Oxidative Stress Poster Session
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Introduction
Several aquatic invertebrates, such as the freshwater crayfish of eastern Canada, have had to adapt to rapid and dramatic fluctuations in PO2. Adaptation to anoxia is a key factor for the survival of Orconectes virilis, a small freshwater crayfish of eastern Canada, and they do so through several biochemical and physiological responses. A rapid increase in respiratory water flow, a reduction in ventilation volume, and the scope of energy produced during anaerobic metabolism could all contribute to survival under anoxic conditions. Adaptation to severe environmental conditions is often associated with metabolic rate depression (Storey and Storey 1990). The metabolic pathways of O. virilis are dramatically influenced by changes in available levels of oxygen. Two key metabolic enzymes involved in glycolysis, pyruvate kinase (PK) and phosphofructokinase (PFK), have been shown to be regulated by phosphorylation in several species (Storey 1984, Michaelidis and Storey 1990, Whitwam and Storey 1990, Fernandez et al. 1994, Benoit et al. 1994, Holwerda et al. 1983, Brooks and Storey 1990, Walsh et al. 1991). This is the case for this species, the regulation by reversible phosphorylation of which was found to be conducted by a cAMP-dependent protein kinase (PKA; Cowan and Storey 1999a). The purification and characterization of PKAc from O. virilis has also shown that this regulatory enzyme has very similar properties characteristic of PKAc found in both vertebrates and invertebrates (Cowan and Storey 1999b).
To further understand the metabolic responses to anoxia by a freshwater crayfish and to assess the connection between tolerance of this stress and changes to metabolic pathways, we focused on several key metabolic enzymes and their maximal activities in response to anoxic conditions. Nineteen regulatory enzymes were assayed in tail muscle and hepatopancreas of O. virilis, comparing enzyme activities under both control and anaerobic conditions.
Materials and Methods
Chemicals and animals
Biochemicals were purchased from Sigma Chemical Co. (St. Louis, MO) or Boehringer-Mannheim Corp. (Montreal, Que.). Distilled, deionized water was used for the preparation of all solutions. Crayfish were obtained from the Ottawa region and arrived in plastic bags filled with their original freshwater. These animals averaged 7.1 +/- 0.3 cm in length and 16.3 +/- 1.6 g in weight. They were kept in an incubator at 10.5°C in plastic tubs containing their original water and bubbled with oxygen. Anoxia was established by bubbling 50% original freshwater / 50% deionized distilled water with N2 (g) in a sealed container, containing enough bubbling water for the number of crayfish to be stressed. The water was gassed for 45 minutes until anoxic conditions were established. Crayfish to be stressed were sealed in the container for 20 hours, removed and immediately dissected. Tissue extraction involved decapitation and removal of the hepatopancreas and the tail muscle, which were then immediately frozen in liquid nitrogen and stored at –70°C.
Enzyme analysis
Samples of frozen tissues were quickly weighed and then homogenized 1:5 w/v in 20 mM imidazole, pH 7.0, 15 mM ß-mercaptoethanol, 50 mM NaF, 2 mM EDTA, 2 mM EGTA, and 20% v/v glycerol with 0.1 mM phenylmethylsulfonyl fluoride added immediately prior to homogenizing. The homogenate was centrifuged at 14000 x g for 25 minutes at 4°C in a Baxter Canlab Biofuge 15 and the supernatant was removed and stored on ice to be used for enzyme analysis.
Maximal activities of metabolic enzymes were assayed at 25°C in a final volume of 250 µl (plus extract volume) using a Dynatech MR5000 microplate reader at an absorbance of 340 nm (unless otherwise noted). Assay conditions were optimized for all enzymes using extracts of O. virilis tail muscle and hepatopancreas. Blanks (minus the most specific substrate) were subtracted. Optimal assay conditions were as follows:
Hexokinase (HK; E.C. 2.7.1.1): 100 mM Tris buffer (pH 8.0), 1 mM EDTA, 2 mM MgCl
Glucokinase (GK; E.C. 2.7.1.2): 100 mM Tris buffer (pH 8.0), 1 mM EDTA, 2 mM MgCl2, 100 mM glucose, 0.2 mM NADP+, 1 mM ATP and 1 unit (U) glucose-6-phosphate dehydrogenase.
