Mechanism of inactivation of the flavoenzyme lactate oxidase by oxalate.

Lactate oxidase from Mycobacterium smegmatic 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 times 10--5 M. The first step is a fast second order reaction with k-1/k1 equals 8.3 times 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 equals 40 min--1; k-2 equals 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 equals 6.8 times 10-3 M--1 s--1, k-1 equals 28 s--1). Upon illumination the enzyme-oxalate Complex II is converted very rapidly 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 comples is, in contrast, very slow and yields a stable N(5) urea adduct of the reduced flavin.