Studies on citrate and malate metabolism in Lycopersicon esculentum

• Main Author: Jeffery, David
• Published: University of Bath 1985
• Subjects: 611; Tomato storage
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.353243

The specific activities of citrate synthase and malate dehydrogenase extracted from mature green fruit of Lycopersicon esculentum, fell 60% during the first two weeks of a twelve week experiment in which the fruit were stored in an atmosphere designed to inhibit ethylene synthesis. Throughout the remainder of the experiment, the specific activities were relatively constant. In the initial two week period, the specific activity of NADP-linked malic enzyme rose by 400%, malic acid concentration fell by 50%, while the concentration of citric acid rose by 20%. Those features of ripening such as the de novo synthesis of lycopene and polygalacturonase, which were thought to depend on ethylene for initiation of response, could not be detected until the fruit were removed to a normal atmosphere. Additionally, citrate synthase and malate dehydrogenase from mature green tomato fruit stored in the presence or absence of ethylene, showed similar trends in specific activity, and the presence of the olefin made no significant difference to the rate of loss of enzyme specific activity. The purification and partial characterisation of citrate synthase from Lycopersicon esculentum is described. The enzyme is a dimer with sub-units of similar size and a total Mr of approximately. 100,000. The characterisation revealed no obvious regulatory features that would easily account for the fall in specific activity. Sub-cellular fractionation studies demonstrated unequivocally that the site of organic acid metabolism was the mitochondrion. Citrate synthase, NAD-dependent isocitrate dehydrogenase and NAD-dependent malic enzyme were shown to be located exclusively in the mitochondrion, while malate dehydrogenase was located both in the cytosol and the mitochondrion. All these enzymes including cytosolic malate dehydrogenase exhibited the co-ordinated fall in specific activity described above. A hypothesis is proposed which includes a novel coarse control of the citric acid cycle and related enzymes, as an early indicator of senescence.