Metabolic Processes in Cytoplasmic Particles of the Avocado Fruit. III. The Operation of the Tricarboxylic Acid Cycle.

The evidence for the operation of the tricarboxylic acid cycle in the metabolism of plant mitochondria has been based mostly on studies of oxidative activity. The oxidation of diand tricarboxylic acids of the cycle was demonstrated in a variety of tissues, such as etiolated seedlings of mung bean (22), pea (10, 31), Avena (32), and Black Valentine bean (4), lupin cotyledons (7), castor-bean endosperm (5, 33), pea internodes (26), cauliflower (15, 16, 17) and broccoli (18) buds, Arum spadix (13, 14), tubers of white potato (28) and of sweet potato (2, 19), avocado fruit (6, 23), spinach leaves (24) and green as well as etiolated pea seedlings and leaves (29, 30). In several of these studies the oxidation rates of acids other than citrate, a-ketoglutarate, succinate or malate were also followed (5, 10, 22, 29, 30, 32). The metabolism of pyruvate by the mediation of the cycle was the subject of investigations dealing with young and rapidly growing tissues (4, 5, 7, 10, 22, 32), and to a lesser extent with mature and stable material (2, 6, 19). Oxidative studies alone do not furnish sufficient proof for the cycling nature and the sequence of the individual reactions. A better insight may be gained by studying the formation of reaction products along with oxidations. This was done only to a limited extent with plant materials. Davies (10) adopted this procedure for a particulate fraction from pea hypocotyl. James and Elliott (14) also cited some chromatographic evidence for mitochondria from Arum spadix. The objective of this study was to investigate the operation of the cycle in avocado mitochondria by following rates of oxidation of acids and formation of products in the presence and absence of inhibitors.