The activities of proline dehydrogenase, glutamate dehydrogenase, aspartate-oxoglutarate aminotransferase and alanine-oxoglutarate aminotransferase in some insect flight muscles.
TL;DR: The flavoprotein-linked enzyme proline dehydrogenase is the first enzyme of the proline oxidation pathway and therefore its activity should be limiting for the operation of this pathway (see Krebs, 1964), and to this end its activity has been measured in the flight muscles of various insects.
Abstract: Many insects contain a high concentration of proline in the haemolymph (see Wyatt, 1961) and, as this amino acid had been shown to be oxidized by homogenates of flight muscle from the housefly (Sacktor, 1955), it was suggested that proline oxidation could provide energy for insect flight (Sacktor, 1961). In the initial period of flight in the tsetse fly Bursell (1963) observed a decrease in the thoracic content of proline and an increase in that of alanine. Further work by Bursell (1965, 1966, 1967) indicated that proline is converted into alanine via oxidation to glutamate, transamination of the latter with pyruvate to form alanine and oxoglutarate (catalysed by alanine-oxoglutarate aminotransferase, EC 2.6.1.2) and the conversion of this oxoglutarate into pyruvate via the latter portion of the tricarboxylic acid cycle followed by decarboxylation of oxaloacetate. Despite the importance of this pathway in the tsetse fly and possibly other insects (see Kirsten, Kirsten & Arese, 1963; Sacktor & WormserShavit, 1966), little work has been carried out on the enzymes of this metabolic pathway. One possible reason for this is the small amount of muscle available from the one insect in which the pathway appears to be of quantitative importance, namely the tsetse fly. The flavoprotein-linked enzyme proline dehydrogenase, which catalyses the oxidation of proline to Al -pyrroline-5-carboxylate (see Brosemer & Veerabhadrappa, 1965; Sacktor & Childress, 1967), is the first enzyme of the proline oxidation pathway and therefore its activity should be limiting for the operation of this pathway (see Krebs, 1964). Thus the maximum activity of this enzyme should provide some indication of the quantitative importance of proline oxidation for the provision of energy in muscle, and to this end its activity has been measured in the flight muscles of various insects. The activities of glutamate dehydrogenase (EC 1.4.1.2), aspartateoxoglutarate aminotransferase (EC 2.6.1.1) and alanine-oxoglutarate aminotransferase (EC 2.6.1.2) have also been measured and are reported in this
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TL;DR: The enzyme activities from insect flight muscle confirm and extend much of the earlier work and indicate the type of fuel that can support insect flight and the maximum activity of hexokinase was found to be higher in red than in white vertebrate muscle.
Abstract: 1. The maximum activities of hexokinase, phosphorylase and phosphofructokinase have been measured in extracts from a variety of muscles and they have been used to estimate the maximum rates of operation of glycolysis in muscle. These estimated rates of glycolysis are compared with those calculated for the intact muscle from such information as oxygen uptake, glycogen degradation and lactate formation. Reasonable agreement between these determinations is observed, and this suggests that such enzyme activity measurements may provide a useful method for comparative investigations into quantitative aspects of maximum glycolytic flux in muscle. 2. The enzyme activities from insect flight muscle confirm and extend much of the earlier work and indicate the type of fuel that can support insect flight. The maximum activity of hexokinase in some insect flight muscles is about tenfold higher than that in vertebrate muscles. The activity of phosphorylase is greater, in general, in vertebrate muscle (particularly white muscle) than in insect flight muscle. This is probably related to the role of glycogen breakdown in vertebrate muscle (particularly white muscle) for the provision of ATP from anaerobic glycolysis and not from complete oxidation of the glucose residues. The activity of hexokinase was found to be higher in red than in white vertebrate muscle, thus confirming and extending earlier reports. 3. The maximum activity of the mitochondrial glycerophosphate dehydrogenase was always much lower than that of the cytoplasmic enzyme, indicating that the former enzyme is rate-limiting for the glycerol 3-phosphate cycle. From the maximum activity of the mitochondrial enzyme it can be calculated that the operation of this cycle would account for the reoxidation of all the glycolytically produced NADH in insect flight muscle but it could account for only a small amount in vertebrate muscle. Other mechanisms for this NADH reoxidation in vertebrate muscle are discussed briefly.
