Molecular Characterization of Rice α-Oxygenase

Long-chain fatty acids can be metabolized to Cn−1 aldehydes by α-oxidation in plants. The reaction mechanism of the enzyme has not been elucidated. In this study, the gene encoding rice α-oxygenase was expressed in E. coli and purified to apparently homogenous state. Site-directed mutagenesis revealed that His-158, Tyr-380, and Ser-558 were essential for the α-oxygenase activity. These residues are conserved in prostaglandin H synthases (PGHS) and known as a heme-ligand, a source of a radical species to initiate oxygenation reaction, and a residue involved in substrate binding, respectively. The rice α-oxygenase activity was inhibited by imidazole but hardly inhibited by nonsteroidal anti-inflammatory drugs, which are known as typical PGHS inhibitions. In addition, peroxidase activity could not be detected with α-oxygenase when palmitic acid 2-hydroperoxide was used as a substrate. From these findings catalytic resemblance between α-oxygenase and PGHS seems to be evident although there still are differences in their substrate recognitions and peroxidation activities.