a midpoint potential of -448+/-3 mV vs. Ag/AgCl (sat. KCl) at scan rates of 10 mV/s and over the pH range 4.0-8.0. At slow scan rates, the reduction process was close to being electrochemically reversible whereas, at faster scan rates, only quasi-reversibility was observed. A correlation was observed between the peak separation (DeltaE) for the cyclic voltammograms and pH over a wide range of scan rates. The variation of DeltaE with pH was at a minimum (optimum reversibility) at pH 7.0 for all scan rates tested. This correlation may suggest that the direct electrochemistry method could possibly provide a means for determining protein or enzyme activity. The electron transfer rate constant, k(s), was determined to be 0.28 s(-1) at pH 7.0 and a small pH dependence was observed. The results obtained in this study demonstrate the facile nature of direct electron transfer for human adrenodoxin, and provide an estimate of the midpoint reduction potential at a pyrolytic graphite electrode via electrostatic immobilisation.