The anaerobic oyster heart: Coupling of glucose and aspartate fermentation

Mitochondrial responses to constant and cyclic hypoxia depend on the oxidized fuel in a hypoxia-tolerant marine bivalve Crassostrea gigas

Sessile benthic organisms like oysters inhabit the intertidal zone, subject to alternating hypoxia and reoxygenation (H/R) episodes during tidal movements, impacting respiratory chain activities and metabolome compositions. We investigated the effects of constant severe hypoxia (90 min at ~ 0% O2 ) followed by 10 min reoxygenation, and cyclic hypoxia (5 cycles of 15 min at ~ 0% O2 and 10 min reoxygenation) on isolated mitochondria from the gill and the digestive gland of Crassostrea gigas respiring on pyruvate, palmitate, or succinate. Constant hypoxia suppressed oxidative phosphorylation (OXPHOS), particularly during Complex I-linked substrates oxidation. It had no effect on mitochondrial reactive oxygen species (ROS) efflux but increased fractional electron leak (FEL). In mitochondria oxidizing Complex I substrates, exposure to cyclic hypoxia prompted a significant drop after the first H/R cycle. In contrast, succinate-driven respiration only showed significant decline after the third to fifth H/R cycle. ROS efflux saw little change during cyclic hypoxia regardless of the oxidized substrate, but Complex I-driven FEL tended to increase with each subsequent H/R cycle. These observations suggest that succinate may serve as a beneficial stress fuel under H/R conditions, aiding in the post-hypoxic recovery of oysters by reducing oxidative stress and facilitating rapid ATP re-synthesis. The impacts of constant and cyclic hypoxia of similar duration on mitochondrial respiration and oxidative lesions in the proteins were comparable indicating that the mitochondrial damage is mostly determined by the lack of oxygen and mitochondrial depolarization. The ROS efflux in the mitochondria of oysters was minimally affected by oxygen fluctuations indicating that tight regulation of ROS production may contribute to robust mitochondrial phenotype of oysters and protect against H/R induced stress.

NMR Metabolite Profiles of the Bivalve Mollusc Mytilus galloprovincialis Before and After Immune Stimulation With Vibrio splendidus

The hemolymph metabolome of Mytilus galloprovincialis injected with live Vibrio splendidus bacteria was analyzed by ¹H-NMR spectrometry. Changes in spectral hemolymph profiles were already detected after mussel acclimation (3 days at 18 or 25 °C). A significant decrease of succinic acid was accompanied by an increase of most free amino acids, mytilitol, and, to a smaller degree, osmolytes. These metabolic changes are consistent with effective osmoregulation, and the restart of aerobic respiration after the functional anaerobiosis occurred during transport. The injection of Vibrio splendidus in mussels acclimated at 18°C caused a significant decrease of several amino acids, sugars, and unassigned chemical species, more pronounced at 24 than at 12 h postinjection. Correlation heatmaps indicated dynamic metabolic adjustments and the relevance of protein turnover in maintaining the homeostasis during the response to stressful stimuli. This study confirms NMR-based metabolomics as a feasible analytical approach complementary to other omics techniques in the investigation of the functional mussel responses to environmental challenges.

Biochemical adaptations of the stout razor clam (Tagelus plebeius) to changes in oxygen availability: resilience in a changing world?

Climate change is producing sea level rise and deoxygenation of the ocean, altering estuaries and coastal areas. Changes in oxygen availability are expected to have consequences on the physiological fitness of intertidal species. In this work we analyze the coping response of the intertidal stout razor clam (Tagelus plebeius (Lightfoot, 1786)) to extreme environmental changes in oxygen concentration. Their biochemical responses to normoxia, hypoxia, and hyperoxia transition at different intertidal level (low–high) were measured through an in situ transplant experiment. The high intertidal level negatively affected the analyzed traits of the T. plebeius populations. The differences in reactive oxygen species production, total oxyradical scavenger capacities, and catalase activity also suggested more stressful conditions at the high level where long-term hypoxia periods occur. Both hypoxia and re-oxygenation provoked re-adjustments in the antioxidant responses and higher lipid oxidative damage (normoxia < hypoxia < re-oxygenation). The observed responses in transplanted clams at the opposite intertidal level suggested the potential acclimation of T. plebeius to cope with new environmental conditions. These findings are discussed within a global changing context where both increasing deoxygenation conditions and sea level rise are predicted to be exacerbated in the area driven by climate change.

