Metabolism of proline in insect flight muscle and its significance in stimulating the oxidation of pyruvate

Energy consumption during insect flight and bioinspiration for MAV design: A review

The excellent biological characteristics of insects provide an important source of inspiration for designing micro air vehicles (MAVs). Insect flight is an incredibly complex and energy-intensive process. Unique insect flight muscles and contraction mechanisms enable flapping at high frequencies. Moreover, the metabolic rate during flight can reach hundreds of times the resting state. Understanding energy consumption during flight is crucial for designing efficient biomimetic aircraft. This paper summarizes the structures and contraction mechanisms of insect flight muscles, explores the underlying metabolic processes, and identifies methods for energy substrate identification and detection, and discusses inspiration for biomimetic MAV design. This paper reviews energy consumption during insect flight, promotes the understanding of insect bioenergetics, and applies this information to the design of MAVs.

Chromosome-level genome assembly and population genomic analyses provide insights into adaptive evolution of the red turpentine beetle, Dendroctonus valens

BACKGROUND

Biological invasions are responsible for substantial environmental and economic losses. The red turpentine beetle (RTB), Dendroctonus valens LeConte, is an important invasive bark beetle from North America that has caused substantial tree mortality in China. The lack of a high-quality reference genome seriously limits deciphering the extent to which genetic adaptions resulted in a secondary pest becoming so destructive in its invaded area.

RESULTS

Here, we present a 322.41 Mb chromosome-scale reference genome of RTB, of which 98% of assembled sequences are anchored onto fourteen linkage groups including the X chromosome with a N50 size of 24.36 Mb, which is significantly greater than other Coleoptera species. Repetitive sequences make up 45.22% of the genome, which is higher than four other Coleoptera species, i.e., Mountain pine beetle Dendroctonus ponderosae, red flour beetle Tribolium castaneum, blister beetle Hycleus cichorii, and Colorado potato beetle Leptinotarsa decemlineata. We identify rapidly expanded gene families and positively selected genes in RTB, which may be responsible for its rapid environmental adaptation. Population genetic structure of RTB was revealed by genome resequencing of geographic populations in native and invaded regions, suggesting substantial divergence of the North American population and illustrates the possible invasion and spread route in China. Selective sweep analysis highlighted the enhanced ability of Chinese populations in environmental adaptation.

CONCLUSIONS

Overall, our high-quality reference genome represents an important resource for genomics study of invasive bark beetles, which will facilitate the functional study and decipher mechanism underlying invasion success of RTB by integrating the Pinus tabuliformis genome.

Flexible Thermal Sensitivity of Mitochondrial Oxygen Consumption and Substrate Oxidation in Flying Insect Species

Mitochondria have been suggested to be paramount for temperature adaptation in insects. Considering the large range of environments colonized by this taxon, we hypothesized that species surviving large temperature changes would be those with the most flexible mitochondria. We thus investigated the responses of mitochondrial oxidative phosphorylation (OXPHOS) to temperature in three flying insects: the honeybee (Apis mellifera carnica), the fruit fly (Drosophila melanogaster) and the Colorado potato beetle (Leptinotarsa decemlineata). Specifically, we measured oxygen consumption in permeabilized flight muscles of these species at 6, 12, 18, 24, 30, 36, 42 and 45°C, sequentially using complex I substrates, proline, succinate, and glycerol-3-phosphate (G3P). Complex I respiration rates (CI-OXPHOS) were very sensitive to temperature in honeybees and fruit flies with high oxygen consumption at mid-range temperatures but a sharp decline at high temperatures. Proline oxidation triggers a major increase in respiration only in potato beetles, following the same pattern as CI-OXPHOS for honeybees and fruit flies. Moreover, both succinate and G3P oxidation allowed an important increase in respiration at high temperatures in honeybees and fruit flies (and to a lesser extent in potato beetles). However, when reaching 45°C, this G3P-induced respiration rate dropped dramatically in fruit flies. These results demonstrate that mitochondrial functions are more resilient to high temperatures in honeybees compared to fruit flies. They also indicate an important but species-specific mitochondrial flexibility for substrate oxidation to sustain high oxygen consumption levels at high temperatures and suggest previously unknown adaptive mechanisms of flying insects’ mitochondria to temperature.