359 citations
TL;DR: This chapter focuses on the various adaptations that make possible the high metabolic rates necessary for flight, which are achieved by a complicated meshwork of neural, hormonal, and biochemical mechanisms.
Abstract: Publisher Summary Actively flying insects achieve the highest metabolic rates known, and they do so in the fraction of a second required to shift from quiescence to flight. This chapter focuses on the various adaptations that make possible the high metabolic rates necessary for flight. Flight depends on the biochemical and mechanical work done by the flight muscles, which must be continually supplied with oxygen and fuel. The work of the muscles is under neural control and therefore the metabolic rate is also under neural control. Hormones participate as part of the biochemical mechanisms by which the neural commands are executed and also as part of the internal milieu supportive of flight. In larger insects, high metabolic rates and the associated heat production result in elevated body temperatures; temperature effects and temperature regulation are thus closely related to flight. Control of flight metabolism is accomplished by a complicated meshwork of neural, hormonal, and biochemical mechanisms. Control of the rate of metabolism is primarily neural, since the frequency of excitation of the flight muscles provides the primary control on the demand for metabolic performance. The metabolic machinery of the muscle is also influenced by these neural signals, since Ca ++ released by excitation modulates enzyme activity. Hormones that control the uptake of substrates and perhaps other biochemical reactions in the muscles also influence muscle metabolism.
220 citations
TL;DR: An insect preference test found that honeybees indeed prefer nectars rich in the amino acid proline, demonstrating that plants often produce proline-rich floral nectar.
Abstract: Plants offer metabolically rich floral nectar to attract visiting pollinators. The composition of nectar includes not only sugars, but also amino acids. We have examined the amino acid content of the nectar of ornamental tobacco and found that it is extremely rich (2 mM) in proline. Because insect pollinators preferentially utilize proline during the initial phases of insect flight and can reportedly taste proline, we determined whether honeybees showed a preference for synthetic nectars rich in proline. We therefore established an insect preference test and found that honeybees indeed prefer nectars rich in the amino acid proline. To determine whether this was a general phenomenon, we also examined the nectars of two insect-pollinated wild perennial species of soybean. These species also showed high levels of proline in their nectars demonstrating that plants often produce proline-rich floral nectar. Because insects such as honeybees prefer proline-rich nectars, we hypothesize that some plants offer proline-rich nectars as a mechanism to attract visiting pollinators.
175 citations
Cites background from "The activities of proline dehydroge..."
...A number of investigations have found that proline is specifically oxidized in insect flight muscle, especially during the first 30 s of insect flight (Balboni 1978; Brosemer and Veerabhadrappa 1965; Crabtree and Newsholme 1970; Njagi et al. 1992)....
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...So, what is the consequence of high levels of proline in nectar? A number of investigations have found that proline is specifically oxidized in insect flight muscle, especially during the first 30 s of insect flight (Balboni 1978; Brosemer and Veerabhadrappa 1965; Crabtree and Newsholme 1970; Njagi et al. 1992)....
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TL;DR: Insect flight is the most energy-demanding activity of animals, with the nervous system and neurohormones controlling the performance and energy metabolism of muscles, and of the fat body, ensuring that the muscles and nerves are supplied with essential fuels throughout flight.