Surviving anoxia: the maintenance of energy production and tissue integrity during anoxia and reoxygenation

The development of anoxia within tissues represents a significant challenge to most animals because of the decreased capacity for aerobic ATP production, the associated loss of essential cellular functions and the potential for detrimental tissue oxidation upon reoxygenation. Despite these challenges, there are many animals from multiple phyla that routinely experience anoxia and can fully recover. In this Review, we integrate knowledge gained from studies of anoxia-tolerant species across many animal taxa. We primarily focus on strategies used to reduce energy requirements, minimize the consequences of anaerobic ATP production and reduce the adverse effects of reactive oxygen species, which are responsible for tissue damage with reoxygenation. We aim to identify common strategies, as well as novel solutions, to the challenges of anoxia exposure. This Review chronologically examines the challenges faced by animals as they enter anoxia, as they attempt to maintain physiological function during prolonged anoxic exposure and, finally, as they emerge from anoxia. The capacity of animals to survive anoxia is also considered in relation to the increasing prevalence of anoxic zones within marine and freshwater environments, and the need to understand what limits survival.

Inducing the Alternative Oxidase Forms Part of the Molecular Strategy of Anoxic Survival in Freshwater Bivalves

Hypoxia in freshwater ecosystems is spreading as a consequence of global change, including pollution and eutrophication. In the Patagonian Andes, a decline in precipitation causes reduced lake water volumes and stagnant conditions that limit oxygen transport and exacerbate hypoxia below the upper mixed layer. We analyzed the molecular and biochemical response of the North Patagonian bivalve Diplodon chilensis after 10 days of experimental anoxia (<0.2 mg O₂/L), hypoxia (2 mg O₂/L), and normoxia (9 mg O₂/L). Specifically, we investigated the expression of an alternative oxidase (AOX) pathway assumed to shortcut the regular mitochondrial electron transport system (ETS) during metabolic rate depression (MRD) in hypoxia-tolerant invertebrates. Whereas, the AOX system was strongly upregulated during anoxia in gills, ETS activities and energy mobilization decreased [less transcription of glycogen phosphorylase (GlyP) and succinate dehydrogenase (SDH) in gills and mantle]. Accumulation of succinate and induction of malate dehydrogenase (MDH) activity could indicate activation of anaerobic mitochondrial pathways to support anoxic survival in D. chilensis. Oxidative stress [protein carbonylation, glutathione peroxidase (GPx) expression] and apoptotic intensity (caspase 3/7 activity) decreased, whereas an unfolded protein response (HSP90) was induced under anoxia. This is the first clear evidence of the concerted regulation of the AOX and ETS genes in a hypoxia-tolerant freshwater bivalve and yet another example that exposure to hypoxia and anoxia is not necessarily accompanied by oxidative stress in hypoxia-tolerant mollusks.

1H NMR-based metabolomics investigation on the effects of petrochemical contamination in posterior adductor muscles of caged mussel Mytilus galloprovincialis

Environmental metabolomics is a high-throughout approach that provides a snapshot of the metabolic status of an organism. In order to elucidate the biological effects of petrochemical contamination on aquatic invertebrates, mussels Mytilus galloprovincialis were caged at the "Augusta-Melilli-Priolo" petrochemical area and Brucoli (Sicily, south Italy), chosen as the reference site. After confirming the elevated concentrations of polycyclic aromatic hydrocarbons (PAHs) and mercury (Hg) in Augusta sediments in our previous work (Maisano et al., 2016a), herein an environmental metabolomics approach based on protonic nuclear magnetic resonance (¹H NMR), coupled with chemometrics, was applied on the mussel posterior adductor muscle (PAM), the main muscular system in bivalve molluscs. Amino acids, osmolytes, energy storage compounds, tricarboxylic acid cycle intermediates, and nucleotides, were found in PAM NMR spectra. Principal Component Analysis (PCA) indicated that mussels caged at the polluted site clustered separately from mussels from the control area, suggesting a clear differentiation between their metabolic profiles. Specifically, disorders in energy metabolism, alterations in amino acids metabolism, and disturbance in the osmoregulatory processes were observed in mussel PAM. Overall, findings from this work demonstrated the usefulness of applying an active biomonitoring strategy for environmental risk assessment, and the effectiveness of metabolomics in elucidating changes in metabolic pathways of aquatic organisms caged at sites differentially contaminated, and thus its suitability to be applied in ecotoxicological studies.