Phosphofructokinase (PFK; E.C. 2.7.1.11): 20 mM imidazole (pH 7.1), 10 mM MgCl2, 50 mM KCl, 2.5 mM ATP, 0.15 mM NADH, 12.5 mM fructose-6-phosphate (F6P), and 1 U each of aldolase, glycerol-3-phosphate dehydrogenase and triosephosphate isomerase.
Aldolase (ALD; E.C. 4.1.2.13): 50 mM imidazole buffer (pH 7.0), 2 mM MgCl2, 0.2 mM fructose-1,6-bisphosphate (FBP), 0.15 mM NADH, 1 U triosephosphate isomerase and 2 U glycerol-3-phosphate dehydrogenase.
Glyceraldehydephosphate dehydrogenase (GAPDH; E.C. 1.2.1.12): 50 mM imidazole (pH 7.2), 20 mM 3-phosphoglycerate, 5 mM MgSO4, 1 mM ATP, 0.15 mM NADH, and 1 unit (U) PGK.
Pyruvate kinase (PK; E.C. 2.7.1.40): 50 mM imidazole (pH 7.0), 50 mM KCl, 5 mM MgCl2, 0.4 mM phosphoenolpyruvate (PEP), 2 mM ADP, 0.15 mM NADH, 0.2% v/v rotenone saturated ethanol and 2 U lactate dehydrogenase.
Lactate dehydrogenase (LDH; E.C. 1.1.1.27): 50 mM imidazole buffer (pH 7.0), 2 mM pyruvate, 0.15 mM NADH.
ATP-citrate lyase (CL; E.C. 4.1.3.8): 200 mM Tris-HCl buffer (pH 8.4), 10 mM ß-mercaptoethanol, 0.2 mM coenzyme A (CoA), 0.15 mM NADH, 10 mM MgCl2, 20 mM potassium citrate, 10 mM ATP, 10 mM MgCl2, and 1 U malate dehydrogenase.
Fatty acid synthetase (FAS): 100 mM potassium phosphate buffer (pH 7.0), 3 mM EDTA, 5 mM dithiothreitol, 0.05 mM malonyl-CoA, 0.032 mM acetyl-CoA, 0.3 mM NADPH and 0.2% v/v rotenone saturated ethanol.
Isocitrate dehydrogenase, NADP+-dependent (IDH; E.C. 1.1.1.42): 50 mM imidazole buffer (pH 7.5), 2 mM MgCl2, 6 mM DL-isocitrate and 0.4 mM NADP+.
Glucose-6-phosphate dehydrogenase (G6PDH; E.C. 1.1.1.49): 60 mM Tris-HCl buffer (pH 7.5), 3.3 mM glucose-6-phosphate, 0.4 mM NADP+ and 6 mM 1 MgCl2.
Malic enzyme (ME; E.C. 1.1.1.40): 75 mM Triethanolamine buffer (pH 7.4), 4 mM MnCl2, 2.5 mM L-malate, 0.2 mM NADP+.
Fructose-1,6-bisphosphatase (FBPase; E.C. 3.1.3.11): 25 mM imidazole buffer (pH 8.0), 5 mM MgSO4, 0.2 mM NADP+, 6 mM FBP and 1 U each of glucose-6-phosphate dehydrogenase and phosphoglucose isomerase.
Phosphoenolpyruvate carboxykinase (PEPCK; E.C. 4.1.1.32): 100 mM imidazole buffer (pH 6.6), 6 mM ß-mercaptoethanol, 50 mM sodium bicarbonate, 1.2 mM PEP, 1.25 mM inosine diphosphate, 1 mM MnCl2, 0.15 mM NADH and 2.5 U malate dehydrogenase. All solutions were degassed prior to assay.
