Aromatic aldehydes as substrates for yeast and yeast alcohol dehydrogenase.

The conversion of benzaldehyde to optically active L-phenylacetyl carbinol by yeast fermentation is a key step in the manufacture of L-ephedrine. ’ Typical fermentation raw materials are molasses, which provides a source of hexoses for glycolysis, and benzaldehyde. L-phenylacetyl carbinol formation is catalyzed by the pyruvate decarboxylase complex.’ In the carboligase reaction, pyruvate is decarboxylated to active acetaldehyde which forms the carbinol product in the presence of benzaldehyde cosub~ t r a t e . ~ ’ ~ Quantitative conversion of benzaldehyde to phenylacetyl carbinol is never achieved and usually some of the benzaldehyde is reduced to benzyl alcoh01.~*~ Benzoic acid may also be produced at very low levek7 It has previously been reported* that Saccharomyces cerevisiae can also catalyze the conversion of 3-methoxy4-hydroxy benzaldehyde and 3,4-methylenedioxybenzaldehyde to the corresponding optically active acyloin compounds ~(-)-3-methoxy-4-hydroxyphenylacetyl carbinol and ~(-)-3,4-methylenedioxyphenylacetyl carbinol respectively. Under these conditions, side reactions involving formation of the corresponding substituted aromatic alcohols are likewise usually formed. Selected strains of Aerobacter aerogenes can also catalyze the conversion of 3,4-dimethoxybenzaldehyde to L( -)-3,6dimethoxyphenylacetyl carbinol.* In a recent study, the reactivity of yeast alcohol dehydrogenase towards a range of aldehydes was tested. Aromatic aldehydes such as benzaldehyde, p -anisaldehyde, m -tolualdehyde, and p -hydroxybenzaldehyde were found not to be substrates.’ We have investigated the conversion of aromatic aldehydes to L-acetyl aromatic carbinols and aromatic alcohols by Saccharomyces cerevisiae and also the conversion of aldehyde to corresponding alcohol by yeast alcohol dehydrogenase. One objective of the investigation was to evaluate the application of this bioconversion system for