## Results and Discussion
Furthermore, we investigated if the addition of sugar would be favourable for biotransformation, as medium containing a high concentration of glucose would facilitate the regeneration of enzyme cofactors. For this purpose, 1% glucose was added to the harvested yeast cells which were re-suspended in phosphate buffer at pH = 7. The results are presented in Table 4. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Strain**} & \multirow{2}{*}{
\begin{tabular}{c} **Time** \\ **[days]** \\ \end{tabular} } & \multicolumn{3}{c}{_anti_} & \multicolumn{3}{c}{_syn_} \\ \cline{3-10}  & & **2a (35,1\({}^{\prime}\)S)** & **2b (36,1\({}^{\prime}\)R)** & **ee [\%]** & **2c (35,1\({}^{\prime}\)R)** & **2d (36,1\({}^{\prime}\)S)** & **ee [\%]** \\ \hline _Yarrowia lipolytica_ & & & & & & & **[\%]** & **[\%]** & **[\%]** \\ P26A & 1 & 0 & 1 & 99 & 99 & 0 & 0.3 & \(>\)99 \\ \hline _Yarrowia lipolytica_ & & & & & & & & \\ AM71 & 1 & 0 & 1 & 99 & 99 & 0 & 0.6 & \(>\)99 \\ \hline _Candida viswanathi_ & 1 & 1 & 65 & 10 & 73 & 0 & 23.9 & \(>\)99 \\ AM120 & 2 & 0 & 67 & 11 & 72 & 0 & 22.4 & \(>\)99 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The biotransformation of \(\alpha\)-acetylbutyrateolactone **1** in buffer solution (pH = 7) with 1% of glucose (in % according to GC). \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Strain**} & \multirow{2}{*}{
\begin{tabular}{c} **Time** \\ **[days]** \\ \end{tabular} } & \multicolumn{3}{c}{_anti_} & \multicolumn{3}{c}{_syn_} \\ \cline{3-10}  & & **2a (35,1\({}^{\prime}\)S)** & **2b (36,1\({}^{\prime}\)R)** & **ee [\%]** & **2c (35,1\({}^{\prime}\)R)** & **2d (36,1\({}^{\prime}\)S)** & **ee [\%]** \\ \hline _Yarrowia lipolytica_ & 1 & 2 & 1.0 & 97 & 98 & 0 & 0 & - \\ P26A & 2 & 0 & 1.3 & 99 & 97 & 0 & 0 & - \\ \hline _Yarrowia lipolytica_ & 1 & 2 & 0.5 & 97 & 99 & 0 & 0 & - \\ AM71 & 2 & 2 & 1 & 97 & 98 & 0 & 0 & - \\ AM71 & 3 & 0 & 2 & 98 & 96 & 0 & 0 & - \\ \hline _Candida viswanathi_ & 1 & 26 & 45 & 15 & 51 & 0 & 14 & \(>\)99 \\ AM120 & 2 & 7 & 60 & 18 & 55 & 0 & 15 & \(>\)99 \\ AM120 & 3 & 0 & 65 & 19 & 56 & 0 & 16 & \(>\)99 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The biotransformation of \(\alpha\)-acetylbutyrateolactone **1** with resting cell cultures (in % according to GC). Glucose supplementation reduced the time required for complete substrate reduction. Unfortunately, the enantioselectivity of the biotransformation by _Y. lipolytica_ did not improve. Small amounts of undesirable products, namely stereoisomers **2a** and **2d**, were also identified. In contrast, the _C. viswanathi_ AM120 strain exhibited greater enantioselectivity with respect to the **2a** enantiomer, but the diastereoselectivity of reduction was decreased. In the succeeding experiment, small amounts of organic solvents were added to the medium. Ethanol, glycerol, hexane and isopropanol were used in various proportions. With respect to carbonyl reductase, isopropanol can be used as a co-substrate for the regeneration of NAD\({}^{+}\) or NADP\({}^{+}\) and it increases substrate solubility [26]. It is also interesting to note that ethanol is a fascinating alternative to glucose as a co-substrate for the regeneration of cofactors. However, it is toxic to the cells at higher concentrations [27]. The influence of solvent addition was investigated on two strains of yeast, _C. viswanathi_ AM120 and _Y. lipolytica_ P26A (Table 5). Due to the previously obtained results, the time for biotransformation was assumed to be 1 day. Our results indicate that the addition of organic solvents did not have a positive effect on the biotransformation by _C. viswanathi_ AM120 and _Y. lipolytica_ P26A. The use of 5% ethanol resulted in a significant slowdown of the biotransformation. Increasing the quantity of ethanol to 10% caused complete inhibition of the reaction. Also, the addition of hexane to the medium reduced the substrate's conversion rate. Supplementation with glycerol or isopropanol had no significant effect on substrate conversion.