Intertidal oysters reach their physiological limit in a future high-CO2 world

Sessile marine molluscs living in the intertidal zone experience periods of internal acidosis when exposed to air (emersion) during low tide. Relative to other marine organisms, molluscs have been identified as vulnerable to future ocean acidification; however, paradoxically it has also been shown that molluscs exposed to high CO2 environments are more resilient compared with those molluscs naive to CO2 exposure. Two competing hypotheses were tested using a novel experimental design incorporating tidal simulations to predict the future intertidal limit of oysters in a high-CO2 world; either high-shore oysters will be more tolerant of elevated PCO2 because of their regular acidosis, or elevated PCO2 will cause high-shore oysters to reach their limit. Sydney rock oysters, Saccostrea glomerata, were collected from the high-intertidal and subtidal areas of the shore and exposed in an orthogonal design to either an intertidal or a subtidal treatment at ambient or elevated PCO2, and physiological variables were measured. The combined treatment of tidal emersion and elevated PCO2 interacted synergistically to reduce the haemolymph pH (pHe) of oysters, and increase the PCO2 in the haemolymph (Pe,CO2) and standard metabolic rate. Oysters in the intertidal treatment also had lower condition and growth. Oysters showed a high degree of plasticity, and little evidence was found that intertidal oysters were more resilient than subtidal oysters. It is concluded that in a high-CO2 world the upper vertical limit of oyster distribution on the shore may be reduced. These results suggest that previous studies on intertidal organisms that lacked tidal simulations may have underestimated the effects of elevated PCO2.

Metabolomic investigations of American oysters using H-NMR spectroscopy

The Eastern oyster (Crassostrea virginica) is a useful, robust model marine organism for tissue metabolism studies. Its relatively few organs are easily delineated and there is sufficient understanding of their functions based on classical assays to support interpretation of advanced spectroscopic approaches. Here we apply high-resolution proton nuclear magnetic resonance ((1)H NMR)-based metabolomic analysis to C. virginica to investigate the differences in the metabolic profile of different organ groups, and magnetic resonance imaging (MRI) to non-invasively identify the well separated organs. Metabolites were identified in perchloric acid extracts of three portions of the oyster containing: (1) adductor muscle, (2) stomach and digestive gland, and (3) mantle and gills. Osmolytes dominated the metabolome in all three organ blocks with decreasing concentration as follows: betaine > taurine > proline > glycine > ß-alanine > hypotaurine. Mitochondrial metabolism appeared most pronounced in the adductor muscle with elevated levels of carnitine facilitating ß-oxidation, and ATP, and phosphoarginine synthesis, while glycogen was elevated in the mantle/gills and stomach/digestive gland. A biochemical schematic is presented that relates metabolites to biochemical pathways correlated with physiological organ functions. This study identifies metabolites and corresponding (1)H NMR peak assignments for future NMR-based metabolomic studies in oysters.

Occurrence and functions of free D-aspartate and its metabolizing enzymes

D-Aspartate is one of a few D-amino acids that attracted attention at an early date, since it was detected in various tissues of mammals as a protein component. The occurrence of free D-aspartate in nature was recognized a little later, and raised questions about its physiological functions and metabolism. This amino acid has been gradually accepted, based on various experimental observations, to be a physiological substrate of D-aspartate oxidase, whose role had been considered enigmatic since its early discovery in the 1940s. Mammalian enzymes that serve to liberate D-aspartyl residue in proteins have been identified. One enzyme hydrolyzes peptide bond at the amino side of D-aspartyl residue in a dipeptide and another enzyme hydrolyzes that at the carbonyl side of the residue in proteins. The first pyridoxal 5'-phosphate-dependent aspartate racemase has been purified and cloned from a bivalve species. The enzyme supports the high contents of D-aspartate comparable to those of L-aspartate in the bivalve, and the enantiomers are consumed when hypoxia is imposed on the bivalve. In some yeast species, assimilation of D-aspartate has been found to depend on inducible D-aspartate oxidase, which also serves to detoxify acidic D-amino acids.