Abstract: Insect flight is the most energy-demanding activity of animals. It requires the coordination and cooperation of many tissues, with the nervous system and neurohormones controlling the performance and energy metabolism of muscles, and of the fat body, ensuring that the muscles and nerves are supplied with essential fuels throughout flight. Muscle metabolism can be based on several different fuels, the proportions of which vary according to the insect species and the stage in flight activity. Octopamine, which acts as neurotransmitter, neuromodulator or neurohormone in insects, has a central role in flight. It is present in brain, ventral ganglia and nerves, supplying peripheral tissues such as the flight muscles, and its concentration in hemolymph increases during flight. Octopamine has multiple effects during flight in coordinating and stimulating muscle contraction and also energy metabolism partly by activating phosphofructokinase via the glycolytic activator, fructose 2,6-bisphosphate. One important muscle fuel is trehalose, synthesized by the fat body from a variety of precursors, a process that is regulated by neuropeptide hormones. Other fuels for flight include proline, glycerol and ketone bodies. The roles of these and possible regulation in some insect species are discussed.
142 citations
01 Jan 1981
TL;DR: Hemolymph amino acids in general, and proline and glutamate in particular, could serve as important sources of substrate for the Krebs cycle.
Abstract: Amino acids have long been recognized to be prominent constituents of the hemolymph and tissue fluids of insects, accounting for more than 30% of the total osmotic activity in advanced orders (Sutcliffe, 1963). Details of the amino acid pattern have been elucidated for many species, and the results have been widely reviewed (Florkin, 1958; Wyatt, 1961; Chen, 1966; Schoffeniels and Gilles, 1970; Jeuniaux, 1971), to reveal a situation of bewildering, but seemingly controlled (Collett, 1976a), complexity. The only feature that stands out as a general, though by no means universal, characteristic of insect hemolymph is the predominance of proline and/ or of glutamate and its amide glutamine (see also more recent investigations of Barrett, 1974; Nurmi and Birt, 1974; Bailey, 1975; Barrett and Friend, 1975; Collett, 1976b; de Kort and Kramer, 1976.) The early work of Winteringham with radioactively labeled substrates indicated a high rate of turnover in amino acid pools (Winteringham and Harrison, 1956; Winteringham, 1958) and suggested that hemolymph amino acids in general, and proline and glutamate in particular, could serve as important sources of substrate for the Krebs cycle (see also reviews by Sacktor, 1961, 1974). Support for this view was provided by later work, which showed a significant depletion of proline during activity in a number of different insects (Bursell, 1963; Corrigan and Kearns, 1963; Kirsten et al, 1963; Ray, 1964; Sacktor and Wormser-Shavit, 1966; Barker and Lehner, 1972; Brouwers and de Kort, 1979). Since then, proline metabolism has been under intensive investigation, and its importance in energy metabolism has become firmly established.
125 citations
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TL;DR: The discovery of a muscular dystrophy in a strain of mice has facilitated the search for biochemical alterations in myopathy and it seems probable that many, at least, of the secondary biochemical changes may be common to various types of muscle disease.
Abstract: Little is known about biochemical abnormalities which accompany or are responsible for the morphological changes which occur in the primary myopathies. A number of such diseases have been characterized in humans (Walton & Nattrass, 1954). The discovery (Michelson, Russell & Harman, 1955) of a muscular dystrophy in a strain of mice, inherited by an autosomal recessive gene, has facilitated the search for biochemical alterations in myopathy. Although this condition may not be identical with any of the types of human muscular dystrophy, its investigation may help to throw light on the biochemistry ofhuman muscular dystrophies. It seems probable that many, at least, of the secondary biochemical changes may be common to various types of muscle disease. A few workers (e.g. Weinstock, Epstein & Milhorat, 1958; Hazzard & Leonard, 1959; White, 1959) have reported altered concentrations of certain muscle enzymes in the mouse myopathy. In view of reported morphological abnormalities in mitochondria of dystrophic mouse muscle (Dr G. W. Pearce, unpublished work; Ross, Pappas & Har-
980 citations
TL;DR: Three sites of regulation—the phosphorylation of fructose 6-phosphate, the cleavage of trehalose, and the phosphorolysis of glycogen—were established in this system in vivo to determine the steps that control glycolysis in vivo, in the transition from a tissue at rest to one performing strenuous work.
