Temperature dependence of arginine kinase reaction in the tail muscle of live Sycionia ingentis as measured in vivo by 31P-NMR driven saturation transfer

Regulation of tail muscle arginine kinase by reversible phosphorylation in an anoxia-tolerant crayfish

Freshwater crayfish, Orconectes virilis, can experience periodic exposures to hypoxia or anoxia due to low water flow (in summer) or ice cover (in winter) in their natural habitat. Hypoxia/anoxia disrupts energy metabolism and triggers mechanisms that to support ATP levels while often also suppressing ATP use. Arginine kinase (AK) (E.C. 2.7.3.3) is a crucial enzyme involved in energy metabolism in muscle, gating the use of phosphagen stores to buffer ATP levels. The present study investigated AK from tail muscle of O. virilis identifying changes to kinetic properties, phosphorylation state and structural stability between the enzyme from aerobic control and 20 h anoxic crayfish. Muscle AK from anoxia-exposed crayfish showed a significantly higher (by 59%) K (m) for L: -arginine and a lower I(50) value for urea than the aerobic form. Several lines of evidence indicated that AK was converted to a high phosphate form under anoxia: (a) aerobic and anoxic forms of AK showed well-separated elution peaks on DEAE ion exchange chromatography, (b) ProQ Diamond phosphoprotein staining showed a 64% higher bound phosphate content on anoxic AK compared with the aerobic form, and (c) treatment of anoxic AK with alkaline phosphatase reduced K (m) L: -arginine to aerobic levels whereas incubation of aerobic AK with protein kinase A catalytic subunit raised the K (m) to anoxic levels. The physiological consequence of anoxia-induced AK phosphorylation may be to suppress AK activity in the phosphagen-synthesizing direction and, together with reduced cellular pH and ATP levels, promote the phosphagen-catabolizing direction under anoxic conditions. This is first time that AK has been shown to be regulated by reversible phosphorylation.

Osmotic effects on arginine kinase function in living muscle of the blue crab Callinectes sapidus

Flux was examined through the reaction catalyzed by arginine kinase in intact blue crab (Callinectes sapidus) muscle during simulated changes in salinity. Isolated dark levator muscles from the swimming leg were superfused with a saline solution that had an osmolarity equivalent to that of the hemolymph under different salinity regimes. Animals were acclimated for 7 days to a salinity of 5, 17 or 35‰, which corresponds to a hemolymph osmolarity of 640, 720 or 960 mosmoll-1, respectively. Experiments were conducted under control conditions, in which the osmolarity of the superfusion medium matched that of the acclimated hemolymph, as well as under hypo- and hyperosmotic conditions. These latter treatments were meant to simulate a rapid change in environmental salinity. Pseudo-first-order unidirectional rate constants and flux rates were measured for arginine kinase in the forward and reverse directions using a 31P-nuclear magnetic resonance saturation transfer method. There were no differences in the rate constants or flux rates among the controls, indicating that arginine kinase function is not modulated by salinity if the animal has had sufficient acclimation time. However, the rate constants and flux rates of arginine kinase varied over a modest 1.7-fold range across the three types of osmotic treatments, although the range for the flux data was reduced when cell volume changes were taken into account. The hyperosmotic treatments led to a reduction in arginine kinase flux, while the hypo-osmotic treatments led to an enhanced arginine kinase flux. We propose that this effect is mediated by an increase in the concentration of perturbing inorganic ions under hyperosmotic conditions and a decrease in the concentration of such ions during the hypo-osmotic treatments.

Sublethal effects of pentachlorophenol in abalone (Haliotis rufescens) veliger larvae as measured by (31)P-NMR

Toxicity tests with larval marine organisms are widely used as water quality-monitoring tools. Often employing a single morphological endpoint, they provide only limited insight into toxic action mechanisms. Since mechanistic observations can provide more sensitive endpoints of toxicity, the goal of this study was to modify existing in vivo (31)P-NMR techniques to evaluate the effects of pentachlorophenol (PCP) on high-energy phosphates in veliger larvae of the red abalone (Haliotis rufescens), a species and life stage commonly used in developmental toxicity tests. Fluxes of phosphoarginine (PA), nucleoside triphosphates (NTPs), sugar phosphates (SPs), and inorganic phosphate (P(i)), as well as in intracellular pH (pH(i)), were monitored via NMR using a flow-through exposure system. The exposure protocol included 1 h of clean seawater, 2 h of PCP (1.2 mg L(-1)) exposure, and 2 h of recovery in clean seawater. During PCP exposure, veligers displayed maximum declines in PA and NTP resonance intensities to 38+/-10% (P<0.01) and 61+/-16% (P<0.05) of reference values (represented by the average resonance intensities taken from 1 h of preexposure spectral acquisition). Inorganic phosphate concentrations rose to 302+/-63% (P<0.01), and pH(i) declined from a reference period average of 7.12+/-0.03 to 7.04+/-0.04 (P<0.05). On exposure to clean seawater, veligers recovered within 2 h, returning to 196+/-53, 89+/-12, and 89+/-13% of the reference intensities for P(i), PA, and NTPs, respectively (P>0.05 for all values), while pH(i) increased to 7.08+/-0.11. When compared with adult abalone, veliger larvae responded similarly to PCP, differing mainly in their rapid recovery (2 h for larvae versus 6 h adults) and more severe acidification (a 0.08 pH unit decline versus a 0.03 pH unit decline). This report describes an application of NMR for small (> or =200 micro) marine organisms using commonly available vertical-bore NMR magnets; it should prove adaptable for use with other similar larval forms.

Temperature dependence of the creatine kinase reaction measured in rat brain in vivo by31P NMR saturation transfer

Creatine kinase (CK) catalyzes the reversible phosphorylation of MgADP by phosphocreatine and thus regulates cellular concentrations of ADP and ATP. The temperature dependence of this reaction has been determined in rat brain in vivo between 30 and 40°C using31P NMR saturation transfer measurements. The pseudo-first-order rate constant for the forward CK reaction, kf, varies little with temperature over this range, with an apparent activation energy Ea= 14.2 ± 4.9 kJ/mol. This is considerably lower than the values of Eafor isolated CK enzymes. However, when changes in [MgADP] and [H+] with temperature are considered, a substrate concentration-independent value of Ea= 65.3 ± 9.7 kJ/mol is obtained for the maximum forward reaction velocity Vmax. This agrees well with literature values for the isolated brain-type isoform of CK.Key words: creatine kinase, activation energy, temperature, brain, rat.