ARABIDOPSIS FORMALDEHYDE DEHYDROGENASE - MOLECULAR-PROPERTIES OF PLANT CLASS-III ALCOHOL-DEHYDROGENASE PROVIDE FURTHER INSIGHTS INTO THE ORIGINS, STRUCTURE AND FUNCTION OF PLANT CLASS-P AND LIVER CLASS-I ALCOHOL DEHYDROGENASES

A glutathione-dependent formaldehyde dehydrogenase (class III alcohol dehydrogenase) has been characterized from Arabidopsis thaliana. This plant enzyme exhibits kinetic and molecular properties in common with the class III forms from mammals, with a K-m for S-hydroxymethylglutat hione of 1.4 mu M, an anodic electrophoretic mobility (pI: 5.3-5.6) an d a cross-reaction with anti-(rat class III alcohol dehydrogenase) ant ibodies. The enzyme structure, deduced from the cDNA sequence, fits in to the complex system of alcohol dehydrogenases and shows that all lif t forms share the class III protein type. The corresponding mRNA is 1. 4 kb and present in all plant organs; a single copy of the gene is fou nd in the genome. The class III structural variability is different fr om that of the ethanol-active enzyme types in both vertebrates (class I) and plants (class P), although class P conserves mon of the class I II properties than class I does. Also the enzymatic properties differ between the two ethanol-active classes. Active-site variability and ex changes at essential residues (Leu/Gly57, Asp/Arg115) may explain the distinct kinetics. These patterns are consistent with two different me tabolic roles fur the ethanol-active enzymes, a more constant function , reduction of acetaldehyde during hypoxia, for class P, and a mon var iable function, the detoxication of alcohols and participation in meta bolic conversions, for class I. A sequence motif, Pro-Xaa-Ile/Val-Xaa- Gly-His-Glu-Xaa-Xaa-Gly, common to all medium-chain alcohol dehydrogen ases is defined.