Made for Each Other: Vector-Pathogen Interfaces in the Huanglongbing Pathosystem

Citrus greening, or huanglongbing (HLB), currently is the most destructive disease of citrus. HLB disease is putatively caused by the phloem-restricted α-proteobacterium 'Candidatus Liberibacter asiaticus'. This bacterium is transmitted primarily by the Asian citrus psyllid Diaphorina citri (Hemiptera: Liviidae). Most animal pathogens are considered pathogenic to their insect vectors, whereas the relationships between plant pathogens and their insect vectors are variable. Lately, the relationship of 'Ca. L. asiaticus' with its insect vector, D. citri, has been well investigated at the molecular, biochemical, and biological levels in many studies. Herein, the findings concerning this relationship are discussed and molecular features of the acquisition of 'Ca. L. asiaticus' from the plant host and its growth and circulation within D. citri, as well as its transmission to plants, are presented. In addition, the effects of 'Ca. L. asiaticus' on the energy metabolism (respiration, tricarboxylic acid cycle, and adenosine triphosphate production), metabolic pathways, immune system, endosymbionts, and detoxification enzymes of D. citri are discussed together with other impacts such as shorter lifespan, altered feeding behavior, and higher fecundity. Overall, although 'Ca. L. asiaticus' has significant negative effects on its insect vector, it increases its vector fitness, indicating that it develops a mutualistic relationship with its vector. This review will help in understanding the specific interactions between 'Ca. L. asiaticus' and its psyllid vector in order to design innovative management strategies.

Proline as a Sparker Metabolite of Oxidative Metabolism during the Flight of the Bumblebee, Bombus impatiens

Several insect species use the amino acid proline as a major energy substrate. Although initially thought to be limited to blood-feeding dipterans, studies have revealed this capability is more widespread. Recent work with isolated flight muscle showed that the bumblebee Bombus impatiens can oxidize proline at a high rate. However, its role as a metabolic fuel to power flight is unclear. To elucidate the extent to which proline is oxidized to power flight and how its contribution changes during flight, we profiled 14 metabolites central to energy and proline metabolism at key time points in flight muscle and abdominal tissues. Ultra-high performance liquid chromatography-electrospray ionization-quadrupole time of flight mass spectrometry (UPLC-ESI-QTOF MS) analysis revealed that proline is likely used as a sparker metabolite of the tricarboxylic acid cycle at the onset of flight, whereby it supplements the intermediates of the cycle. Carbohydrates are the major energy substrates, which is evidenced by marked decreases in abdominal glycogen stores and a lack of alanine accumulation to replenish flight muscle proline. The time course of fuel stores and metabolites changes during flight highlights homeostatic regulation of energy substrates and patterns of changes in metabolic intermediates within pathways. This study clarifies the role of proline and carbohydrate metabolism during flight in hymenopterans, such as B. impatiens.

Dramatic changes in mitochondrial substrate use at critically high temperatures: a comparative study using Drosophila

Ectotherm thermal tolerance is critical to species distribution, but at present the physiological underpinnings of heat tolerance remain poorly understood. Mitochondrial function is perturbed at critically high temperatures in some ectotherms, including insects, suggesting that heat tolerance of these animals is linked to failure of oxidative phosphorylation (OXPHOS) and/or ATP production. To test this hypothesis, we measured mitochondrial oxygen consumption rate in six Drosophila species with different heat tolerance using high-resolution respirometry. Using a substrate-uncoupler-inhibitor titration protocol, we examined specific steps of the electron transport system to study how temperatures below, bracketing and above organismal heat limits affect mitochondrial function and substrate oxidation. At benign temperatures (19 and 30°C), complex I-supported respiration (CI-OXPHOS) was the most significant contributor to maximal OXPHOS. At higher temperatures (34, 38, 42 and 46°C), CI-OXPHOS decreased considerably, ultimately to very low levels at 42 and 46°C. The enzymatic catalytic capacity of complex I was intact across all temperatures and accordingly the decreased CI-OXPHOS is unlikely to be caused directly by hyperthermic denaturation/inactivation of complex I. Despite the reduction in CI-OXPHOS, maximal OXPHOS capacity was maintained in all species, through oxidation of alternative substrates - proline, succinate and, particularly, glycerol-3-phosphate - suggesting important mitochondrial flexibility at temperatures exceeding the organismal heat limit. Interestingly, this failure of CI-OXPHOS and compensatory oxidation of alternative substrates occurred at temperatures that correlated with species heat tolerance, such that heat-tolerant species could defend 'normal' mitochondrial function at higher temperatures than sensitive species. Future studies should investigate why CI-OXPHOS is perturbed and how this potentially affects ATP production rates.

