Mechanism of Inactivation of the Flavoenzyme Lactate Oxidase by Oxalate

Abstract

Lactate oxidase from Mycobacterium smegmatis is strongly inhibited by oxalate. The inhibition is reversible in the absence and irreversible in the presence of light. Oxalate is bound to the enzyme in a two-step process with an over-all Kd of 1.6 x 10-5 M. The first step is a fast second order reaction with k-1/k1 = 8.3 x 10-3 M, leading to a Complex I. This complex is then reversibly converted to a different Complex II in a slow first order reaction (k2 = 40 min-1; k-2 = 0.07 min-1), which is accompanied by major spectral perturbations of the flavin spectrum. With oxamate, two steps could not be demonstrated, and its binding is described by a single step reversible process, which is second order in oxamate (k1= 6.8 x 103 M-1 S-1, k-1 = 28 s-1). Upon illumination the enzyme-oxalate Complex II is converted very rapidly into a reduced flavoenzyme with carbonate covalently bound at position N(5) of the flavin. Slow hydrolysis in the dark under anaerobic conditions subsequently yields free reduced enzyme. The light reaction of the oxamate complex is, in contrast, very slow and yields a stable N(5) urea adduct of the reduced flavin.