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Estimation of the fructose diphosphatase-phosphofructokinase substrate cycle in the flight muscle of Bombus affinis.

ABSTRACT: 1. Substrate cycling of fructose 6-phosphate through reactions catalysed by phosphofructokinase and fructose diphosphatase was estimated in bumble-bee (Bombus affinis) flight muscle in vivo. 2. Estimations of substrate cycling of fructose 6-phosphate and of glycolysis were made from the equilibrium value of the (3)H/(14)C ratio in glucose 6-phosphate as well as the rate of (3)H release to water after the metabolism of [5-(3)H,U-(14)C]glucose. 3. In flight, the metabolism of glucose proceeded exclusively through glycolysis (20.4mumol/min per g fresh wt.) and there was no evidence for substrate cycling. 4. In the resting bumble-bee exposed to low temperatures (5 degrees C), the pattern of glucose metabolism in the flight muscle was altered so that substrate cycling was high (10.4mumol/min per g fresh wt.) and glycolysis was decreased (5.8mumol/min per g fresh wt.). 5. The rate of substrate cycling in the resting bumble-bee flight muscle was inversely related to the ambient temperature, since at 27 degrees , 21 degrees and 5 degrees C the rates of substrate cycling were 0, 0.48 and 10.4mumol/min per g fresh wt. respectively. 6. Calcium ions inhibited fructose diphosphatase of the bumble-bee flight muscle at concentrations that were without effect on phosphofructokinase. The inhibition was reversed by the presence of a Ca(2+)-chelating compound. It is proposed that the rate of fructose 6-phosphate substrate cycling could be regulated by changes in the sarcoplasmic Ca(2+) concentration associated with the contractile process.

SUBMITTER: Clark MG

PROVIDER: S-EPMC1177847 | biostudies-other | 1973 Jun

REPOSITORIES: biostudies-other

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Project description:The only exogenous substrates oxidized by mitochondria isolated from the flight muscle of the Japanese beetle (Popillia japonica) are proline, pyruvate and glycerol 3-phosphate. The highest rate of oxygen consumption is obtained with proline. The oxidation of proline leads to the production of more NH3 than alanine, indicating a functioning glutamate dehydrogenase (EC 1.4.1.2). Studies of mitochondrial extracts confirm the presence of a very active glutamate dehydrogenase, and this enzyme is found to be activated by ADP and inhibited by ATP. These extracts also show high alanine aminotransferase activity (EC 2.6.1.2) and a uniquely active "malic' enzyme (EC 1.1.1.39). The "malic' enzyme is activated by succinate and inhibited by ATP and by pyruvate. It is suggested that the input of tricarboxylate-cycle intermediate from proline oxidation is balanced by the formation of pyruvate from malate, and the complete oxidation of the majority of the pyruvate. Studies of the steady-state concentrations of mitochondrial CoASH and CoA thioesters during proline oxidation show a high succinyl (3-carboxypropionyl)-CoA content which falls on activating respiration with ADP. There is a concomitant rise in CoASH. However, the reverse transition, from state-3 to state-4 respiration, causes only very slight changes in acylation. The reasons for this are discussed. Studies of the mitochondrial content of glutamate, 2-oxoglutarate, malate, pyruvate, citrate and isocitrate during the same phases of proline oxidation give results consistent with control at the level of glutamate dehydrogenase and isocitrate dehydrogenase during proline oxidation, with the possibility of further control at "malic' enzyme. During the oxidation of pyruvate all of the tricarboxylate-cycle intermediates and NAD(P)H follow the pattern of changes described in the blowfly (Johnson & Hansford, 1975; Hansford, 1974) and isocitrate dehydrogenase is identified as the primary site of control.?2OAuthor

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Estimation of the fructose diphosphatase-phosphofructokinase substrate cycle in the flight muscle of Bombus affinis.

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