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Aldehyde dehydrogenase from Pseudomonas testosteroni was purified to homogeneity. The enzyme has a pH optimum of 8.2, uses a wide range of aldehydes as substrates and cationic dyes (Wurster's blue, phenazine methosulphate and thionine), but not anionic dyes (ferricyanide and 2.6-dichloroindophenol), NAD(P)+ or 02, as electron acceptors. Haem c and pyrroloquinoline quinone appeared to be absent but the common cofactors of molybdenum hydroxylases were present. Xanthine was not a substrate and allopurinol was not an inhibitor. Alcohols were inhibitors only when turnover of the enzyme occurred in aldehyde conversion. The enzyme has a relative molecular mass of 186000, consists of two subunits of equal size ( M , 92000), and 1 enzyme molecule contains 1 FAD, 1 molybdopterin cofactor, 4 Fe and 4 S. It is a novel type of NAD(P)+-independent aldehyde dehydrogenase since its catalytic and physicochemical properties are quite different from those reported for already known aldehyde-converting enzymes like haemoprotein aldehyde dehydrogenase (EC 1.2.99.3), quinoprotein alcohol dehydrogenases (EC 1.1.99.8) and molybdenum hydroxylases.Aldehydes are toxic compounds, omnipresent in the natural environment mainly as a result of combustion processes. They are also intermediates in the biological degradation of alkanes, alcohols, and several xenobiotics. Both facts probably explain the general occurrence and the variety of microbial aldehyde-oxidizing activities. One of these is dyelinked aldehyde dehydrogenase activity, frequently encountered in cell extracts of alkane-grown or alcohol-grown bacteria. Since purification of this activity from methanotrophic bacteria resulted in a haem-c-containing protein [I], dyelinked aldehyde dehydrogenase has been classified as a haemoprotein. It should be realized, however, that it is far from clear whether haem c is the only prosthetic group and whether all dye-linked aldehyde dehydrogenases are haemoproteins. Thus, for the enzyme from acetic acid bacteria it has been stated that it is quite probably a quinoprotein containing pyrroloquinoline quinone (PQQ) [2]. Although no unequivocal evidence for the presence of PQQ was given and the quantity of it in the enzyme has not been reported, the cofactor seems at least suited to catalyze such a reaction since several quinoprotein alcohol dehydrogenases are also able to oxidize aldehydes very efficiently [3].

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