A-Hydroxyacid oxidase

Limited amino acid sequence information has shown that long-chain a-hydroxyacid oxidase from rat kidney is also related to these FMN-containing oxidoreductases (55). It is likely that several further members of this family remain to be identified. The flavodehydrogenase domain shows no sequence similarity to the lactate dehydrogenase from bacteria and higher eukaryotes that utilize NAD as a substrate. Yeasts lack such an enzyme and the substrate specificity of flavocytochrome 62 has presumably evolved independently of the NAD-linked dehydrogenases. 

P. are particularly abundant in liver and kidney cells, e.g. a single rat liver cell contains 350-400 P. The half-life of P. is 1.5-2 days. Other cell types usually contain smaller P. with homogeneous contents (microperoxisomes). In addition to catalase, P. contain, inter alia, o-amino acid oxidase, a-hydroxyacid oxidase and urate oxidase. The latter enzyme is often present as a large crystal in the otherwise homogeneous matrix. These enzymes are particularly important in the oxidative degradation of metabolic intermediates (e.g. purine bases) and in the formation of carbohydrates from amino acids and other materials. [L.J. Olsen J.J.Harada Peroxisomes and Their Assembly in Higher Plants Annu. Rev. Plant Physiol. Plant Mol. BioL 46 (1995) 123-146)... 

Phillips, D. R., Duley, J. A., Fennell, D. J., and Holmes, R. S., 1976, The self-association of L-a-hydroxyacid oxidase, Biochim. Biophys. Acta 4t27 679. 

With D-amino acid oxidase, one of the diagnostic tests for a carbanion mechanism was that ethane nitronate acted as a substrate (110). However, with flavocytochrome 62 there is no evidence of electron transfer between ethane nitronate and the enzyme, rather this compound behaves as a competitive inhibitor (96). The reason why there is no electron transfer in the case of flavocytochrome 62 is unclear, but one possibility is that the carbanion is not correctly oriented. This absence of electron transfer in no way disproves a carbanion mechanism. Indeed, other flavoenzymes, such as long-chain hydroxyacid oxidase also fail to utilize ethane nitronate (115). So, apart from the ethane nitronate result, there is substantial evidence to support a carbanion mechanism in flavocytochrome 62 The question now is how do the electrons transfer to the oxidized FMN There are essentially three possibilities (1) there is a nucleophilic attack by the substrate carbanion at flavin N5, forming a covalent bond, with subsequent cleavage resulting in reduced flavin (2) there is a one-electron transfer to the flavin followed by collapse of the radical pair to again form a... 

Duley J.A. and R.S. Holmes, 1976. L-alpha-Hydroxyacid oxidase isozymes. Purification and molecular properties. Eur. J. Biochem. 63, 163-173. 

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