Oxidation and Decarboxylation of Amino Acids by Squash Preparations.

Only a moderate amount of information is presently available on the oxidation and decarboxylation of amino acids by higher plant tissues. In conjunction with other studies on amino acids, it became desirable to find out whether amino acids in plant tissues undergo degradation at rates which vary sufficiently to account for the resuilts of Wood and Cruickshank (17) and Kemble and Mlacpherson (6). These investigators have shown that certain amino acids are degraded in detached leaves at rates which make it unlikely that these amino acids are available for normal protein synthesis. The dehydrogenase which catalyzes the oxidative deamination of glutamic acid has been found in many plants (2) and a tryptophan oxidase has been reported to occur in pea-seedling tissue (16). Only glutamic acid has been demonstrated to be decarboxylated by plant tissue preparations (12), although the occurrence in plant tissues of histamine, hvdroxytyramine, and putrescine, the decarboxylation products respectively of histidine, dihydroxyphenylalanine, and ornithine, has been reported (10, 17). The enzyme source in the present study was an extract of acorn squash prepared essentially bv the method of Schales et al (11). The ovary wall of the squash was homogenized in a Waring blendor at 40 C in M/15 phosphate buffer of pH 5.8. The brei was filtered through muslin to remove cell wall debris, and the filtrate used without further treatment. Decarboxvlation and oxidation rates were measured manometrically at 350 C by standard techniques (13). For decarboxylation studies, the reaction system consiste(l of 4 ml of squash extract containing 20 mg of protein, and 1 ml of a solution containing 33 micromoles of the desired substrate. Both aerobic and anaerobic (nitrogen) conditions were employed. For studies on oxidation, the protein concentration was two times that noted above. The reaction mixture consisted of 2 ml of squash extract, 0.5 ml of a solution containing 33 micromoles of the desired substrate, 1 ml of phosphate buffer of pH 7.0, and 0.2 ml of 10 % KOH in the center well of the Warburg vessel. In order to determine the cellular location of glutamic decarboxylase, the enzyme preparation was fractionated by centrifugation into three portions: material sedimented for 15 minutes at 10,000 x g, probably ntclear and mitochondrial (7), material sedimented for 15 minttes at 100,000 x g which is