AMPHIBIAN ALCOHOL-DEHYDROGENASE, THE MAJOR FROG LIVER-ENZYME - RELATIONSHIPS TO OTHER FORMS AND ASSESSMENT OF AN EARLY GENE DUPLICATION SEPARATING VERTEBRATE CLASS-I AND CLASS-III ALCOHOL DEHYDROGENASES

Submammalian alcohol dehydrogenase structures can be used to evaluate the origins and functions of the different types of the mammalian enzyme. Two avian forms were recently reported, and we now define the major amphibian alcohol dehydrogenase. The enzyme from the liver of the Green frog Rana perezi was purified, carboxymethylated, and submitted to amino acid sequence determination by peptide analysis of six different digests. The protein has a 375-residue subunit and is a class I alcohol dehydrogenase, bridging the gap toward the original separation of the classes that are observable in the human alcohol dehydrogenase system. In relation to the human class I enzyme, the amphibian protein has residue identities exactly halfway (68%) between those for the corresponding avian enzyme (74%) and the human class III enzyme (62%), suggesting an origin of the alcohol dehydrogenase classes very early in or close to the evolution of the vertebrate line. This conclusion suggests that these enzyme classes are more universal among animals than previously realized and constitutes the first real assessment of the origin of the duplications leading to the alcohol dehydrogenase classes. Functionally, the amphibian enzyme exhibits properties typical for class I but has an unusually low K(m) for ethanol (0.09 mM) and K(i) for pyrazole (0.15-mu-M) at pH 10.0. This correlates with a strictly hydrophobic substrate pocket and one amino acid difference toward the human class I enzyme at the inner part of the pocket. Coenzyme binding is highly similar, while subunit-interacting residues, as in other alcohol dehydrogenases, exhibit several differences. The frog enzyme has a lower pI than mammalian class I alcohol dehydrogenases, showing that electrophoretic migration is not a reliable indicator of the class distinction. The pI difference is explained by amino acid substitutions resulting in three more negative charges in the frog than in the human class I gamma-1 subunit. In conclusion, the amphibian enzyme allows a rough positioning of the divergence of the alcohol dehydrogenase classes, shows that the class I type is widespread in vertebrates, and functionally conforms with greater variations at the substrate-binding than the coenzyme-binding site.