Redox titrations of the nickel ion in active hydrogenase from Methanobacterium thermoautotrophicum and Chromatium vinosum were performed in the absence of artificial redox mediators, by variation of the H2-partial pressure. These experiments revealed a redox behaviour of the nickel ion which differed remarkably from previous redox titrations in the presence of redox mediators. Notably the EPR signal of the species earlier characterized as monovalent nickel with bound hydrogen, behaved as an n = 2 redox component upon reduction under varying H2-partial pressures. The EPR signal was not a transient one and persisted upon removal of hydrogen. Possible redox processes to explain these observations are discussed. A similar behaviour of nickel was also observed in enzyme as present in intact cells of M. thermoautotrophicum. These results suggest that nickel hydrogenases possess a second site for reaction with H2.
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