Extracellular and Intracellular Acid‐Base Status with Regard to the Energy Metabolism in the OysterCrassostrea gigasduring Exposure to Air

The acid-base status of extra- and intracellular fluids was studied in relation to the anaerobic energy metabolism in the adductor muscle, mantle, gills, and heart of the marine bivalve Crassostrea gigas after exposure to air for periods of 2, 4, 8, 12, 24, and 48 h. Such exposure was found to cause a significant reduction in the pH in the hemolymph (pH(e)) within the first 4 h. The decrease in the pHe was accompanied by elevated Pco2 values, causing [HCO3-] to rise (respiratory acidosis). Thereafter, the pHe fell at a lower rate, and this fall was partially compensated for by a further increase in [HCO3-] in the hemolymph. The increase in the [Ca] levels in the hemolymph indicates a mobilization of Ca2+ from CaCO3 and the involvement of bicarbonates in the buffering of pHe. The main anaerobic end-products that accumulated in the tissues during the first stages of anaerobiosis were alanine and succinate, at a ratio of about 2 : 1. Later on, propionate and acetate were also accumulated at significant rates. In contrast to the adductor muscle, gills, and mantle, opine production in the heart was significant after 12-24 h of exposure to air. Determination of intracellular pH (pHi) revealed that there is a close relationship between the rate of anaerobic end-product accumulation and the extent of intracellular acidosis in the adductor muscle, mantle, and gills. On the contrary, accumulation of anaerobic end-products in the heart did not cause any significant change in its pHi. The intracellular nonbicarbonate, nonphosphate buffering value (beta (NB,NPi)) was determined to be higher in the heart than in the other three tissues and thus probably plays a crucial role in stabilizing heart pHi during exposure to air.

PETER HOCHACHKA: Adventures in Biochemical Adaptation

Peter Hochachka was one of the most creative forces in the field of comparative physiology during the past half-century. His career was truly an exploratory adventure, in both intellectual and geographic senses. His broad comparative studies of metabolism in organisms as diverse as trout, tunas, oysters, squid, turtles, locusts, hummingbirds, seals, and humans revealed the adaptable features of enzymes and metabolic pathways that provide the biochemical bases for diverse lifestyles and environments. In its combined breadth and depth, no other corpus of work better illustrates the principle of "unity in diversity" that marks comparative physiology. Through his publications, his stimulating mentorship, his broad editorial services, and his continuous-and highly infectious-enthusiasm for his field, Peter Hochachka served as one of the most influential leaders in the transformation of comparative physiology.

Exposure to air, but not seawater, increases the glutamine content and the glutamine synthetase activity in the marsh clamPolymesoda expansa

Polymesoda expansa spends a considerable portion of its life exposed to air in mangrove swamps where salinity fluctuates greatly. Thus, the aim of this study was to evaluate the effects of aerial exposure (transfer from 10‰ brackish water directly to air) or salinity changes (transfer from 10‰ brackish water directly to 30‰ seawater) on nitrogen metabolism in P. expansa. We concluded that P. expansa is non-ureogenic because carbamoyl phosphate (CPS) III activity was undetectable in the adductor muscle, foot muscle, hepatopancreas and mantle when exposed to brackish water (control), seawater or air for 17 days. It is ammonotelic as it excretes nitrogenous wastes mainly as ammonia in brackish water or seawater. After transfer to seawater for 17 days, the contents of total free amino acids(TFAA) in the adductor muscle, foot muscle, hepatopancreas and mantle increased significantly. This could be related to an increase in protein degradation because exposure to seawater led to a greater rate of ammonia excretion on days 15 and 17, despite unchanged tissue ammonia contents. Alanine was the major free amino acid (FAA) in P. expansa. The contribution of alanine to the TFAA pool in various tissues increased from 43–48% in brackish water to 62–73% in seawater. In contrast, in clams exposed to air for 17 days there were no changes in alanine content in any of the tissues studied. Thus, the functional role of alanine in P. expansa is mainly connected with intracellular osmoregulation. Although 8.5–16.1% of the TFAA pool of P. expansa was attributable to glutamine, the glutamine contents in the adductor muscle, foot muscle,hepatopancreas and mantle were unaffected by 17 days of exposure to seawater. However, after exposure to air for 17 days, there were significant increases in ammonia content in all these tissues in P. expansa, accompanied by significant increases in glutamine content (2.9-, 2.5-, 4.5- and 3.4-fold,respectively). Simultaneously, there were significant increases in glutamine synthetase activities in the adductor muscle (1.56-fold) and hepatopancreas(3.8-fold). This is the first report on the accumulation of glutamine associated with an upregulation of glutamine synthetase in a bivalve species in response to aerial exposure, and these results reveal that the evolution of glutamine synthesis as a means for detoxification of ammonia first occurred among invertebrates.

