Comparative Study of 17O and 18O Isotope Effects As a Probe for Dioxygen Activation: Application to the Soybean Lipoxygenase Reaction

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