Autoxidation versus biotransformation of alpha-pinene to flavors with Pleurotus sapidus: regioselective hydroperoxidation of alpha-pinene and stereoselective dehydrogenation of verbenol.

The enzymatic conversion of alpha-pinene to verbenols, verbenone, and minor volatile flavors was studied using submerged cultured cells, lyophilisate, and microsomal fractions of the edible basidiomycete Pleurotus sapidus . The similarity of the product range obtained by the bioconversions with the range of products found after autoxidation of alpha-pinene at 100 degrees C suggested similar initial pinene radicals. Extracts of the bioconversions were analyzed using thin layer chromatography with hydroperoxide staining and cool on-column capillary gas chromatography-mass spectrometry. Two isomer alpha-pinene hydroperoxides were identified as the key intermediates and their structures confirmed by comparison with synthesized reference samples and by microchemical reduction to (Z)- and (E)-verbenol. When the biocatalysts were supplemented with one of the verbenols, only the (Z)-isomer was oxidized, indicating the activity of a highly stereospecific monoterpenol dehydrogenase. The structural comparison of subunits shows that fungal oxifunctionalization reactions of some common terpene substrates, such as (+)-limonene or (+)-valencene, might likewise be catalyzed by dioxygenases rather than by CYP450 enzymes, as previously assumed.