Cytochrome c Oxidase at Full Thrust: Regulation and Biological Consequences to Flying Insects

Flight dispersal represents a key aspect of the evolutionary and ecological success of insects, allowing escape from predators, mating, and colonization of new niches. The huge energy demand posed by flight activity is essentially met by oxidative phosphorylation (OXPHOS) in flight muscle mitochondria. In insects, mitochondrial ATP supply and oxidant production are regulated by several factors, including the energy demand exerted by changes in adenylate balance. Indeed, adenylate directly regulates OXPHOS by targeting both chemiosmotic ATP production and the activities of specific mitochondrial enzymes. In several organisms, cytochrome c oxidase (COX) is regulated at transcriptional, post-translational, and allosteric levels, impacting mitochondrial energy metabolism, and redox balance. This review will present the concepts on how COX function contributes to flying insect biology, focusing on the existing examples in the literature where its structure and activity are regulated not only by physiological and environmental factors but also how changes in its activity impacts insect biology. We also performed in silico sequence analyses and determined the structure models of three COX subunits (IV, VIa, and VIc) from different insect species to compare with mammalian orthologs. We observed that the sequences and structure models of COXIV, COXVIa, and COXVIc were quite similar to their mammalian counterparts. Remarkably, specific substitutions to phosphomimetic amino acids at critical phosphorylation sites emerge as hallmarks on insect COX sequences, suggesting a new regulatory mechanism of COX activity. Therefore, by providing a physiological and bioenergetic framework of COX regulation in such metabolically extreme models, we hope to expand the knowledge of this critical enzyme complex and the potential consequences for insect dispersal.

Diet-MEF2 interactions shape lipid droplet diversification in muscle to influence Drosophila lifespan

The number, size, and composition of lipid droplets can be influenced by dietary changes that shift energy substrate availability. This diversification of lipid droplets can promote metabolic flexibility and shape cellular stress responses in unique tissues with distinctive metabolic roles. Using Drosophila, we uncovered a role for myocyte enhancer factor 2 (MEF2) in modulating diet-dependent lipid droplet diversification within adult striated muscle, impacting mortality rates. Muscle-specific attenuation of MEF2, whose chronic activation maintains glucose and mitochondrial homeostasis, leads to the accumulation of large, cholesterol ester-enriched intramuscular lipid droplets in response to high calorie, carbohydrate-sufficient diets. The diet-dependent accumulation of these lipid droplets also correlates with both enhanced stress protection in muscle and increases in organismal lifespan. Furthermore, MEF2 attenuation releases an antagonistic regulation of cell cycle gene expression programs, and up-regulation of Cyclin E is required for diet- and MEF2-dependent diversification of intramuscular lipid droplets. The integration of MEF2-regulated gene expression networks with dietary responses thus plays a critical role in shaping muscle metabolism and function, further influencing organismal lifespan. Together, these results highlight a potential protective role for intramuscular lipid droplets during dietary adaptation.

Proline as a fuel for insect flight: enhancing carbohydrate oxidation in hymenopterans

Bees are thought to be strict users of carbohydrates as metabolic fuel for flight. Many insects, however, have the ability to oxidize the amino acid proline at a high rate, which is a unique feature of this group of animals. The presence of proline in the haemolymph of bees and in the nectar of plants led to the hypothesis that plants may produce proline as a metabolic reward for pollinators. We investigated flight muscle metabolism of hymenopteran species using high-resolution respirometry performed on permeabilized muscle fibres. The muscle fibres of the honeybee, Apis mellifera, do not have a detectable capacity to oxidize proline, as those from the migratory locust, Locusta migratoria, used here as an outgroup representative. The closely related bumblebee, Bombus impatiens, can oxidize proline alone and more than doubles its respiratory capacity when proline is combined with carbohydrate-derived substrates. A distant wasp species, Vespula vulgaris, exhibits the same metabolic phenotype as the bumblebee, suggesting that proline oxidation is common in hymenopterans. Using a combination of mitochondrial substrates and inhibitors, we further show that in B. impatiens, proline oxidation provides reducing equivalents and electrons directly to the electron transport system. Together, these findings demonstrate that some bee and wasp species can greatly enhance the oxidation of carbohydrates using proline as fuel for flight.