Adventures in oxygen metabolism

Peter W. Hochachka led a grand life of science adventure and left as his legacy a whole new field--biochemical adaptation. Oxygen was at the core of Peter's career and his laboratory made major contributions to our understanding of how animals deal with variation in oxygen availability in many forms. He analyzed the molecular mechanisms that support facultative anaerobiosis, studied muscle exercise metabolism for high speed flight, swimming and running, investigated mammalian diving on many trips to the Antarctic to study Weddell seals, and probed the metabolic and genetic adaptations that provide optimal hypoxia tolerance for humans residing at high altitudes. His work illuminated both biochemical and physiological mechanisms that are used to optimize aerobic metabolism, to compensate for hypoxic insults, and to conserve energy by strong metabolic rate depression under anoxia. His articles, books and lectures galvanized the field with leading-edge insights and theories and he consistently challenged comparative biochemists to use their unique model systems to explore the range and breadth of animal strategies of biochemical adaptation. Lessons drawn from my training in Peter's laboratory have led me on continuing explorations of adaptations in enzyme function, signal transduction, gene expression, and antioxidant defenses ranging over systems of anoxia tolerance, freezing survival, estivation, and mammalian hibernation.

Nucleotides modulate the activity of aspartate racemase of Scapharca broughtonii

The activity of D-aspartate racemase purified from Scapharca broughtonii has been found to depend markedly on some nucleotides. Purine nucleoside monophosphates enhanced the enzyme activity, which was, on the contrary, lowered by purine nucleoside triphosphates and not affected by pyrimidine nucleotides. AMP produced the highest increase of seven-fold in the enzyme activity at 6 mM and a half-maximum increase at approximately 3.8 mM. ATP caused a half-maximum decrease in the activity at approximately 1.4 mM and the remaining activity was lower than 7% at saturating ATP concentrations. AMP and ATP both brought about changes in V(max) and not in K(m). Analysis of the effect of AMP and ATP suggests that each of them has its own primary binding site, which is different from the substrate-binding site. In view of these effects of the nucleotides, the roles of the racemase and D-aspartate in energy metabolism under anoxic conditions are discussed.

Metabolic enzyme activities across an altitudinal gradient: an examination of pikas (genus Ochotona)

Changes in metabolic enzyme activities were examined in three species of pikas that occur over a range of altitudes. Because these closely related mammals live in comparable ecosystems and face similar environmental factors regardless of altitude, modifications of metabolic machinery are probably due to differences in oxygen availability. Citrate synthase (CS), beta-hydroxyacyl CoA dehydrogenase (HOAD) and lactate dehydrogenase (LDH) activities were measured in heart, diaphragm, vastus lateralis, gastrocnemius and soleus muscles. Additionally, the activity levels of both M-LDH (skeletal muscle type) and H-LDH (heart type) isozymes were quantified in tissue samples. Pikas from high altitude had greater CS and HOAD activities in heart and diaphragm when compared with pikas from low altitude, while activity levels did not differ in skeletal muscles. The increase in oxidative enzyme activities in tissues with high metabolic demand is thought to enhance oxygen utilization when oxygen availability is low and may reflect greater metabolic demand on heart and diaphragm tissue. Pikas from high altitude were also found to have greater total LDH activities in all tissues examined. High altitude animals had dramatically higher H-LDH activity (2.3-3.8 times greater) while M-LDH activity was more comparable (1.8 times lower to 1.7 times greater) when compared with low altitude animals. High total LDH activity enables pikas to perform short bouts of anaerobic activity, while high levels of H-LDH isozymes may serve to enhance lactate removal and decrease recovery time in animals living at high altitude.

