Rice a-oxygenase (R alpha O) catalyzes the insertion of O-2 into the C-alpha-H bond of various fatty acids. The mechanism is thought to involve a tyrosyl radical as the oxidant on the basis of comparisons to the structurally homologous cyclooxygenase enzymes. Kinetic and spectroscopic results presented here for the wild-type R alpha O and the Tyr379Phe mutant indicate an irreversible H. abstraction mechanism and support the involvement of the proposed catalytic Tyr.. In addition, very large, weakly temperature dependent deuterium kinetic isotope effects (similar to 50) are observed, consistent with extensive nuclear tunneling. R alpha O, thus, presents a novel example where such quantum effects are associated with an amino acid radical-utilizing enzyme.