COMPARATIVE-STUDY OF O-17 AND O-18 ISOTOPE EFFECTS AS A PROBE FOR DIOXYGEN ACTIVATION - APPLICATION TO THE SOYBEAN LIPOXYGENASE REACTION

We present the first comparative study of O-17 and O-18 isotope effect s in a dioxygen-dependent enzymatic reaction. The binding of dioxygen to soybean lipoxygenase is expected to be near the diffusive limit, in troducing the possibility of a magnetic isotope effect with O-17 react ing faster than O-16. However, the experimental data indicate a purely mass effect among the O-16, O-17, and O-18 isotopes. A reaction mecha nism for soybean lipoxygenase is proposed in which dioxygen undergoes reversible reaction with a substrate derived radical, followed by a sl ow release of the bound lipid hydroperoxide.