Biochemical and evolutionary aspects of anaerobically functioning mitochondria

Mitochondria are usually considered to be the powerhouses of the cell and to be responsible for the aerobic production of ATP. However, many eukaryotic organisms are known to possess anaerobically functioning mitochondria, which differ significantly from classical aerobically functioning mitochondria. Recently, functional and phylogenetic studies on some enzymes involved clearly indicated an unexpected evolutionary relationship between these anaerobically functioning mitochondria and the classical aerobic type. Mitochondria evolved by an endosymbiotic event between an anaerobically functioning archaebacterial host and an aerobic alpha-proteobacterium. However, true anaerobically functioning mitochondria, such as found in parasitic helminths and some lower marine organisms, most likely did not originate directly from the pluripotent ancestral mitochondrion, but arose later in evolution from the aerobic type of mitochondria after these were already adapted to an aerobic way of life by losing their anaerobic capacities. This review will focus on some biochemical and evolutionary aspects of these fermentative mitochondria, with special attention to fumarate reductase, the synthesis of the rhodoquinone involved, and the enzymes involved in acetate production (acetate : succinate CoA-transferase and succinyl CoA-synthetase).

The electron transport chain in anaerobically functioning eukaryotes

Many lower eukaryotes can survive anaerobic conditions via a fermentation pathway that involves the use of the reduction of endogenously produced fumarate as electron sink. This fumarate reduction is linked to electron transport in an especially adapted, anaerobically functioning electron-transport chain. An aerobic energy metabolism with Krebs cycle activity is accompanied by electron transfer from succinate to ubiquinone via complex II of the respiratory chain. On the other hand, in an anaerobic metabolism, where fumarate functions as terminal electron acceptor, electrons are transferred from rhodoquinone to fumarate, which is the reversed direction. Ubiquinone cannot replace rhodoquinone in the process of fumarate reduction in vivo, as ubiquinone can only accept electrons from complex II and cannot donate them to fumarate. Rhodoquinone, with its lower redox potential than ubiquinone, is capable of donating electrons to fumarate. Eukaryotic fumarate reductases were shown to interact with rhodoquinone (a benzoquinone), whereas most prokaryotic fumarate reductases interact with the naphtoquinones menaquinone and demethylmenaquinone. Fumarate reductase, the enzyme essential for the anaerobic functioning of many eukaryotes, is structurally very similar to succinate dehydrogenase, the Krebs cycle enzyme catalysing the reverse reaction. In prokaryotes these enzymes are differentially expressed depending on the external conditions. Evidence is now emerging that also in eukaryotes two different enzymes exist for succinate oxidation and fumarate reduction that are differentially expressed.

Rhodoquinone and complex II of the electron transport chain in anaerobically functioning eukaryotes

Many anaerobically functioning eukaryotes have an anaerobic energy metabolism in which fumarate is reduced to succinate. This reduction of fumarate is the opposite reaction to succinate oxidation catalyzed by succinate-ubiquinone oxidoreductase, complex II of the aerobic respiratory chain. Prokaryotes are known to contain two distinct enzyme complexes and distinct quinones, menaquinone and ubiquinone (Q), for the reduction of fumarate and the oxidation of succinate, respectively. Parasitic helminths are also known to contain two different quinones, Q and rhodoquinone (RQ). This report demonstrates that RQ was present in all examined eukaryotes that reduce fumarate during anoxia, not only in parasitic helminths, but also in freshwater snails, mussels, lugworms, and oysters. It was shown that the measured RQ/Q ratio correlated with the importance of fumarate reduction in vivo. This is the first demonstration of the role of RQ in eukaryotes, other than parasitic helminths. Furthermore, throughout the development of the liver fluke Fasciola hepatica, a strong correlation was found between the quinone composition and the type of metabolism: the amount of Q was correlated with the use of the aerobic respiratory chain, and the amount of RQ with the use of fumarate reduction. It can be concluded that RQ is an essential component for fumarate reduction in eukaryotes, in contrast to prokaryotes, which use menaquinone in this process. Analyses of enzyme kinetics, as well as the known differences in primary structures of prokaryotic and eukaryotic complexes that reduce fumarate, support the idea that fumarate-reducing eukaryotes possess an enzyme complex for the reduction of fumarate, structurally related to the succinate dehydrogenase-type complex II, but with the functional characteristics of the prokaryotic fumarate reductases.

