Molecular forms of L-alpha-hydroxy acid oxidase from rat kidney

The role of a beta barrel loop 4 extension in modulating the physical and functional properties of long-chain 2-hydroxy-acid oxidase isozymes

Peroxisomal long-chain 2-hydroxy-acid oxidase, an FMN-dependent enzyme, catalyzes the oxidation of a variety of L-2-hydroxy acids into keto acids at the expense of oxygen. We recently reported the cloning and sequencing of its CDNA and the existence of a weakly expressed isozyme [Belmouden, A., Le, K. H. D., Lederer, F. & Garchon, H. J. (1993) Eur. J. Biochem. 214, 17-251. This isozyme, beta 2 differs from the major one in having a three-residue insertion, -VRK-, in loop 4 of the beta 8 alpha 8 barrel. In the crystal structures of homologous flavocytochrome beta 2, and glycolate oxidase, the corresponding region of loop 4 is disordered. We now report on the constitutive high-level expression of isozymes beta 1, and beta 2 in Escherichia coli under control of the lambda pL promoter, and on the influence of the E. coli genetic background and the growth medium on the expression level. We describe the properties of isozyme beta 2 and compare them with those of pure isoform beta 1. The visible spectra of the purified enzymes differ in the position of the near-ultraviolet band of the prosthetic group. pH titration studies indicate that the FMN ionizes at N3 at a lower pH than free flavin and that there is a small pKa difference between the isozymes. To our knowledge, the only other known case of a lowered pKa for the protein-bound flavin is that of glycolate oxidase. In the CD spectra of the FMN region, a marked difference between isozymes in the 270-300-nm region appears to be related to the pKa difference for the N3-H bond. Kinetic parameters for a number of substrates and inhibitors are indistinguishable within the limits of experimental error, with the exception of values for kcat for mandelate (the most active substrate), Km for hydroxyhippurate (a new substrate), Ki for cinnamate and oxalate, and Kd for sulfite. The differences are no larger than twofold. The foregoing comparison between isozymes beta 1 and beta 2 shows that the naturally engineered insertion in loop 4 exerts some influence on the flavin spectral properties and the active-site reactivity. Since the corresponding loop 4 regions in the three-dimensional structures of flavocytochrome 2 and glycolate oxidase are 1.5-2.0 nm removed from the flavin, it would appear either that loop 4 has a very different conformation in hydroxy-acid oxidase, or that it may interact with the active site due to mobility.

Rat kidney L-2-hydroxyacid oxidase. Structural and mechanistic comparison with flavocytochrome b2 from baker's yeast

Hydroxyacid oxidase from rat kidney is an FMN-dependent enzyme that catalyzes the oxidation of L-alpha-hydroxy acids as well as, more slowly, that of L-alpha-amino acids. We report here a modified purification method for the enzyme, which is found to possess one cofactor per subunit of Mr 39,000. Determination of its N-terminal sequence suggests the protein is homologous to spinach glycolate oxidase and baker's yeast lactate dehydrogenase. In the presence of a hydroxy acid and of bromopyruvate, under anaerobic conditions, the enzyme is found to catalyze both transhydrogenation and reductive bromide ion elimination. It had previously been observed that hydroxyacid oxidase could not catalyze chloride elimination from chlorolactate in the presence of oxygen [Cromartie, T.H., & Walsh, C.T. (1975) Biochemistry 14, 3482-3490]. The behavior of this enzyme toward halogeno substrates is therefore similar to that of baker's yeast L-lactate dehydrogenase and in part different from that of Mycobacterium smegmatis lactate oxidase and porcine kidney D-amino-acid oxidase. These findings can be rationalized on the basis of a common mechanism for all these enzymes, implying formation of a carbanion as a first step, with different rate-limiting steps in the overall reaction.