Studies on linoleic acid 8R-dioxygenase and hydroperoxide isomerase of the fungus Gaeumannomyces graminis.

Linoleic acid is sequentially converted to 7S,8S-dihydroxy-9Z,12Z-octadecadienoic acid by the 8R-dioxygenase and hydroperoxide isomerase of the fungus Gaeumannomyces graminis, which is a common pathogen of wheat. The objective of this study was to separate and characterize the two enzyme activities. The isomerase activity was found mainly in the microsomal fraction of the mycelia and the 8R-dioxygenase in the cytosol. The 8R-dioxygenase could be partially purified by ammonium sulfate precipitation, gel filtration, ion exchange chromatography or isoelectric focusing. The 8R-dioxygenase was unstable during purification, but it could be stabilized by glutathione, glutathione peroxidase and ethylenediaminetetraacetic acid. Several protease inhibitors reduced the enzyme activity. Gel filtration with Sephacryl S-300 showed that most 8R-dioxygenase activity was eluted with the front with little retention. Isoelectric focusing in the presence of ethylene glycol (20%) indicated an isoelectric point of pl 6.1-6.3. The enzyme was retained on strong anion exchange columns at pH 7.4 and could be eluted with 0.3-0.5 M NaCl. Incubation of the enzyme with 0.1 mM linoleic acid led to partial inactivation, which may indicate product inhibition. Paracetamol and the lipoxygenase inhibitor ICI 230,487 at 30 microM inhibited the 8R-dioxygenase by 44 and 58%, respectively. 8R-hydroperoxy-9Z,12Z-octadecadienoic acid was isolated from incubations of linoleic acid with the partially purified enzyme or with the cytosol in the presence of p-hydroxymercuribenzoate. The hydroperoxide was rapidly converted by the hydroperoxide isomerase in the microsomal fractions to 7S,8S-dihydroxy-9Z,12Z-octadecadienoic acid.(ABSTRACT TRUNCATED AT 250 WORDS)