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Metabolic Processes in Cytoplasmic Particles of the Avocado Fruit. IX. The Oxidation of Pyruvate and Malate during the Climacteric Cycle ^1,2

Department of Botany and Plant Biochemistry, University of California, Los Angeles, California 90024

Mitochondria isolated from preclimacteric avocado fruit oxidize pyruvate at a much lower rate than those separated from climacteric fruit. The external addition of thiamine pyrophosphate (TPP) increased the rate of pyruvate oxidation in both cases.

The study of the influence of TPP on the rate of oxidation of malate by mitochondria obtained from both preclimacteric and climacteric fruit indicated that the effect of this cofactor could be understood by assuming that malate was converted to pyruvate. TPP stimulation of malate oxidation was prevented by arsenite, an inhibitor of keto acid oxidation. The addition of glutamate increased the rate of malate oxidation through the transamination of oxaloacetate. This suggests that the rate of oxidation of malate is highly dependent upon mechanisms which remove oxaloacetate efficiently.

Incubation of mitochondria from preclimacteric fruit with malate-U-¹⁴C resulted in the labeling of oxaloacetate and the accumulation of labeled pyruvate. Addition of TPP to this system induced the rapid formation of citrate. This conversion was completely inhibited by arsenite.

The results indicate that the ability to carry out the oxidative decarboxylation of -ketoacids improves as the ripening process progresses. The idea was advanced that TPP available to the mitochondria plays an important controlling role.

⁴ Present address: Glasshouse Crops Research Institute, Littlehampton, Sussex, England.

¹ Preliminary reports of this work have appeared (10,16).

² This investigation was supported in part by Public Health Service Research Grant GM-08224 from the National Institute of General Medical Sciences, by the Cancer Research Coordinating Committee of the University of California, and bv a Kellogg Foundation Fellowship to one of us (G.E.H.).