l-Hydroxy acid oxidase.

In previous communications (l-4) the properties and isolation of the l-amino acid oxidase of rat kidney were described. Quite accidentally, it was found that solutions of the enzyme were able to catalyze the oxidation of Z-hydroxy acids, and further study disclosed that this ability ran parallel with l-amino acid oxidase activity from the first crude extract to the final electrophoretically homogeneous preparation of the enzyme. The experiments which are reported below support the view that the same enzyme exercises these two catalytic functions and that the same prosthetic group and active groups are involved in both cases. In a sense, the enzyme is incapable of distinguishing between an amino acid and a hydroxy acid of the 1 series. There are present in practically all animal tissues enzymes such as the lactic and malic dehydrogenases which specifically oxidize certain I-hydroxy acids. These enzymes can be distinguished from the I-hydroxy acid oxidase in two respects : they are specific for only one hydroxy acid and they specifically require diphosphopyridine nucleotide as oxidizing agent (5). By contrast, the enzyme isolated from rat kidney oxidizes a large variety of ol-hydroxy acids, does not require or react with diphosphopyridine nucleotide, and contains flavin monophosphate as prosthetic group. It is clear that the enzyme is distinct from other hydroxy acid oxidizing systems. Thus far hydroxy acid oxidase activity has been found only in conjunction with amino acid oxidase activity. The structural analogy between the or-amino and cu-hydroxy acids makes it appear very probable that the mechanism of oxidation, involving the amino or hydroxyl hydrogen as well as the hydrogen attached to the a-carbon atom, is t.he same in both classes of substrates. This is borne out by the fact that a-hydroxyisobutyric acid, which has no hydrogen on the a-carbon atom, is not oxidized. It was of interest to determine whether the d-amino acid oxidase of animal tissues was also active towards the d isomers of the cY-hydroxy acids. Using the enzyme from pig kidney, we have tested dl-lactate and dlhydroxy caproate, with negative results. Apparently the ability to oxidize hydroxy acids is not a general property of amino acid oxidases.