Metabolic depression in land snails: in vitro analysis of protein kinase involvement in pyruvate kinase control in isolated Otala lactea tissues

Isolated tissues from the land snail Otala lactea were used to examine the relationship between protein kinase activity and phosphorylation-induced changes associated with metabolic depression. Hepatopancreas and foot muscle were removed from active and estivating land snails and incubated in vitro under aerobic and anoxic conditions. Pyruvate kinase (PK), cAMP-dependent protein kinase (PKA), and protein kinase second messenger compounds (cyclic AMP and inositol 1,4,5-triphosphate) were measured after incubating the tissues for 4 hours. Pyruvate kinase from the hepatopancreas of active snails was phosphorylated during anoxic incubations as indicated by changes in the I50 value for L-alanine. However, measurements of PKA activity and of cellular cAMP concentrations suggested that PKA activity was lower in these incubated tissues. When foot muscle was used as the tissue source, incubation under anoxic conditions produced no changes in PK activity even though PKA activity was drastically reduced. Analysis of changes in inositol 1,4,5-triphosphate concentrations after tissue incubation showed that they were not consistent with changes in PK activity in either organ. These results suggest that PKA and Ca2+/phospholipid-dependent protein kinase C do not phosphorylate PK during anoxia in land snails. The differences between values measured in incubated tissues and those measured in vivo suggest that isolated O. lactea tissues are not a good in vitro model system for studying metabolic changes associated with depressed metabolism.

Control of glycolytic enzyme binding: effect of changing enzyme substrate concentrations on in vivo enzyme distributions

The effect of changing concentrations of glycolytic intermediates on the binding of phosphofructokinase, aldolase and pyruvate kinase to cellular particulate matter was investigated. Concentrations of glycolytic intermediates were altered by adding 2 mM iodoacetic acid (IAA) to an incubation medium containing tissues isolated from the channelled whelk Busycon canaliculatum. Iodoacetic acid inhibited glyceraldehyde 3-phosphate dehydrogenase activity causing a 100-400 fold increase in the concentration of fructose 1,6-bisphosphate as well as 3-20 fold increases in glucose 6-phosphate, fructose 6-phosphate, and dihydroxyacetone phosphate levels depending on the experimental protocol. Cellular pH values were not statistically different in the presence of IAA. Measurement of enzyme binding to particulate matter showed that the binding of phosphofructokinase, aldolase and pyruvate kinase was unaffected by iodoacetic acid under any experimental condition. These results show that changes in the tissue concentrations of enzyme substrates and products do not regulate enzyme binding to particulate matter in the cell.

Biomphalaria glabrata: respiration, calcium and end products of carbohydrate metabolism

1. Lactic acid and succinic acid (end products of anaerobiosis) also occur under aerobic conditions in the haemolymph and excretion products of Biomphalaria glabrata. This phenomenon has been investigated in more detail. 2. Experiments on oxygen uptake, and analyses of organic acid, amino acid and calcium were carried out under various aerating conditions, various temperatures and in various water qualities. 3. No differences were found in the concentrations of the organic acids and calcium in the haemolymph under different aerating conditions. 4. Neither snail-conditioned water, nor artificial crowding effects played a role in the initiation of anaerobic respiration. 5. A low exposure temperature (4 degrees C) initiated anaerobic respiration in spite of the aeration.

Purification and properties of tauropine dehydrogenase from the shell adductor muscle of the ormer, Haliotis lamellosa

