Synergistic activation of the human red cell calcium ATPase by magnesium and vanadate

Magnesium can protect against vanadium-induced lipid peroxidation in the hepatic tissue

The protective effect of magnesium as magnesium sulfate (MS) on sodium-metavanadate- (SMV-) induced lipid peroxidation (LPO) under in vivo and in vitro conditions was studied. The 18-week SMV intoxication (Group II, 0.125 V_end/mL) enhanced spontaneous malondialdehyde (MDA) generation in rat liver, compared with the control (Group I) and MS-supplemented animals (Group III, 0.06 Mg_end/mL). Coadministration of SMV with MS (Group IV, SMV-MS) caused a return of the MDA level to the control value range. The effect seems to result from the Mg_end-independent action and its antagonistic interaction with V_end. The in vitro treatment of liver supernatants (LS) obtained from all the tested animals groups with selected exogenous concentrations of Fe_exg or V_exg exhibited enhanced MDA production, compared with spontaneously formed MDA. It also showed Mg_exg-stimulating effect on LPO (LS I, Group I) and revealed that the changes in the MDA generation in LS IV (Group IV) might have resulted from the synergistic interactions of V_end with Fe_exg and V_exg and from the antagonistic interactions of Mg_end with Fe_exg and V_exg. The findings allow a suggestion that adequate Mg intake for a specific period in the conditions of SMV exposure may prevent V-induced LPO in the liver.

Selected haematological and biochemical parameters of blood in rats after subchronic administration of vanadium and/or magnesium in drinking water

The purpose of these studies was to evaluate the effect of selected vanadium and magnesium doses on certain haematological and biochemical blood parameters in rats. Outbred 2-month-old, albino male Wistar rats received for a period of 6 weeks, as a sole drinking liquid, the following water solutions: group II, sodium metavanadate (SMV) at a concentration of 0.125 mg V/mL; group III, magnesium sulphate (MS) at a concentration of 0.06 mg Mg/mL; and group IV, SMV-MS solution at the same concentrations. The control group received at this time deionized water to drink. It was calculated that group II ingested with drinking water about 10.7 mg V/kg b. w./24 h, group III 6 mg Mg/kg b. w./24 h, and group IV about 9 mg V and 4.5 mg Mg/kg b. w./24 h. The exposure to vanadium alone (group II) led to a statistically significant decrease in body weight gain, food and fluid intakes. Moreover, in the same group of rats a statistically significant decrease in the RBC count, Hb concentration, MCV, and MCH values was demonstrated. Additionally, a statistically significant decrease in the plasma L-ascorbic acid concentration and a significant increase in MDA concentration in blood in this group were found. Instead, after the administration of magnesium alone (group III), a statistically significant decrease in the fluid intake and in the L-ascorbic acid concentration in plasma was noted. Furthermore, in the same group of rats a statistically significant increase in Hb level and in the plasma magnesium concentration was demonstrated. Two-way analysis of variance (ANOVA) did not reveal the interactions between V and Mg.

Effects of magnesium plus vanadate on partial reactions of the Ca(2+)-ATPase from human red cell membranes

Under conditions in which pretreatment with Mg2+ plus vanadate activate the Ca(2+)-ATPase, the initial rate of phosphorylation of the enzyme increased from 141 to 259 pmol/mg protein per s while the steady-state level of phosphoenzyme lowered from 1.9 to 1.1 pmol/mg protein. The drop in phosphoenzyme level was caused by incubation and washing during treatment rather than by vanadate. The data allowed to estimate a turnover number for the enzyme that raised by 170% after pretreatment. The results show that the activation of the Ca(2+)-ATPase by Mg2+ plus vanadate is due to changes in the kinetic properties of the enzyme.

Activation of the human red cell calcium ATPase by calcium pretreatment

Some kinetic parameters of the human red cell Ca(2+)-ATPase were studied on calmodulin-free membrane fragments following preincubation at 37 degrees C. After 30 min treatment with EGTA (1 mM) plus dithioerythritol (1 mM), a Vmax of about 0.4 mumol Pi/mg x hr and a Ks of 0.3 microM Ca2+ were found. When Mg2+ (10 mM) or Ca2+ (10 microM) were also added during preincubation, Vmax, but not Ks was altered. Ca2+ was more effective than Mg2+, thus increasing Vmax to about 1.3 mumol P/mg x hr. The presence of both Ca2+ and Mg2+ during pretreatment decreased Ks to 0.15 microM, while having no apparent effect on Vmax. Conversely, addition of ATP (2 mM) with either Ca2+ or Ca2+ plus Mg2+ increased Vmax without affecting Ks. Preincubation with Ca2+ for periods longer than 30 min further increased Vmax and reduced Ks to levels as low as found with calmodulin treatment. The Ca2+ activation was not prevented by adding proteinase inhibitors (iodoacetamide, 10 mM; leupeptin, 200 microM; pepstatin A, 100 microM; phenylmethanesulfonyl fluoride, 100 microM). The electrophoretic pattern of membranes preincubated with or without Mg2+, Ca2+ or Ca2+ plus Mg2+ did not differ significantly from each other. Moreover, immunodetection of Ca(2+)-ATPase by means of polyclonal antibodies revealed no mobility change after the various treatments. The above stimulation was not altered by neomycin (200 microM), washing with EGTA (5 mM) or by both incubating and washing with delipidized serum albumin (1 mg/ml), or omitting dithioerythritol from the preincubation medium.(ABSTRACT TRUNCATED AT 250 WORDS)

Irreversible effects of calcium ions on the plasma membrane calcium pump

The calcium pump of human red cells can be irreversibly activated by preincubation of the membranes in the presence of calcium ions, with a pattern reminiscent of that produced by controlled trypsin attack. With 1 mM Ca2+, the activity of the basal enzyme increases three to fourfold over 30 to 60 min, to levels about half those obtained in the presence of calmodulin. On the whole, the effect occurs slowly, with a very low Ca2+ affinity at 37 degrees C and is unaffected by serine-protease inhibitors. The activation caused by 1 mM Ca2+ is little affected by leupeptin (a thiol-protease inhibitor) and that obtained at 10 microM Ca2+ is not inhibited. Preincubations at 0 degrees C also lead to activation, to a level up to half that seen at 37 degrees C, and the effect is not affected by leupeptin or antipain. No activation is observed by preincubating soluble purified Ca,Mg-ATPase in Ca(2+)-containing solutions at 37 degrees C. Instead, calcium ions protect the detergent-solubilized enzyme from thermal inactivation, the effect being half-maximal between 10 and 20 microM Ca2+. We conclude that the activation of the membrane-bound Ca,Mg-ATPase by Ca2+ should result from an irreversible conformational change in the enzyme and not from attack by a membrane-bound protease, and that this change presumably arises from the release of inhibitory particles existing in the original membrane preparations.