Dietary L-arginine accelerates pupation and promotes high protein levels but induces oxidative stress and reduces fecundity and life span in Drosophila melanogaster

L-Arginine, a precursor of many amino acids and of nitric oxide, plays multiple important roles in nutrient metabolism and regulation of physiological functions. In this study, the effects of L-arginine-enriched diets on selected physiological responses and metabolic processes were assessed in Drosophila melanogaster. Dietary L-arginine at concentrations 5-20 mM accelerated larval development and increased body mass, and total protein concentrations in third instar larvae, but did not affect these parameters when diets contained 100 mM arginine. Young (2 days old) adult flies of both sexes reared on food supplemented with 20 and 100 mM L-arginine possessed higher total protein concentrations and lower glucose and triacylglycerol concentrations than controls. Additionally, flies fed 20 mM L-arginine had higher proline and uric acid concentrations. L-Arginine concentration in the diet also affected oxidative stress intensity in adult flies. Food with 20 mM L-arginine promoted lower protein thiol concentrations and higher catalase activity in flies of both sexes and higher concentrations of low molecular mass thiols in males. When flies were fed on a diet with 100 mM L-arginine, lower catalase activities and concentrations of protein thiols were found in both sexes as well as lower low molecular mass thiols in females. L-Arginine-fed males demonstrated higher climbing activity, whereas females showed higher cold tolerance and lower fecundity, compared with controls. Food containing 20 mM L-arginine shortened life span in both males and females. The results suggest that dietary L-arginine shows certain beneficial effects at the larval stage and in young adults. However, the long-term consumption of L-arginine-enriched food had unfavorable effects on D. melanogaster due to decreasing fecundity and life span.

Remembering Components of Food in Drosophila

Remembering features of past feeding experience can refine foraging and food choice. Insects can learn to associate sensory cues with components of food, such as sugars, amino acids, water, salt, alcohol, toxins and pathogens. In the fruit fly Drosophila some food components activate unique subsets of dopaminergic neurons (DANs) that innervate distinct functional zones on the mushroom bodies (MBs). This architecture suggests that the overall dopaminergic neuron population could provide a potential cellular substrate through which the fly might learn to value a variety of food components. In addition, such an arrangement predicts that individual component memories reside in unique locations. DANs are also critical for food memory consolidation and deprivation-state dependent motivational control of the expression of food-relevant memories. Here, we review our current knowledge of how nutrient-specific memories are formed, consolidated and specifically retrieved in insects, with a particular emphasis on Drosophila.

The Drosophila estrogen-related receptor directs a metabolic switch that supports developmental growth

Metabolism must be coordinated with development to provide the appropriate energetic needs for each stage in the life cycle. Little is known, however, about how this temporal control is achieved. Here, we show that the Drosophila ortholog of the estrogen-related receptor (ERR) family of nuclear receptors directs a critical metabolic transition during development. dERR mutants die as larvae with low ATP levels and elevated levels of circulating sugars. The expression of active dERR protein in mid-embryogenesis triggers a coordinate switch in gene expression that drives a metabolic program normally associated with proliferating cells, supporting the dramatic growth that occurs during larval development. This study shows that dERR plays a central role in carbohydrate metabolism, demonstrates that a proliferative metabolic program is used in normal developmental growth, and provides a molecular context to understand the close association between mammalian ERR family members and cancer.

Identification of two arginases generated by alternative splicing in the silkworm, Bombyx mori

Arginase (EC 3.5.3.1) catalyzes the hydrolysis of arginine to ornithine and urea. Here, we have cloned two arginase cDNAs from the silkworm, Bombyx mori. The analysis of exon/intron structures showed that the two mRNAs named bmarg-r and bmarg-f were generated from a single gene by alternative usage of exons. The bmarg-r and bmarg-f were predicted to encode almost the same amino acid sequences, except that the latter had additional ten N-terminal residues. Recombinant bmARG-r and bmARG-f in Escherichia coli cell lysates were roughly similar to each other in enzymatic characteristics, which did not show large difference from those of arginases assayed by using tissue extracts. Differential RT-PCR experiments and tissue distribution analyses of arginase activity indicated that the bmarg-r gene is expressed in the male reproductive organs, especially in the glandula lacteola and vesicular seminalis, from which it is secreted to the seminal fluid and transferred to the female during copulation, whereas the bmarg-f gene is expressed in the larval and adult nonreproductive organs including the fat body and muscle, where the produced arginase proteins are considered to stay in the cells. Thus, the two silkworm arginase isoforms may have a difference in whether or not the product is excreted out of the cells in which it is synthesized.

Virulent Hessian fly larvae manipulate the free amino acid content of host wheat plants

Gall-forming insects induce host plants to form specialized structures (galls) that provide immature life stages of the insect access to host plant nutrients and protection from natural enemies. Feeding by larvae of the Hessian fly (Mayetiola destructor Say) causes susceptible host wheat plants to produce a gall-like nutritive tissue that supports larval growth and development. To determine if changes in host plant free amino acid levels are associated with virulent Biotype L Hessian fly larval feeding, we quantified free amino acid levels in crown tissues of susceptible Newton wheat plants 1, 4, and 7 days after Hessian fly egg hatch. Hessian fly-infested susceptible plants were more responsive than resistant plants or uninfested controls, showing higher concentrations of alanine, glutamic acid, glycine, phenylalanine, proline, and serine 4 days after egg hatch. This 4-day post-hatch time point corresponds to the maturation of nutritive tissue cells in susceptible plants and the onset of rapid larval growth. By 7 days after egg hatch, when virulent second instars are actively feeding on the contents of nutritive tissue cells, the aromatic amino acids phenylalanine and tyrosine were more abundant compared to uninfested controls, but the levels of other free amino acids were no longer elevated. Changes in free amino acid abundance described in this report were associated with increased levels of mRNA encoded by wheat genes involved in amino acid synthesis and transport.

