HUMAN-LIVER PI-ALCOHOL DEHYDROGENASE - KINETIC AND MOLECULAR-PROPERTIES

A new and distinctive form of human liver alcohol dehydrogenase, π-ADH, has recently been purified to homogeneity [Bosron, W. F., Li, T.-K., Lange, L. G., Dafeldecker, W. P., & Vallee, B. L. (1977) Biochem. Biophys. Res. Commun. 74, 85-91]. Its general characteristics, i.e., molecular weight of 78 000, dimeric structure, and zinc content, 4 g-atoms per mol, are all similar to those of other mammalian alcohol dehydrogenases. However, its amino acid composition differs from that of both horse liver ADH and previous preparations of human liver ADH containing a mixture of isoenzymes. The kinetics of π-ADH follow an ordered bibi mechanism with cofactor adding first to form a binary enzyme complex. In contrast to the other molecular forms, π-ADH is less stable in vitro, exhibits a more limited substrate specificity, has higher KM values for ethanol and acetaldehyde, both approximately 30 mM at pH 7.5, and is much less sensitive to inhibition by pyrazole and 4-methylpyrazole, with KI values of 30 and 2 mM, respectively. Hence, differentiation of ADH-independent ethanol oxidizing pathways in man cannot be based solely upon the lack of inhibition of alcohol oxidation by pyrazole or 4-methylpyrazole, the inhibitors most commonly employed for such purposes. Significantly, π-ADH exhibits markedly lower K1 values toward other pyrazole derivatives, 4-bromo-, 4-nitro-, or 4-pentylpyrazole ranging from 4 to 27 μM. Hence, these pyrazole derivatives may be suitable for quantitative inhibitor studies of all molecular forms of human liver ADH, including π-ADH. © 1979, American Chemical Society. All rights reserved.