The common reaction in yeast cells is reduction of acetaldehyde to ethanol. In vitro the enzyme is generally assayed and used in a more alkaline pH region, a condition which favors a shift of equilibrium towards the oxidation of ethanol.

Characteristics of Alcohol Dehydrogenase from Yeast:

Molecular weight: 141,000 comprised of four subunits of 35,000 each (Büchner and Sund 1969).

Composition: A metalloenzyme containing four tightly bound zinc atoms per molecule (Vallee and Hoch 1955). Per subunit, there are two distinct active site sulfhydryl groups which can be distinguished on the basis of differential reactivity with iodoacetate and butyl isocyanate (Twu, Chin, and Wold 1973). A histidine residue is considered to have an essential role (Dickenson and Dickinson 1975).

Optimum pH: For the oxidation of ethanol, pH 8.6-9.0 (the enzyme becomes increasingly unstable at higher pH). For the reduction of acetaldehyde a pH nearer to 7.0 is considered optimum. This reaction is kinetically complex with pH being only one factor determining optimum conditions.

Extinction coefficient: = 12.6.

Isoelectric point: 5.4 (Sund and Theorell 1963).

Activators: Sulfyhydryl activating reagents, mercaptoethanol, dithiothreitol, cysteine, etc., and heavy metal chelating reagents.

Specificity: Yeast ADH which has a more narrow specificity than that of liver enzyme, accepts ethanol, is somewhat active on the straight chain primary alcohols, and acts to a very limited extent on certain secondary and branched chain alcohols (Dickinson and Dalziel 1967). NADP does not serve as coenzyme.

Inhibitors: Heavy metals and -SH reagents. See Sund and Theorell (1963) for a more comprehensive list; also Anderson and Reynolds (1966), Anderson et al. (1966), Atkinson et al. (1967), Fiddick and Heath (1967), Rashed and Rabin (1968).

Stabilizers: Dilute solution of the enzyme may be stabilized by serum albumin, gelatin, and/or glutathione or cysteine. At pH values below 6.0 and above 8.5 the enzyme is increasingly unstable. More concentrated solutions of the enzyme in high purity water, near neutrality, are stable several days at 5 °C.

Stability: Lyophilized preparations, stored refrigerated, are stable 6-12 months. Crystalline suspensions in ammonium sulphate are stable 6 months at 2 - 8 °C.