A glia-neuron alanine/ammonium shuttle is central to energy metabolism in bee retina

It has been proposed that glial cells may supply carbon fuel to neurons and also that there are fluxes of ammonium from neurons to glia. We have investigated both these proposals in Apis retinal slices, in which virtually all the mitochondria are in the photoreceptor neurons. Normally the superfusate contained no substrate of energy metabolism; addition of glucose or alanine did not increase oxygen consumption (QO2), confirming that the neurons received adequate substrate from glycogen in the glia. 1,4-Dideoxy-1,4-imino-D-arabinitol (DAB, 100 microm), an inhibitor of glycogen phosphorylase, progressively decreased QO2. This decrease was reversed by alanine but not glucose. Ammonium-sensitive microelectrodes did not detect significant extracellular [NH(4)(+)] ([NH(4)(+)](e)) in slices superfused with normal superfusate. Removal of Cl(-), necessary for cotransport of NH(4)(+) into the glia, increased [NH(4)(+)](e) so that at the end of 2 min photostimulation mean [NH(4)(+)](e) was 0.442 mM (S.E.M. = 0.082 mM, n = 16). In 0 Cl(-), [NH(4)(+)](e) was reduced by 2-(methylamino)isobutyrate (MeAIB) an inhibitor of alanine transport. MeAIB also blocked oxidation of alanine in the presence of DAB, but did not decrease QO2 in normal superfusate. Lactate (l and d) and pyruvate (but not glucose) increased QO2 in DAB and decreased [NH(4)(+)](e) in 0 Cl(-). These results strengthen the evidence that in superfused retinal slices, glucose is metabolized exclusively in the glia, which supply alanine to the neurons, and that ammonium returns to the glia. They also show that another fuel (perhaps lactate) can be supplied by the glia to the neurons.

Energy metabolism in orchid bee flight muscles: carbohydrate fuels all

The widely accepted idea that bees fuel flight through the oxidation of carbohydrate is based on studies of only a few species. We tested this hypothesis as part of our research program to investigate the size-dependence of flight energetics in Panamanian orchid bees. We succeeded in measuring rates of O(2) consumption and CO(2) production in vivo during hovering flight, as well as maximal activities (V(max) values) in vitro of key enzymes in flight muscle energy metabolism in nine species belonging to four genera. Respiratory quotients (ratios of rates of CO(2) production to O(2) consumption) in all nine species are close to 1.0. This indicates that carbohydrate is the main fuel used for flight. Trehalase, glycogen phosphorylase and hexokinase activities are sufficient to account for the glycolytic flux rates estimated from rates of CO(2) production. High activities of other glycolytic enzymes, as well as high activities of mitochondrial oxidative enzymes, are consistent with the estimated rates of carbohydrate-fueled oxidative metabolism. In contrast, hydroxyacylCoA dehydrogenase, an enzyme involved in fatty acid oxidation, was not detectable in any species. Thoracic homogenates displayed ADP-stimulated oxidition of pyruvate + proline, but did not oxidize palmitoyl l-carnitine + proline as substrates. A metabolic map, based on data reported herein and information from the literature, is presented. The evidence available supports the hypothesis that carbohydrate serves as the main fuel for flight in bees.

Age-associated decline in mitochondrial respiration and electron transport in Drosophila melanogaster

The principal objective of the present study was to identify specific alterations in mitochondrial respiratory functions during the aging process. Respiration rates and the activities of electron transport chain complexes were measured at various ages in mitochondria isolated from thoraces of the fruit fly, Drosophila melanogaster, which consist primarily of flight muscles. The rates of state 3 respiration (ADP-stimulated), RCRs (respiratory control ratios) and uncoupled respiration rates decreased significantly as a function of age, using either NAD+- or FAD-linked substrates; however, there were no differences in state 4 respiration (ADP-depleted) rates. There was also a significant age-related decline in the activity of cytochrome c oxidase (complex IV), but not of the other mitochondrial oxidoreductases examined. Exposure of mitochondria isolated from young flies to low doses of KCN or NaAz (sodium azide), complex IV inhibitors, decreased cytochrome c oxidase activity and increased the production of H2O2. Collectively, these results support the hypothesis that impairment of mitochondrial respiration may be a causal factor in the aging process, and that such impairment may result from and contribute to increased H2O2 production in vivo.

