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A Study of an aldehyde dehydrogenase from Pseudomonas aeruginosa Von Tigerstrom, Richard G. C.

Abstract

An aldehyde dehydrogenase was found in cell extracts of Pseudomonas aeruginosa ATCC 9027 grown on several carbon sources. It was present in highest concentration in cell extracts after growth of the organism on ethylene glycol or ethanol . The enzyme from ethanol-grown cells was purified by protamine sulfate, ammonium sulfate, acetone, and isoelectric precipitation, ion exchange chromatography and gel filtration. After an eighteen-to twenty-fold purification with a twenty-three per cent yield of activity a homogeneous preparation was obtained, as evidenced by ultracentrifugation, electrophoresis, and other criteria. The enzyme was found to be unstable in crude preparations. This instability was overcome by the use of bisulfite buffer. The enzyme oxidizes a wide variety of aldehydes. The products of glycolaldehyde and glyceraldehyde oxidation were identified as the free acids. The pH optimum for the reaction was found to be between pH 8.0 and 8.6. The enzyme is more active with NAD⁺ as the hydrogen acceptor than with NADP⁺. Potassium or ammonium was found to be essential for activity. Less activity was obtained in the presence of rubidium. Aldehyde dehydrogenases from five other species of Pseudomonas were also activated by potassium. Michaelis constants for aldehyde substrates, NAD⁺, NADP⁺, and the activating ions were determined. In addition to the activating ion, a reducing agent was required for enzymatic activity. It could be replaced, in part, by EDTA or o-phenanthroline. No inhibition was observed with EDTA, but o-phen-anthroline inhibited the enzyme reaction in the presence of a reducing agent. However, zinc was not found to be present in the purified aldehyde dehydrogenase. Aldehyde dehydrogenase also was inhibited by iodoacetamide, iodoacetate, arsenite,Cu⁺⁺ , and p-chloromercuribenzoate. Enzymatic activity also was lost when trypsin was added to the enzyme preparation. This loss of activity and the inhibition by the alkylating agents were specifically prevented by the addition of the activating ion and NAD⁺ to the enzyme preparation. Some protection from digestion by trypsin was afforded by potassium alone. However, in the absence of NAD⁺ potassium accelerated the rate of inhibition by alkylating agents. A molecular weight of 200,000 was determined for aldehyde dehydrogenase by several methods. At low ionic strength the enzyme underwent a partial dissociation with loss of enzymatic activity. This dissociation could be reversed by increasing the salt concentration. Dissociation and association of the enzyme into subunits of approximately equal size could be followed in the ultracentrifuge and on starch gel electrophoresis. The dissociated form of the enzyme was isolated after starch gel electrophoresis and found to be completely inactive. Full enzymatic activity was obtained only when the associated enzyme was protected from oxidation. The enzyme was soluble below its isoelectric point (pH 4.8) but denatured as evidenced by sedimentation, diffusion, and viscosity studies. The molecular weight of the enzyme preparation at pH 3.0 was estimated to be approximately one-half of that found at pH 7.0.Aldehyde dehydrogenase contained relatively large amounts of all common amino acids. The lowest amount was obtained for cysteic acid: 23 to 24 residues per mole. Studies with ¹⁴C-iodoacetamide showed that the enzyme was completely inhibited when approximately three moles of iodoacetamide were taken up per mole of enzyme. Following chymotryptic digestion of labelled aldehyde dehydrogenase, a fraction containing a large percentage of the radioactivity was partially purified by ion exchange chromatography and gel filtration. This fraction contained one peptide species, or several very similar peptide species, probably derived from the active site of the enzyme.

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