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Effects of calcium ions and adenosine diphosphate on the activities of NAD+-linked isocitrate dehydrogenase from the radular muscles of the whelk and flight muscles of insects.

ABSTRACT: 1. The activity of NAD+-linked isocitrate dehydrogenase from the radular muscle of the whelk is higher than those in many vertebrate muscles and only slightly lower than in the flight muscles of insects. The enzyme activity from the whelk (Buccinum undatum) is stable for several hours after homogenization of the radular muscle, whereas that from insect flight muscle is very unstable. Consequently, the enzyme from the whelk muscle is suitable for a systematic investigation of the effects of Ca2+ and ADP. 2. The sigmoid response of the enzyme activity to isocitrate concentration is markedly increased by raising the Ca2+ concentration from 0.001 to 10 muM, but it is decreased by ADP. The inhibitory effect of Ca2+ is most pronounced at pH7.1; it is not observed at pH 6.5. Similar effects are observed for the enzyme from the flight muscle of the locust (Schistocerca gregaria) and the water bug (Lethocerus cordofanus). The percentage activation by ADP of the enzyme from either the whelk or the insects is greater at 10 muM-Ca2+, and 50% of the maximum activation is obtained at 0.10 and 0.16 mM-ADP for the enzyme from whelk and locust respectively at this Ca2+ concentration. At 10 muM-Ca2+ in the absence of added ADP, the apparent Km for isocitrate is markedly higher than in other conditions. Ca2+ concentrations of 0.01, 0.1 and 0.2 muM cause 50% inhibition of maximum activity of the enzyme from the muscles of the whelk, locust and water bug respectively. 3. Recent work has indicated that mitochondria may play a complementary role to the sarcoplasmic reticulum in the control of the distribution of Ca2+ in muscle. The opposite effects of Ca2+ on the activities of isocitrate dehydrogenase and mitochondrial glycerol phosphate dehydrogenase from muscle tissue are consistent with the hypothesis that changes in the intracellular distribution of Ca2+ control the activities of these two enzymes in order to stimulate energy production for the contraction process in the muscle. Although both enzymes are mitochondrial, glycerol phosphate dehydrogenase resides on the outer surface of the inner membrane and responds to sarcoplasmic changes in Ca2+ concentration (i.e. an increase during contraction), whereas the isocitrate dehydrogenase resides in the matrix of the mitochondria and responds to intramitochondrial concentrations of Ca2+ (i.e. a decrease during contraction). It is suggested that changes in intramitochondrial Ca2+ concentrations are primarily responsible for regulation of the activity of NAD+-isocitrate dehydrogenase in order to control energy formation for the contractile process. However, when the muscle is at rest, changes in intramitochondrial concentrations of ADP may regulate energy formation for non-contractile processes.

SUBMITTER: Zammit VA

PROVIDER: S-EPMC1172770 | biostudies-other | 1976 Mar

REPOSITORIES: biostudies-other

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Project description:1. The activities of citrate synthase, NAD+-linked and NADP+-linked isocitrate dehydrogenase were measured in muscles from a large number of animals, in order to provide some indication of the importance of the citric acid cycle in these muscles. According to the differences in enzyme activities, the muscles can be divided into three classes. First, in a number of both vertebrate and invertebrate muscles, the activities of all three enzymes are very low. It is suggested that either the muscles use energy at a very low rate or they rely largely on anaerobic glycolysis for higher rates of energy formation. Second, most insect flight muscles contain high activities of citrate synthase and NAD+-linked isocitrate dehydrogenase, but the activities of the NADP+-linked enzyme are very low. The high activities indicate the dependence of insect flight on energy generated via the citric acid cycle. The flight muscles of the beetles investigated contain high activities of both isocitrate dehydrogenases. Third, other muscles of both vertebrates and invertebrates contain high activities of citrate synthase and NADP+-liniked isocitrate dehydrogenase. Many, if not all, of these muscles are capable of sustained periods of mechanical activity (e.g. heart muscle, pectoral muscles of some birds). Consequently, to support this activity fuel must be supplied continually to the muscle via the circulatory system which, in most animals, also transports oxygen so that energy can be generated by complete oxidation of the fuel. It is suggested that the low activities of NAD+-linked isocitrate dehydrogenase in these muscles may be involved in oxidation of isocitrate in the cycle when the muscles are at rest. 2. A comparison of the maximal activities of the enzymes with the maximal flux through the cycle suggests that, in insect flight muscle, NAD+-linked isocitrate dehydrogenase catalyses a non-equilibrium reaction and citrate synthease catalyses a near-equilibrium reaction. In other muscles, the enzyme-activity data suggest that both citrate synthase and the isocitrate dehydrogenase reactions are near-equilibrium.

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Effects of calcium ions and adenosine diphosphate on the activities of NAD+-linked isocitrate dehydrogenase from the radular muscles of the whelk and flight muscles of insects.

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