Superoxide and hydrogen peroxide production by Drosophila mitochondria

Drosophila melanogaster is a key model organism for genetic investigation of the role of free radicals in aging, but biochemical understanding is lacking. Superoxide production by Drosophila mitochondria was measured fluorometrically as hydrogen peroxide, using its dependence on substrates, inhibitors, and added superoxide dismutase to determine sites of production and their topology. Glycerol 3-phosphate dehydrogenase and center o of complex III in the presence of antimycin had the greatest maximum capacities to generate superoxide on the cytosolic side of the inner membrane. Complex I had significant capacity on the matrix side. Center i of complex III, cytochrome c, and complex IV produced no superoxide. Native superoxide generation by isolated mitochondria was also measured without added inhibitors. There was a high rate of superoxide production with sn-glycerol 3-phosphate as substrate; two-thirds mostly from glycerol 3-phosphate dehydrogenase on the cytosolic side and one-third on the matrix side from complex I following reverse electron transport. There was little superoxide production from any site with NADH-linked substrate. Superoxide production by complex I following reverse electron flow from glycerol 3-phosphate was particularly sensitive to membrane potential, decreasing 70% when potential decreased 10 mV, showing that mild uncoupling lowers superoxide production in the matrix very effectively.

Proline can be utilized as an energy substrate during flight of Aedes aegypti females

In order to determine whether proline can be utilized as fuel during flight of Aedes aegypti, proline, alanine, and glutamine concentrations were monitored at 0, 30 and 60 min after flight using sugar-fed males and females, and blood meal-fed females. In sugar-fed and blood meal-fed females, flight lead to a significant decrease in proline and a significant increase in glutamine concentration in both hemolymph and thorax. Only during flight after a blood meal was a significant increase in the alanine concentration observed in hemolymph. After flight, the proline alanine and glutamine levels in the hemolymph and thorax from males did not change significantly. In addition, activities of enzymes related to amino acid metabolism were assayed in homogenates of cephalothorax and thorax from both sexes, and in fat body and midgut from females. In both sexes, the activities of all the enzymes studied were significantly higher in thorax than in cephalothorax. The levels of the enzymes involved in proline oxidation were higher in thorax than in fat body and midgut. These results suggest that proline can be used as an energy substrate for flight muscle of Ae. aegypti females. However, the elevation in glutamine levels observed in hemolymph and thorax after flight has not been reported in other insects that fuel flight using proline and may suggest an additional mechanism for shuttling ammonia between flight muscle and fat body is present in mosquitoes.

Cold tolerance and proline metabolic gene expression in Drosophila melanogaster

Treatment of Drosophila melanogaster adults with an inhibitor of protein synthesis led to a decrease in intrinsic cold-shock tolerance, but no difference in the rapid cold hardening response, which is apparent only if a period at 4 degrees C precedes the cold stress. Increases in energy reserves, including proline, were found in lines of flies selected for resistance to chilling injury. Since an increase in proline levels has been associated with overwintering in insects, and for salt and cold tolerance in plants, an RNase protection assay was developed to assess changes in transcript abundance for two genes encoding enzymes important for proline metabolism, pyrroline 5-carboxylate reductase and proline oxidase. The mRNA levels did not change in response to low temperature, but the high level of pyrroline 5-carboxylate reductase transcript is consistent with the interpretation that a large proline pool is important for Drosophila metabolism and survival during cold stress.

Amino acids and osmolarity in honeybee drone haemolymph

In the haemolymph of honeybee drones, concentrations of free amino acids were higher than in worker haemolymph, with different relative proportions of individual amino acids. The overall concentration of free amino acids reached its highest level at the 5th day after adult drone emergence, and after the 9th day only minor changes in the concentration and distribution of free amino acids were observed. This coincides with the age when drones reach sexual maturity and change their feeding behaviour. Levels of essential free amino acids were high during the first 3 days of life and thereafter decreased. Osmolarity was lowest at emergence (334 +/- 42 mOsm), increased until the age of 3 days (423 +/- 32 mOsm) and then stayed generally constant until the 16th day of life. Only 25-day-old drones had significantly higher osmolarity (532 +/- 38 mOsm). The overall change in osmolarity during a drone's lifetime was about 40%.

