Journal of the Japanese Society for Horticultural Science
Online ISSN : 1880-358X
Print ISSN : 0013-7626
ISSN-L : 0013-7626
Studies on the Metabolism of Free Amino Acids during Maturation and Ripening of Tomato Fruits
IV. On the Respiratory Regulation Associated with Glu-OAA-transamination in Tomato Mitochondria during the Development of the Climacteric
Hiroyuki YAMANAKAKazuo CHACHINKuniyasu OGATAShozo KOBAYASHI
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1974 Volume 43 Issue 2 Pages 199-206

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Abstract

In this study, the respiratory regulation in tomato mitochondria was investigated in relation to some systems for the removal of Oxaloacetate (OAA).
Mitochondria were extracted from outer locular wall tissue of tomato fruit. The wall tissues were grated gently in an isolation medium with a stainless-steel grater, and then mitochondria suspension were prepared according to a method of KU et al. The oxidative activity in the isolated mitochondria was measured polarographically using oxygen electrode (Yanagimoto-PO 100). Respiratory control ratios (state 3/ state 4) during the oxidation of malate and succinate in mitochondria were 2.5 to 3.0. The oxidative activity of mitochondria with succinate as substrate was high at mature green stage and decreased during ripening. On the other hand, O2-uptake of whole fruit was low at mature green stage and increased from mature green to breaker and then showed a peak at breaker or light pink. From this, it is supposed that the activity of mitochondria in whole fruit may be suppressed by some inhibitory mechanisms at mature green stage. The inhibition of succinate and malate oxidation by the addition of OAA was observed in mitochondria isolated from tomato fruit. The OAA content in tomato fruit was high at mature green stage and decreased during ripening of fruits. It seems that the oxidative activity of mitochondria in intact fruit may be controlled by the level of OAA.
Glutamate-OAA transaminase (GOT) activity and the influence of thiamine pyrophosphate (TPP) on the oxidation of malate in mitochondria and citrate synthase activity were determined as the factors for the removal of OAA.
OAA inhibition of succinate and malate oxidation in mitochondria isolated from tomatoes was relieved by the addition of glutamic acid throughout ripening period, and in this reaction aspartic acid was produced. It is assumed that a transamination reaction between glutamate and OAA gave rise to aspartate and α-ketoglutarate and removed the inhibitory effect of OAA. There would be a action of GOT in tomato mitochondria. TPP is an absolute requirement for the oxidative decarboxylation of pyruvate to acetyl-CoA. The addition of TPP to mitochondria in the presence of pyruvate resulted in a considerable acceleration of the rate of oxidation of malate. This result suggests that TPP may be the limiting factor for the oxidation of both malate and pyruvate. It was found that the effect of TPP in the presence of pyruvate on the oxidation of malate in tomato mitochondria was high at mature green stage and decreased during ripening. Citrate synthase was high at mature green stage and gradually decreased during ripening.
It is possible to conclude from the results above that the low O2-uptake of tomato fruits at mature green stage may be due to OAA inhibition of mitochondria oxidation, and that the activity of GOT, the effect of TPP in the presence of pyruvate and citrate synthase serving to accelerate the removal of OAA would result in the decrease of OAA level in the whole fruits and the increase of O2-uptake of the whole fruits at the stage of mature green to breaker. The increased glutamate content in tomatoes during ripening may also play a role in the removal of OAA through GOT.

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