Glutamate dehydrogenase (GDH; E.C. 1.4.1.2): 100 mM Triethnolamine buffer (pH 8.0), 5 mM EDTA, 200 mM ammonium carbonate, 7.5 mM 2-oxoglutarate, 0.15 mM NADH, and 1 mM ADP.
Serine dehydratase (SDH; E.C. 4.2.1.13): 200 mM potassium phosphate buffer (pH 8.0), 2 mM EDTA, 100 mM L-serine, 0.11 mM pyridoxal phosphate, 0.15 mM NADH, 0.2% v/v rotenone saturated ethanol, and 1 U lactate dehydrogenase.
Glutamate-oxaloacetate transaminase (GOT; E.C. 2.6.1.1): 500 mM imidazole buffer (pH 7.8), 30 mM ß-mercaptoethanol, 15 mM 2-oxoglutarate, 250 mM L-aspartate, 0.11 mM pyridoxal phosphate, 0.15 mM NADH and 1 U malate dehydrogenase (NAD+ dependent).
Glutamate-pyruvate transaminase (GPT; E.C. 2.6.1.2): 500 mM imidazole buffer (pH 7.3), 30 mM ß-mercaptoethanol, 15 mM 2-oxoglutarate, 500 mM L-alanine, 0.11 mM pyridoxal phosphate, 0.15 mM NADH and 1 U lactate dehydrogenase.
Branched chain alpha-ketoacid dehydrogenase (KADH, E.C.): 30 mM potassium phosphate buffer (pH 7.5), 1 mM dithiothreitol, 2 mM MgCl2, 0.1 % Triton X-100, 10 U LADH, 0.4 mM CoA, 0.4 mM TPP, 0.2 mM NAD+, 2 mM alpha-ketoisovalerate
Data analysis
Statistical analysis used the Student’s t-test.
Results
Table 1 shows the significant changes to maximal activities of metabolic enzymes assessed in two tissues (tail muscle and hepatopancreas) of control and 20 hours anoxic O. virilis. Activities are presented as milliunits per gram wet weight. Table 2 shows enzyme maximal activities in tissues from control animals or, if no effect of anoxia occurred, shows combined values for tissues from control and anoxic animals.
Table 1.
Significant changes to maximal activities of enzymes in tail muscle and hepatopancreas of O. virilis.
Control Anoxic
mU/gww mU/gww
Tail Muscle
ALD 30645 +/- 3630 (6) 20400 +/- 1690 (6) *
CL 457 +/- 42.7 (7) 186 +/- 27.3 (5) **
ME 496 +/- 50.4 (4) 390 +/- 10.9 (6) *
GPT 27.2 +/- 5.87 (5) 47.2 +/- 6.19 (6) *
HK 20.1 +/- 3.86 (4) 55.1 +/- 6.92 (4) *
GK 31.6 +/- 6.14 (8) 0.513 +/- 0.086 (8) **
KADH 67.5 +/- 5.55 (7) 9.03 +/- 2.36 (5) **
SDH 7 92 +/- 47.6 (6) 327 +/- 40.4 (6) **
PFK 9312 +/- 543 (7) 6551 +/- 180 (7) **
Hepatopancreas
ALD 265 +/- 10.2 (7) 429 +/- 43.0 (6) **
LDH 19146 +/- 1630 (7) 24067 +/- 805 (6) *
FBP 45.3 +/- 6.17 (6) 65.0 +/- 3.94 (7) *
PK 6325 +/- 518 (6) 4523 +/- 411 (6) *
G6PDH 93.2 +/- 17.4 (5) 304 +/- 38.9 (7) **
GPT 171 +/- 16.1 (7) 320 +/- 30.9 (8) **
GK 32.5 +/- 4.35 (6) 75.6 +/- 8.79 (5) **
KADH 21.5 +/- 2.87 (6) 39.9 +/- 5.14 (5) *
SDH 624 +/- 63.0 (6) 953 +/- 38.6 (6) **
PFK 246 +/- 29.3 (6) 116 +/- 6.23 (6) **
Data are mU per gram wet weight, means +/- SEM, n = 4-8. *Significantly different from corresponding control values, P<0.05. **Significantly different from corresponding control values, P<0.005.
