No abstract

Nunsmulu poljworpha CBS 4732 grown on a variety of substrates contained very high activities of enzymes catalyzing the NADH-linked reduction of dihydroxyacetone, acetoin, diacetyl, acetol, methylglyoxal and acetone. The enzymes catalyzing these reductions have been purified and their kinetic properties are described. Three different enzymes were found responsible for the above-mentioned activities, namely: (1) dihydroxyacetone reductase; (2) acetone reductase; and (3) alcohol dehydrogenase.So far, the physiological function of dihydroxyacetone reductase and acetone reductase is obscure. The kinetic properties of dihydroxyacetone reductase and the regulation of the synthesis of this enzyme suggest that it does not function as a glycerol dehydrogenase.

The substrate specificity of alcohol dehydrogenase (ADH) from Hansenula polymorpha and Candidu utilis has been compared with that of the classical ADH from baker's yeast. Cell-free extracts of H. polymorpha and C. utiiis exhibited a much higher ratio of butanol to ethanol oxidation than baker's yeast ADH. This was also observed with the purified enzymes. The ratio ofactivities with ethanol and butanol was pH-dependent. With the baker's yeast enzyme the activity strongly decreased with increasing chain length, whereas the enzymes from H. polymorpha and C. utilis showed a high reactivity with long-chain alcohols. In addition, the affinity constant for ethanol was more than tenfold lower than that of the baker's yeast enzyme. The purified preparations yielded several protein bands on polyacrylamide slab gels, each of which showed activity with both ethanol and butanol.KEY WORD -Alcohol dehydrogenase.

Cundida utilis CBS 621 contained four different enzymes capable of reducing carbonyl compounds such as dihydroxyacetone. acetoin, diacetyl, acetol, methylglyoxal and acetone, namely alcohol dehydrogenase, acetone reductase, dihydroxyacetone reductase and 2,3-butanediol dehydrogenase. The dihydroxyacetone reductase of C. utilis did not oxidize glycerol, thus providing evidence that this enzyme cannot function as a glycerol-2-dehydrogenase during growth of the yeast on glycerol. This enzyme may, however, play a role in the assimilation of 2,3-butanediol by C. utilis. The organism also contained a separate 2,3-butanediol dehydrogenase which was unable to reduce dihydroxyacetone. Both dihydroxyacetone reductase and 2,3-butanediol dehydrogenase were present at very high activities during growth of C . ufilis on a variety of substrates, including 2,3-butanediol.KEY WORDS -Dihydroxyacetone reductase; 2,3-butanediol dehydrogenase.

The biochemistry and physiology of 2,3-butanediol metabolism has been studied in a number of selected yeast species. Cundida utilis CBS 621 exhibited diauxic growth on 2,3-butanediol. The first phase wascharacterized by the utilization of the two optically active stereoisomers and associated accumulation of acetoin. In the second phase of growth the meso-form of 2,3-butanediol was utilized together with acetoin. An attempt is made to explain these phenomena on the basis of the substrate specificity of the two enzymes which oxidize 2,3-butanediol in C . utilis. Although whole cells oxidized acetoin and diacetyl at high rates, attempts to identify the enzymes responsible for these oxidations were unsuccessful. In C. utilis and other yeasts metabolism of 2,3-butanediol probably involves a cleavage of the substrate into C2-units which are assimilated by the glyoxylate cycle. In the few yeasts which have been found to grow on 2,3butanediol differences may be encountered with respect to the substrate specificity for the three stereoisomers of 2,3butanediol. For example, Candida salmanticensis CBS 5121 showed no diauxic growth and utilized only two of three stereoisomers.