Mechanism of phenylacetylcarbinol synthesis by yeast.

The enzymatic mechanism by which microorganisms synthesize acetoin and other ketols has been the subject of much experimentation. Many bacteria form acetoin by the intermediate formation of a-acetolactate from two moles of pyruvate, followed by decarboxylation (Dolin and Gunsalus, 1951; Discherl and Hofermann, 1951; Happold and Spencer, 1952; Juni, 1952a). Yeasts, however, utilize free aldehyde. This fact and the demonstrated inability of brewers' yeast to decarboxylate a-acetolactate to acetoin (Juni, 1952b) indicate that the formation of ketols by yeast is through a mechanism distinct from that by bacteria. Neuberg and Hirsch (1921) suggested that a special enzyme, "carboligase", brings about the formation of ketols in yeast. Neuberg and Hirsch (1921) and Discherl (1931) proposed the condensation of acetaldehyde or benzaldehyde with a "nascent" adlehyde to yield the corresponding ketol. Isotopic carbon studies by Gross and Werkman (1947) and by Juni (1952b) substantiate that a condensation of this general type occurs. Gross and Werkman (1947) found that the addition of C18 labeled acetaldehyde to a cell-free yeast juice in the presence of pyruvate resulted in the labeling of all carbon atoms of the acetoin formed. The heaviest labeling occurred in the carbinol end of the molecule. The addition of pyruvic acid-2-C4 and unlabeled acetaldehyde to a cell-free enzyme preparation of brewers' yeast yielded acetoin with the heaviest labeling in the carbonyl portion of acetoin (Juni, 1952b). In both cases, the presence of labeled carbon in all carbon atoms could be accounted for by oxidation or reduction of some of the acetoin to a symmetrical molecule. Singer and Pensky (1951) have reported that a purified a-carboxyiase of wheat germ will synthesize acetoin from acetaldehyde, pyruvate, or both, when diphosphothiamin and Mg++ are