201 citations
TL;DR: The amounts of carbohydrate which are synthesized vary within very wide limits—between almost nil and perhaps 200 g per day in the case of the human adult—and this will bring me to the question of how the rate of gluconeogenesis is regulated and adjusted to changing needs.
Abstract: I am using the term gluconeogenesis in this lecture to denote any new formation of carbohydrate from non-carbohydrates. These non-carbohydrates include amino acids, as well as the lactate continuously produced in the body, e.g. in blood cells and in the exercised muscles. When lactate is the precursor of carbohydrate the formation of glucose from it constitutes a re-formation rather than a new formation as the lactate has been derived from glucose. But as the enzymic mechanisms of glucose formation from lactate and from amino acids are essentially the same, it is reasonable to treat them jointly. Gluconeogenesis is a biosynthetic process of major importance. I intend to review first some aspects of the physiological role of gluconeogenesis. This will lead to the fact that the amounts of carbohydrate which are synthesized vary within very wide limits—between almost nil and perhaps 200 g per day in the case of the human adult—and this will bring me to the main subject: the question of how the rate of gluconeogenesis is regulated and adjusted to changing needs.
181 citations
TL;DR: The activity of glycerol kinase was measured in a variety of muscles from vertebrates and invertebrates in an attempt to explain the large variation in the activity of this enzyme in different muscles.
Abstract: 1. Glycerol kinase (EC 2.7.1.30) activity was measured in crude extracts of skeletal muscles by a radiochemical method. The properties of the enzyme from a number of different muscles are very similar to those of the enzyme from rat liver. Glycerol kinase from locust flight muscle was inhibited competitively by l-3-glycerophosphate with a Ki of 4·0×10−4m. 2. The activity of glycerol kinase was measured in a variety of muscles from vertebrates and invertebrates in an attempt to explain the large variation in the activity of this enzyme in different muscles. 3. In vertebrates glycerol kinase activities were generally higher in red muscle than in white muscle; the highest activities (approx. 0·2μmole/min./g. fresh wt.) were found in the red breast muscle of some birds (e.g. pigeon, duck, blue tit) whereas the activities in the white breast muscle of the pheasant and domestic fowl were very low (approx. 0·02μmole/min./g.). 4. On the basis of glycerol kinase activities, muscles from insects can be classified into three groups: muscles that have a low enzyme activity, i.e. 1·5μmoles/min./g. (e.g. bees, wasps, some blowflies). 5. The function of glycerol kinase in vertebrate and insect muscles that possess a low or intermediate activity is considered to be the removal of glycerol that is produced from lipolysis of triglyceride or diglyceride by the muscle. Therefore in these muscles the activity of glycerol kinase is related to the metabolism of fat, which is used to support sustained muscular activity. A possible regulatory role of glycerol kinase in the initiation of triglyceride or diglyceride lipolysis is discussed. 6. The function of glycerol kinase in the insect muscles that possess a high activity of the enzyme is considered to be related to the high rates of glycolysis that these muscles can perform. The oxidation of extramitochondrial NADH, and therefore the maintenance of glycolysis, is dependent on the functioning of the glycerophosphate cycle; if at any stage of flight (e.g. at the start) the rate of mitochondrial oxidation of l-3-glycerophosphate was less than the activity of the extramitochondrial glycerophosphate dehydrogenase, this compound would accumulate, inhibit the latter enzyme and inhibit glycolysis. It is suggested that such excessive accumulation of l-3-glycerophosphate is prevented by hydrolysis of this compound to glycerol; the latter would have to be removed from the muscle when the accumulation of l-3-glycerophosphate had stopped, and this would explain the presence of glycerol kinase in these muscles and its inhibition by l-3-glycerophosphate.
116 citations
61 citations