Electron-spin-resonance evidence for interaction of protons with Mo(V) in reduced forms of xanthine oxidase

It was found by They con- cluded that direct hydrogen transfer from sub- strate to enzyme takes place in the catalytic reaction. Since proton-Mo(V) interaction does not seem to have been observed, either in other enzymes or in low-molecular-weight complexes of the metal, the chemical nature of the species responsible for the signal is of the utmost interest. We now present further ESR data on this signal ('a,fl'), together with results on two additional Mo(V) signals from the enzyme. These both appear slowly, so it is clear that the enzyme species respon-