Table 2.
Maximal activities of enzymes in tail muscle and hepatopancreas of O. virilis.
Tail Muscle Hepatopancreas
mU/gram wet weight mU/gram wet weight
HK 20.1 +/- 3.86 (4) 5.43 +/- 0.719 (8) a
GK 31.6 +/- 6.14 (8) 32.5 +/- 4.35 (6)
PFK 9312 +/- 543 (7) 246 +/- 29.3 (6)
ALD 30645 +/- 3630 (6) 265 +/- 10.2 (7)
GAPDH 7788 +/- 544 (15) a 8777 +/- 1055 (16) a
PK 58014 +/- 6057 (10) a 6325 +/- 518 (6)
LDH 144101 +/- 8992 (14) a 19146 +/- 1630 (7)
GDH 21788 +/- 1498(14) a 3743 +/- 180 (15) a
GPT 27.2 +/- 5.87 (5) 171 +/- 16.1 (7)
GOT 63083 +/- 1475 (16) a 18031 +/- 654 (16) a
SDH 792 +/- 47.6 (6)a 624 +/- 63.0 (6)
KADH 67.5 +/- 5.55 (7) 21.5 +/- 2.87 (6)
G6PDH 62.7 +/- 10.4 (13) a 93.2 +/- 17.4 (5)
FBPASE 11.4 +/- 0.78 (13) a 45.3 +/- 6.17 (6)
PEPCK 796 +/- 36.8 (13) a 229 +/- 25.7 (12) a
ME 496 +/- 50.4 (4) 359 +/- 24.2 (14) a
CL 457 +/- 42.7 (7) 126 +/- 8.92 (14) a
FAS 83.8 +/- 5.80 (13) a 121 +/- 16.8 (13) a
IDH 16900 +/- 1217 (14) a 929 +/- 108 (10) a
Values are mU/gram wet weight, means +/- SEM, with n in brackets and represent data for control crayfish alone or where indicated by (a) data for control and anoxic crayfish are combined. Data were combined when no significant difference was found between the values from the two states.
In general, anoxia had no effect on enzyme maximal activities of GAPDH, PEPCK, IDH, FAS, GOT, or GDH in either tissue, and out of the 19 activities detected GK, ALD, PFK, AND GPT were changed significantly between control and anoxic animals in both tissues. In liver 9 out of 19 enzymes were affected during anoxia exposure, as was the case for tail muscle. Hence, anoxic conditions seemed to cause a specific and important reorganization of enzyme activities in both hepatopancreas and tail muscle.
Tail Muscle
The effect of anoxia on the maximal activities of the 9 enzymes in crayfish tail muscle which were significantly affected by anaerobic conditions are depicted in Table 1. Activity of the rate-limiting enzyme of glycolysis, PFK, decreased by 30 % during anoxia and that of the initial step of glycolysis, GK, decreased by 98 %. The maximal activity of ALD also decreased by 33 % during anoxia. The activity of one other enzyme of glycolysis, HK, was most strongly affected, increasing during anoxia by 2.7-fold. CL activity fell, down 59 % compared to controls, as did ME, down 21 %. Tail muscle GPT activities represented the only other muscle enzyme activity that rose during anoxia, up 74 %. Both KADH and SDH activities dropped with respect to controls, down 87 and 59 %, respectively.
