Autoxidation versus Biotransformation of α-Pinene to Flavors with Pleurotus sapidus: Regioselective Hydroperoxidation of α-Pinene and Stereoselective Dehydrogenation of Verbenol

The enzymatic conversion of α-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 α-pinene at 100 °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 α-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.