Free amino acids in the hemolymph of the cockroach, Blaberus discoidalis

The concentrations of free amino acids in the hemolymph of adult males of the cockroach Blaberus discoidalis were determined by automated amino acid analysis. Eighteen of the twenty standard amino acids as well as two nonstandard amino acids (hydroxyproline and ornithine) were found. Aspartic acid and cysteine were not present. Proline levels were up to 100-fold higher than those of any other amino acid. Total amino acid concentration was comparable to those reported for most other cockroaches. The reported amino acid profile formed the basis for an organ culture medium which sustained metabolic viability of isolated B. discoidalis neuroendocrine glands and brain-gland complexes for at least 24 hr.

Protein phosphorylation on serine, threonine, and tyrosine residues modulates membrane-protein interactions and transcriptional regulation in Salmonella typhimurium

There exists a plethora of tyrosine kinases that play essential roles in regulation of eukaryotic proteins. Several dual specificity kinases that phosphorylate proteins on threonine, serine, and tyrosine residues also play critical roles in eukaryotic phosphorylation cascades. In contrast, very few prokaryotic proteins have been shown to be phosphorylated on tyrosine residues, and the functions of the rare examples remain obscure. Furthermore, no dual specificity kinases have been described in prokaryotes. Our results indicate that PutA protein from the bacterium Salmonella typhimurium autophosphorylates on several threonine, serine, and tyrosine residues. PutA protein both represses the proline utilization (put) operon and degrades proline to glutamate. These two opposing functions are regulated by the availability of proline and the membrane sites needed for the proline dehydrogenase activity of PutA protein. In addition, these functions are modulated by phosphorylation of PutA protein. The rate of dephosphorylation of PutA protein is determined by the availability of proline and membranes. Dephosphorylated PutA protein has a higher DNA binding affinity than the phosphorylated protein and thus may prevent toxic overexpression of PutA protein in the absence of available membrane sites.

Hydrogen peroxide release by mitochondria increases during aging

The effect of aging on the release of H2O2 by mitochondria was studied in the housefly in order to elucidate the causes of previously observed age-related increase in the level of oxidative stress. Intact flight muscle mitochondria of the housefly, supplemented with alpha-glycerophosphate, produce 1-2 nmol H2O2/min per mg protein, even in the absence of respiratory inhibitors. The rate of H2O2 secretion progressively increases approximately 2-fold during aging of the fly. Neither uncoupling of oxidative phosphorylation nor mechanical damage to mitochondria during the isolation procedure appear to be responsible for the age-related increase in H2O2 production. Activities of NADH-ferricyanide reductase, succinate-ubiquinone reductase, and NADH-, succinate- and alpha-glycerophosphate-cytochrome c reductases, were approximately 2-fold higher in mitochondria from the old than those from the young flies. However, the concentration of enzymatically reducible ubiquinone remained unchanged with age. Infliction of damage by exposure of mitochondria to free radical-generating systems in vitro caused an increase in the rate of H2O2 generation. Glutaraldehyde, an intermolecular crosslinking agent, induced an increase in the rate of H2O2 generation by mitochondria. Results of this study demonstrate that aging in the housefly is associated with an increase in the rate of H2O2 generation by mitochondria probably due, at least in part, to self-inflicted damage by mitochondria. Intermolecular cross-linking in the inner mitochondrial membrane can contribute towards the increased H2O2 generation.

Catalytic transfer of hydride ions from NADPH to oxygen by the interconversions of proline and delta 1-pyrroline-5-carboxylate

Studies in reconstituted systems indicate that the interconversions of proline and delta 1-pyrroline-5-carboxylate can constitute a shuttle which transfers reducing equivalents into mitochondria as proline and oxidizing potential out as delta 1-pyrroline-5-carboxylate. The studies reported here determine if the transfer of reducing equivalents can be stoichiometrically greater than the utilization of shuttle intermediates. First, 3HOH production from [5-3H]proline was used to quantitate proline oxidation under conditions where delta 1-pyrroline-5-carboxylate could recycle back to proline in a system containing mitochondrial particles and pyrroline-5-carboxylate reductase. In parallel incubations the metabolic fate of [U-14C]proline was determined. 3HOH production continued to increase while no net change occurred in 14C-labeled proline and delta 1-pyrroline-5-carboxylate indicating catalytic activity in this system. In another system that contained intact mitochondria and pyrroline-5-carboxylate reductase, proline oxidation was quantitated by both [U-14C]proline recoveries and 3HOH production from [5-3H]proline. Proline oxidation was the same by both methods in incubations lacking NADPH and pyrroline-5-carboxylate reductase. However, it was significantly underestimated by [U-14C]proline recoveries as compared to 3HOH production in incubations containing all shuttle components. These results indicate catalytic functioning of the proposed shuttle in a reconstituted system containing intact mitochondria. This shuttle may function at specific times to catalytically generate cytosolic NADP+ and in turn regulate enzymes limited by [NADP+]. We suggest that the proposed shuttle may function to increase ribose-5-phosphate synthesis by the oxidative limb of the pentose phosphate pathway and inturn increase PP-ribose-P and purine synthesis during the initiation of cell growth.

