| Abstrakt: |
Bioreduction of acetophenone (ACP) to phenethyl alcohol (PEA) by baker's yeast (Saccharomyces cerevisiae), which is highly enantioselective, can be carried out entirely in a resting state using stored carbohydrate, suggesting that a high degree of chemical tolerance might be possible. However, viability and catalytic activity of precultured cells decline steeply within 24 h at initial ACP concentrations >0.2% (17 mM). Viability of cells at 0.4% ACP was 1/4 the viability at 0.2% ACP as determined by vital staining, and <1% based on colony-forming ability. This sensitivity was observed in suspensions with a cell content of nearly 30% (w/v). Longterm PEA production is strongly dependent on viability, indicating that the cumulative yield per batch of cells is maximized by maintaining a very Iow concentration of substrate (∼0.2%). However, nonviable cells (CFU ml[sup -1] < 1% cells ml[sup -1]) can achieve PEA yields up to 1/3 the maximum, an amount representing initial absorption of ACP without further uptake. Regarding population adaptability, when cells surviving the most selective (toxic) concentration of ACP (0.6%) were subcultured in an ACP-free medium and re-reacted, the 24-h percent viabilities (vital staining) and colony-forming frequencies exceeded those of non-selected cells. However, the surviving cells represented only a small fraction (∼1%) of the recultured progeny. Even at ACP concentrations as Iow as 0.25% (w/v), surviving cells were unreliable in transmitting and maintaining ACP-tolerance. In addition, there was no evidence that the chemical yield of recultured ACP-tolerant cells (amount of PEA relative to initial amount of ACP) can consistently exceed the maximum yield of an equivalent density of previously unreacted (non-selected) cells. These results indicate that over a broad range of substrate concentrations, rapid replacement of cells may be more cost-effective than maintenance or reuse of viable cells. [ABSTRACT FROM AUTHOR] |
