## 4 Discussion  
The biocatalysts used in this study performed the reduction of the double bond in the carvone ring as the first step, followed by the reduction of the carbonyl group. This course of transformation is well known in biotransformations observed in plant cell cultures, e.g., _Nicotiana tabacum_[14], _Mentha piperita_[23], _Medigo sativa_, and _Astasia longa_[24]. In the paper by [PERSON] et al. [25], the transformation of (4\(R\))-(-)-carvone (**1**) via comminuted carrot was described. The acquired results differed from those presented by us in this paper, where the main difference was related to the stereospecificity of the reduction of the double bond in the carvone ring. While the dihydrocarvone obtained by [PERSON] et al. had the configuration (1\(S\), 4\(R\)), in our results, the configuration was (1\(R\), 4\(R\))-dihydrocarvone (**3**). The degree of substrate conversion was also different; we established 18%, while [PERSON] et al. achieved 60%. Another difference is that in the work of [PERSON] et al., the principal product of biotransformation was (1\(S\), 2\(R\), 4\(R\))-neoisodihydrocarveol, which was obtained after 8 h with very high enantioselectivity (95%). The reduction of the carbonyl group in both studies advanced in the same way--\(R\)-alcohol was formed, contrary to [PERSON]'s rule. Such a sizeable discrepancy in the results can be explained by the use of another carrot variety and the prevailing climatic conditions in our latitudes. The results from [PERSON] et al. were more compatible with biotransformations carried out with yeast [25] than with the use of plant cultures [23]. In conclusion, the biotransformations executed in our study demonstrated that the first step of the transformation was the regioselective reduction of the double bond in the ring of both carvone stereoisomers (4\(R\))-(-)-(**1**) and (4\(S\))-(+)-carvone (**6**), which delivered the appropriate dihydrocarvones. The identical stereochemistry of reduction was observed in both carvone stereoisomers. The hydrogen atom attack at C-1 was from the Si side, while it was at C-6 from the _re_ side (Figure 7). Figure 6: Biotransformation of (4\(S\))-(+)-carvone (**6**) via a comminuted potato. The enzymatic hydrogenation reactions of the double bond are catalyzed primarily by enoate reductases (ERs, E.C. 1.3.1.x), which are dependent on flavin mononucleotide. Most of them belong to the \"Old Yellow Enzyme\" family (OYE) and are involved in the metabolism of fatty acids or in the detoxification process [26]. Although a number of enzymes in this group have been isolated and tested even with the use of microorganisms; in practice, entire cultures are utilized. This is caused by enzyme instability as their special sensitivity even to trace amounts of oxygen can effectively deactivate the enzyme [27]. Thus, the use of an isolated enzyme requires a system of cofactor regeneration. During enzymatic hydrogenation, usually the _trans_ addition of hydrogen to the double bond is observed. This reaction occurs more readily when the double bond is coupled to a second double bond or when it is activated by the presence of electron withdrawing substituents. It is worth noting that, unlike enzymes isolated from microorganisms, known plant reductases catalyze _cis_-additions [23]. In this study, the _trans_ or _cis_ addition was examined depending on the carvone enantiomer. In the case of (4_R_)-(-)-carvone (**1**), compound (**2**) was formed (_trans_ product), while from (45)-(+)-carvone (**6**), _cis_-dihydroxcarvone (**7**) was formed. In the biotransformation of (4_R_)-(-)-carvone (**1**) and (45)-(+)-carvone (**6**), apart from the hydrogenation products, the corresponding alcohols were also obtained. The reduction of substrate (**1**) provided alcohol \(R\); consequently, it ran against [PERSON]'s rule. The reaction of the carbonyl group reduction was catalyzed by dehydrogenases or corresponding reductases. As a result of the desymmetrization of the substrate molecule, optically pure products are often obtained.