Regulation of pyruvate oxidation in blowfly flight muscle mitochondria: requirement for ADP

Blowfly (Phormia regina) flight muscle mitochondria oxidized pyruvate ( + proline) in the presence of either ADP (coupled respiration) or carbonylcyanide-p-trifluoromethoxyphenylhydrazone (FCCP-uncoupled respiration). There was an absolute requirement for ADP (Km = 8.0 microM) when pyruvate oxidation was stimulated by FCCP in the presence of oligomycin. This requirement for ADP was limited to the oxidation of pyruvate; uncoupled alpha-glycerolphosphate oxidation proceeded maximally even in the absence of added ADP. Atractylate inhibited uncoupled pyruvate oxidation whether added before (greater than 99%) or after (95%) initiation of respiration with FCCP. In the presence of FCCP, oligomycin, and limiting concentrations of ADP (less than 110 microM), there was a shutoff in the uptake of oxygen. This inhibition of respiration was completely reversed by the addition of more ADP. Plots of net oxygen uptake as a function of the limiting ADP concentration were linear; the observed ADP/O ratio was 0.22 +/- 0.025. An ADP/O ratio of 0.2 was predicted if phosphorylation occurred only at the succinyl-CoA synthetase step of the tricarboxylate cycle. Experiments performed in the presence of limiting concentrations of ADP, and designed to monitor changes in the mitochondrial content of ADP and ATP, demonstrated that the shutoff in oxygen uptake was not due to the presence of a high intramitochondrial concentration of ATP. Indeed, ATP, added to the medium prior to the addition of FCCP, inhibited uncoupled pyruvate oxidation; the apparent KI was 0.8 mM. These results are consistent with the hypothesis that it is the intramitochondrial ATP/ADP ratio that is one of the controlling factors in determining the rate of flux through the tricarboxylate cycle. Changes in the mitochondrial content of citrate, isocitrate, alpha-ketoglutarate, and malate during uncoupled pyruvate oxidation in the presence of a limiting concentration of ADP were consistent with the hypothesis that the mitochondrial NAD + -linked isocitric dehydrogenase is a major site for such control through the tricarboxylate cycle.

Transfer of reducing equivalents into mitochondria by the interconversions of proline and delta 1-pyrroline-5-carboxylate

Direct evidence is presented for a proline cycle using a cell-free experimental system which sequentially transfers 3H from [1-3H]glucose to NADP+ to delta 1-pyrroline-5-carboxylate and yields [3H]proline. The formation of [3H]proline depends on the presence of NADP, delta 1-pyrroline-5-carboxylate, and the enzymes glucose-6-phosphate dehydrogenase and delta 1-pyrroline-5-carboxylate reductase. The production of [3H]proline from unlabeled proline in the presence of mitochondria provides direct evidence for one complete turn of a proline cycle which transfers reducing equivalents produced by glucose oxidation in the pentose pathway into mitochondria. In this cycle, proline is oxidized to delta 1-pyrroline-5-carboxylate by mitochondrial proline oxidase. delta 1-pyrroline-5-carboxylate is released from mitochondria and is recycled back to proline by delta 1-pyrroline-5-carboxylate reductase with concomitant oxidation of NADPH. At the maximal rate observed, 60% of delta 1-pyrroline-5-carboxylate produced is recycled back to proline. This cycle provides a mechanism for transferring reducing equivalents from NADPH into mitochondria and is linked to glucose oxidation in the pentose pathway by NADPH turnover.

The distribution of pyrroline carboxylate reductase and proline oxidase in the larva of the blowfly, Aldrichina grahami

P5C reductase and proline oxidase in the larva of the blowfly, Aldrichina grahami, were found to be localized mainly in the fat body mitochondrial matrix and the muscle, respectively.

Proline synthesis from glutamate in the mitochondria isolated from a blowfly, Aldrichina grahami

Proline biosynthesis from glutamate was demonstrated in a cell free system prepared from blowfly abdomen. The biosynthetic activity was found mainly in the mitochondrial fraction. The biosynthesis of proline from glutamate required ATP, NADPH and Mg++ as cofactors.