The coenzyme requirement and enzyme inhibitors of pineapple indoleacetic acid oxidase.

E&acts of pineapple stem tissue constitute the most potent known source of indoleacetic acid oxidase, despite the presence of an inhibitor which is difficult to remove without simultaneously removing some other factor necessary for maximal activity (I). Dialysis invariably lowers the enzyme activity as compared with photolysis, and even prolonged photolysis is never completely effective in removing the naturally occurring inhibitor from the enzyme extract. Since the inhibitor was presumed to be a polyphenol, and since the enzyme extracts were rich in both peroxidase and catalase, the addition of hydrogen peroxide was tried as a means of destroying the inhibitor in pineapple indoleacetic acid oxidase preparations. The phenolic compounds were indeed oxidized, but the resulting enzyme preparation was devoid of indoleacetic acid oxidase activity despite the rapid and complete disappearance of peroxide from the solution. Addition of small amounts of boiled extract restored activity, demonstrating that the tissue contained an essential cofactor as well as the enzyme inhibitor for the indoleacetic acid oxidase. Subsequent work has shown that two different cinnamic acid derivatives were involved as modifiers of the enzyme (2). This paper will present evidence that one of these naturally occurring cinnamic acid derivatives, either p-coumaric acid or the more prevalent depside of p-coumaric acid and quinic acid found in the tissues (3), is a coenzyme for pineapple indoleacetic acid oxidase. Knowledge of the coenzyme requirement has made possible the full activation of the enzyme system and thus the study of enzyme inhibitors. A previous report (2) has given evidence that a derivative of ferulic acid is the naturally occurring inhibitor of pineapple indoleacetic acid oxidase. This paper will present evidence on the inhibitory action of ferulic acid and related compounds.