Previous measurements of the kinetics of oxidation of linoleic acid by soybean lipoxygenase 1 have indicated very large deuterium isotope effects, but have not been able to distinguish the primary isotope effect from the α-secondary effect. To address this question, singly deuterated linoleic acid was prepared, and enantiomerically resolved using the enzyme itself. Noncompetitive measurements of the primary deuterium isotope effect give a value of ca. 40 which is temperature-independent. The enthalpy of activation is low and isotope-independent, and there is a large isotope effect on the Arrhenius prefactor. A very large apparent secondary isotope effect (ca. 2.1) is measured with deuterium in the primary position, but a greatly reduced value (1.1) is observed with protium in the primary position. Mutagenesis of the active site leads to a significant reduction in k_cat and perturbed isotope effects, in particular, a …