Tauropine dehydrogenase (tauropine:NAD oxidoreductase) was purified from the shell adductor muscle of the ormer, Haliotis lamellosa. The enzyme was found to utilize stoichiometrically NADH as co-enzyme and pyruvate and taurine as substrates producing tauropine [rhodoic acid; N-(D-1-carboxyethyl)-taurine]. The enzyme was purified to a specific activity of 463 units/mg protein using a combination of ammonium sulphate fractionation, ion-exchange and affinity chromatography. The relative molecular mass was 38,000 +/- 1000 when assessed by gel filtration on Ultrogel AcA 54 and 42,000 +/- 150 by electrophoresis on 5-10% polyacrylamide gels in the presence of 1% sodium dodecyl sulphate; the data suggest a monomeric structure. Tauropine and pyruvate were found to be the preferred substrates. Among the amino acids tested for activity with the enzyme, only alanine is used as an alternative substrate, but with a rate less than 6% of the enzyme activity with taurine. Of the oxo acids tested, 2-oxobutyrate and 2-oxovalerate were also found to be substrates. Apparent Km values for the substrates NADH, pyruvate and taurine are 0.022 +/- 0.003 mM, 0.64 +/- 0.07 mM and 64.7 +/- 5.4 mM, respectively, at pH 7.0 and for the products, NAD+ and tauropine, are 0.29 +/- 0.01 mM and 9.04 +/- 1.27 mM, respectively, at pH 8.3. Apparent Km values for both pyruvate and taurine decrease with increasing co-substrate (taurine or pyruvate) concentration. NAD+ and tauropine were found to be product inhibitors of the forward reaction. NAD+ was a competitive inhibitor of NADH, whereas tauropine gave a mixed type of inhibition with respect to pyruvate and taurine. Succinate was found to inhibit non-competitively with respect to taurine and pyruvate with an apparent Ki value in the physiological range of this anaerobic end product. The inhibition by L-lactate, not an end product in the ormer, was competitive with respect to pyruvate. The physiological role or tauropine dehydrogenase during anaerobiosis is discussed.

Purification and properties of aerobic and anoxic forms of pyruvate kinase from the hepatopancreas of the channelled whelk, Busycotypus canaliculatum

Aerobic and anoxic variants of pyruvate kinase (termed PK-aer and PK-anx) from the hepatopancreas of the gastropod mollusc, Busycotypus canaliculatum, were purified to apparent homogeneity with final specific activities of 14 and 2.3 units/mg protein, respectively. Both enzymes were homotetramers of the same molecular weight. The enzymes also showed equivalent affinities for ADP (0.22 mM) and very similar affinities for Mg2+, Mn2+, K+, and NH4+. PK-aer and PK-anx differed strongly, however, in maximal enzyme velocity (Vmax 9-fold higher for PK-aer), in affinity for P-enolpyruvate (PEP0.5 = 0.38 mM for PK-aer and 1.1 mM for PK-anx), and in the effects of activators and inhibitors on the enzymes. PK-aer was much more strongly stimulated by fructose-1,6-P2 and aspartate as activators (a 19- and 32-fold activation of enzyme velocity at subsaturating PEP levels versus only 4.1- and 2.6-fold activation for PK-anx, respectively). K alpha for fructose-1,6-P2 was 3-fold lower (0.16 microM) for PK-aer than for PK-anx (0.48 microM), but K alpha for aspartate was the same for both enzymes (1.5 mM). Activators decreased the PEP0.5 (to 0.05 mM for PK-aer and 0.07 mM for PK-anx), relieved inhibitions by alanine, Mg ATP, ADP, and Pi, and, when added together, showed a strong synergistic activation of PK-aer (but not PK-anx). The kinetic differences between PK-aer and PK-anx are similar to those of the dephosphorylated versus phosphorylated forms of PK from other sources, including those of red muscle PK of B. canaliculatum, and indicate that the change in enzyme form brought about during anaerobiosis may be due to enzyme phosphorylation. The powerful activation of hepatopancreas PK by aspartate is a novel regulatory control of the enzyme. Aspartate is one of the substrates of anaerobic energy production in marine molluscs and its effects on the enzyme may be important in a tissue where inactivation of PK can occur for one of two reasons: anaerobiosis or gluconeogenesis.

The anaerobic molluscan heart: adaptation to environmental anoxia. Comparison with energy metabolism in vertebrate hearts

The hearts of many bivalve and gastropod molluscs are resistant to exposure to hypoxic and anoxic conditions. Glycogen and aspartate are simultaneously fermented leading to the accumulation of alanine, succinate and alanopine/strombine. Lactate is not a major end product of anaerobic metabolism in molluscan hearts. In contrast, vertebrate hearts respond to hypoxia by the fermentation of glycogen leading to lactate formation. There is some evidence for aspartate and glutamate breakdown in vertebrate hearts during anoxia. However, the quantitative contribution of this process to energy production is small. The differences in modes of energy production in molluscan and vertebrate hearts may reflect adaptations to long-term as opposed to short-term anoxia.