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Metabolism of Indole‐3‐acetaldehyde. III. Some Characteristics of the Aldehyde Oxidase of Avena Coleoptiles
Author(s) -
RAJAGOPAL R.
Publication year - 1971
Publication title -
physiologia plantarum
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.351
H-Index - 146
eISSN - 1399-3054
pISSN - 0031-9317
DOI - 10.1111/j.1399-3054.1971.tb03491.x
Subject(s) - chemistry , acetaldehyde , sephadex , ferricyanide , enzyme , enzyme assay , cofactor , biochemistry , stereochemistry , ethanol
Coleoptiles of Avena contain a soluble enzyme system, capable of oxidizing indoleacetaldehyde (lAAld) to indoleacetic acid (IAA). There is a gradient in the concentration of the enzyme along the length of the coleoptile and the first inter‐node. The top 5 mm segment of each organ is relatively richer in this enzyme than the rest of the tissue. The enzyme was purified 17.7‐fold by fractional precipitation with ammonium sulphate followed by gel filtration on Sephadex. Optimal pH for lAAld oxidation is ca . 4.4. Activity of the enzyme is normally oxygen obligatory. But, in the absence of oxygen, phenazine methosulphate (PMS) serves as hydrogen acceptor for aldehyde oxidation, but not some other dyes tried. Approximately one mole of oxygen was consumed for each mole of IAA formed. Formation of H 2 O 2 could not be detected. Added H 2 O 2 inhibited the reaction. Prolonged dialysis progressively inactivated the enzyme. Added NAD, NADP, FMN, FAD, cytochrome c , cyanoco‐balamin, folic acid and ascorbic acid did not restore the lost activity. But 10 −3 M cysteine restored about 60 % of the lost activity. The enzyme is an acidic protein, isoelectric at pH 4.05. For lAAld, under the conditions of experimentation, a Km of 3.45 × 10 −4 M was calculated. Besides lAAld, indole‐3‐aldehyde and phenylacetaldehyde served as substrates, but not acetaldehyde, propionaldehyde, salicylaldehyde, xanthine, hypoxanthine or catechol. Cyanide, dithionite and mercapto‐ethanol totally inactivated the enzyme depending upon the concentration and duration of treatment. X‐ray irradiation up to a dosage of 2900 r promoted the lAAld oxidizing activity of cell‐free preparations made from irradiated coleoptiles. As yet, no cofactor requirements have been found for the activity. The enzyme is unlikely to be a pyridino‐ or a flavoprotein.