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-hydroxymethylglutathione of 1.4 microM, an 
anodic electrophoretic mobility (pI: 5.3-5.6) and a cross-reaction with 
anti-(rat class III alcohol dehydrogenase) antibodies. The enzyme structure, 
deduced from the cDNA sequence, fits into the complex system of alcohol 
dehydrogenases and shows that all life 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 found in the genome. The class III structural variability is 
different from that of the ethanol-active enzyme types in both vertebrates 
(class I) and plants (class P), although class P conserves more of the class III 
properties than class I does. Also the enzymatic properties differ between the 
two ethanol-active classes. Active-site variability and exchanges at essential 
residues (Leu/Gly57, Asp/Arg115) may explain the distinct kinetics. These 
patterns are consistent with two different metabolic roles for the 
ethanol-active enzymes, a more constant function, reduction of acetaldehyde 
during hypoxia, for class P, and a more variable function, the detoxication of 
alcohols and participation in metabolic conversions, for class I. A sequence 
motif, Pro-Xaa-Ile/Val-Xaa-Gly-His-Glu-Xaa-Xaa-Gly, common to all medium-chain 
alcohol dehydrogenases is defined.