Hepatopancreas
Anoxia induced significant changes in the activities of ten enzymes in hepatopancreas (Table 1). Of these, only the activity of the glycolytic enzymes, PFK and PK, decreased during anoxia, dropping by 53 and 28 %, respectively. However, activity of three other glycolytic enzymes, GK, ALD, and LDH, increased by 132, 62, and 26 %. Activities of a gluconeogenic enzyme, FBP, also increased significantly during anoxia, up 43 % from control values. In addition, G6PDH activities increased 3.3-fold, KADH activities by 1.9-fold and GPT and SDH rose 87 and 53 % from their corresponding control values.
Discussion and Conclusion
After twenty hours of anoxia, the metabolic make-up of crayfish organs had changed considerably. Organ-specific patterns were apparent in the responses of metabolic enzymes in both tissues. In tail muscle, exposure to anoxia altered the maximal activities of almost half of the metabolic enzymes assayed, as was the case in hepatopancreas, indicating that both tissues undergo a profound metabolic reorganization that impact on numerous pathways of intermediary metabolism. There could be several influences at work during anoxia exposure that could differentially affect the metabolic make-up of different organs. The first priority is to maintain glucose supply and this is done by depleting glycogen reserves, increasing gluconeogenesis from glycerol or from amino acids supplied by the breakdown of muscle protein, and switching most organs to a primary dependence on lipid oxidation. Thus, during anoxia it is expected that the crayfish tissues adapt to suppress glycolysis and fatty acid synthesis and to increase protein catabolism, gluconeogenesis, and lipid oxidation. Another influence on metabolic reorganization is metabolic arrest. Overall suppression of metabolic rate has been linked with reversible phosphorylation controls on key enzymes (Storey and Storey, 1990; Cowan and Storey 1998) but may also involve specific changes in the maximal activities of selected enzymes to suppress nonessential metabolic functions during long term dormancy. These factors can impact on the metabolic make-up of crayfish organs to optimize enzymatic pathways to meet the challenges of the dormant state.
During anoxia the maximal activities of nine metabolic enzymes changed significantly in tail muscle. Activities of three enzymes of glycolysis, GK, ALD, and PFK, were reduced; notably PFK is the rate-limiting enzyme of the pathway so this suggests that glycolytic rate was suppressed. PFK activities are much higher in tail muscle than HK activities. Reduced CL activity in tail muscle during anoxia may be linked with low rates of fatty acid synthesis in the anaerobic state. GPT activity was also greatly enhanced in tail muscle (by 74 %). This indicates that some protein catabolism must have taken place, and that anaerobic crustaceans must rely on the fermentation of glutamate during periods of anoxia exposure.
In hepatopancreas, changes in the activities of metabolic enzymes were also consistent with changes in fuel use patterns in the anaerobic state. The strong trend among those enzymes that changed was for elevated enzyme activities (8 out of 10 cases) (Figure 2). Activities of two glycolytic enzymes were reduced in hepatopancreas of anoxic crayfish, consistent with carbohydrate sparing. PFK and PK activities were reduced or inhibited whereas gluconeogenesis was promoted through FBPase activity. In crayfish hepatopancreas, activities clearly favour gluconeogenesis during anoxia. Significantly increased levels of G6PDH during anoxia may be linked to the enhanced GK activities found in hepatopancreas under the same conditions. Amino acid oxidation also appears to be facilitated in hepatopancreas of anoxic crayfish with elevated levels of SDH and GPT.
Several changes to metabolic pathway potential occur in order to optimize the crayfish ability to survive conditions without environmental oxygen. Anoxia also affects the activities or protein levels of several key regulatory enzymes including protein kinase A, protein phosphatase types 1 and 2, numerous tyrosine kinase and phosphatases, MAPKs, and a variety of transcription factors involved in regulating induced gene espression (Cowan and Storey 1999a, 1999b, 1999c). The future directions of studies into anoxia-tolerance in the freshwater crayfish will lead us into completing the picture of what actually occurs during metabolic rate depression in O. virilis, including an analysis of amino acid levels and glycogen storage in tissues, assays for glycogen phosphorylase, and levels of phosphagen arginine phosphate.
References
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