Mammalian liver alcohol dehydrogenases

Abstract

Literature on the properties of liver alcohol dehydrogenase (ADH) from man, horse and rat is reviewed and discussed under two major headings: 1) physical and chemical properties of ADH and 2) structure-function relationship in isoenzymes. Under the first heading are discussed: molecular weight, subunit composition catalytic sites per molecule, sulfhydryl groups, end groups, amino acid composition, role of Zn++ in the structure and function, coenzyme specificity and binding, conformational changes, substrate specificity, catalytic mechanism and recent results from x-ray crystallography of horse liver ADH. The physicochemical properties of ADH from man, horse and rat are for the most part similar. All three enzymes have identical molecular weights, similar amino acid compositions, consist of two subunits, and are all metalloenzymes containing Zn++: horse and human ADH contain one coenzyme binding site per subunit; no results are available for the rat ADH. ADH catalyses interconversion of a large variety of saturated and unsaturated aliphatic and aromatic alcohols and the corresponding aldehydes and ketones utilizing NAD(H). The physiological role of ADH is uncertain. ADH readily combines with reduced coenzymes to form binary complexes with low dissociation constants (10-7 to 10-8M); in the ternary complexes with coenzymes and substrate-competitive inhibitors, these constants are even lower. In the presence of suitable inhibitors, the enzymes can be titrated by coenzymes employing fluorometric and spectrophotometric procedures. The rate of the overall reaction catalyzed by ADH is determined by the dissociation rates of coenzymes, the slowest steps in the reaction sequence. Under the second heading are discussed: liver ADH isoenzymes of horse, man, rat, rhesus monkey and other species, and the significance of steroid activity which accounts for the distinct substrate specificity of some isoenzymes. ADH from horse liver is a heterogeneous enzyme consisting of subunits of distinct substrate specificity and primary structure. The difference in the amino acid sequence between subunit E (active with classical ADH substrates, but not with steroids) and subunit S (active also with steroids) amounts to six amino acids out of 374. Human ADH is also heterogeneous, and at least five genes code for polypeptides which, by dimerization, form different isoenzymes. Experimental evidence suggests that rat ADH is a single unique protein which, like horse liver ADH, SS, is active with steroids. The physiological significance of steroid activity of ADHs is unknown. (Four tables with comparative data and one figure are presented).