`In crystallo' substrate binding triggers major domain movements and reveals magnesium as a co-activator of Trypanosoma brucei pyruvate kinase

The active site of pyruvate kinase (PYK) is located between the AC core of the enzyme and a mobile lid corresponding to domain B. Many PYK structures have already been determined, but the first `effector-only' structure and the first with PEP (the true natural substrate) are now reported for the enzyme from Trypanosoma brucei. PEP soaked into crystals of the enzyme with bound allosteric activator fructose 2,6-bisphosphate (F26BP) and Mg(2+) triggers a substantial 23° rotation of the B domain `in crystallo', resulting in a partially closed active site. The interplay of side chains with Mg(2+) and PEP may explain the mechanism of the domain movement. Furthermore, it is apparent that when F26BP is present but PEP is absent Mg(2+) occupies a position that is distinct from the two canonical Mg(2+)-binding sites at the active site. This third site is adjacent to the active site and involves the same amino-acid side chains as in canonical site 1 but in altered orientations. Site 3 acts to sequester Mg(2+) in a `priming' position such that the enzyme is maintained in its R-state conformation. In this way, Mg(2+) cooperates with F26BP to ensure that the enzyme is in a conformation that has a high affinity for the substrate.

Magnesium: the forgotten electrolyte

Magnesium (Mg++), Potassium (K+) and Calcium (CA++) are important electrolytes in keeping a stable electrical status. The purpose of this study was to measure them in critically ill patients.

METHODS

We evaluated the electrolytes in 28 consecutive patients. Eighteen were females and 10 males with mean age of 62 +/- 5 years.

RESULTS

The admission diagnosis in 95% of the cases was congestive heart failure. Sixty-four percent of the patients had subnormal values of Mg++, 53% subnormal values of K+, and 28% subnormal values of CA++. Fourteen percent showed lower values of the three electrolytes and 35% only of Mg++ and K+ concomitantly. Twenty-eight percent showed prolonged QTC interval. All patients with prolonged QTC interval had low Mg++ and K+ levels. Twenty five percent of the patients showed atrial fibrillation, 25% ventricular tachycardia, and 3% junctional tachycardia. The ventricular tachycardia group had more electrolyte abnormalities than those with atrial fibrillation. None of the patients received Mg++ replacement during critical management while 50% received K+ replacement.

CONCLUSION

This data shows physician overlook the Importance of Mg++ and K+ deficiency in critically ill patients.

Effect of magnesium on gastrointestinal transit time in normal and diabetic rats: possible mechanism of action

BACKGROUND

Many gastrointestinal complications in diabetes are connected to neurohumoral dysfunction resulting in abnormalities of intestinal motility, secretion and absorption. Minerals have been reported as essential cofactors for basic cellular reactions but there is dearth of information on effect of Magnesium on gastrointestinal transit time (GITT) and the mechanism of action.

METHODS

Sixty male albino Wistar rats (180 - 200g) were grouped into twelve of five animals each. Group 1 (control) received 0.2ml saline. Groups 2-6 were normal rats treated with magnesium sulphate (as magnesium) (500mg/kg), adrenaline (0.5mg/kg), magnesium (500mg/kg) and adrenaline (0.5mg/kg), prazosin (1mg/kg) and both magnesium (500mg/kg) and prazosin (1mg/kg) respectively. Groups 7 - 12 were diabetic rats treated as in groups 1- 6. Diabetes was induced intraperitoneally with alloxan (120mg/kg bwt).

RESULTS

There was significant (p<0.05) reduction in GITT index in normal rats treated with magnesium, prazosin and combination of magnesium and prazosin compared with control. Treatment with adrenaline alone produced significant increase in GITT. However treatment with both magnesium and adrenaline produced significant reduction compared with control. This reduction in GITT was similar to that obtained in magnesium only and prazosin only treated groups. Diabetic groups showed significant reduction in GITT in all treated groups except the adrenaline only treated group which produced significant increase in GITT.

CONCLUSION

The significant reduction in GITT produced by magnesium in both normal and diabetic animals was comparable to that produced by prazosin (an á-adrenoceptor antagonist) indicating that magnesium may be inhibiting gastrointestinal smooth muscle contraction through á-adrenoceptor antagonist pathway.

[Magnesium and cardiac function]

Free magnesium ion (Mg(2 + )) is involved in numerous processes of cardiac function. However, mechanism of regulation by Mg(2 + ) has not been fully understood. Extracellular Mg(2 + ) can act on the external surface of the cell membrane, whereas intracellular Mg(2 + ) can exert its effects via many different sites : various enzymes, intracellular organella and internal surface of the cell membrane. In this article, we will briefly review the extracellular and intracellular effects of Mg(2 + ) on each step of E-C coupling of cardiac myocytes, in an attempt to integrate them into cardiac function.

[Shortage and metabolic disturbance of magnesium in diabetic patients and significance of magnesium replacement therapy]

It is reported that shortage of oral magnesium intake increase the incidence of diabetes. In addition, magnesium replacement therapy improves insulin resistance and glycemic control. Low levels of magnesium in the venous blood induce the disturbances of auto-phosphylation on the insulin receptor and deteriorate insulin resistance. Since magnesium is closely related to evolution and development of metabolic syndrome including diabetes mellitus and so on, magnesium is expected as potentially effective ingredient of drug therapy in the future perspectives.

[Magnesium and pregnancy]

Blood levels of total Mg, ionized Mg and intracellular ionized Mg of platelet were lowered in the 2nd trimester of gestation and thereafter. Urinary excretion of Mg does not change during the whole period of pregnancy. According to our results of animal experiments, intestinal absorption of Mg does not increase during pregnancy compared with non-pregnant state. These data suggest pregnant women tend to become magnesium deficiency. This is reflected in the fact that Mg metabolic parameters of pregnant women with preeclampsia are relatively reduced compared with those of healthy gravidae. In the obstetrical practice Mg has other significance, because magnesium sulfate is frequently used as a drug of choice in the therapy of threatened premature delivery and eclampsia. Mg plays important roles in physiology and pharmacology during gestation.

[Magnesium for nutrient]

Magnesium is essential nutrient. Here is magnesium for nutrient. Magnesium has a large number of biological or biochemical functions. It's distribution, absorption, deficiency, excess disorder, requirement, supply sources, and especially status and issue of magnesium intake in Japanese are reviewed.

[Significance of the Magnesium in the hard tissues such as Bone and Teeth]

In latest year, the relationship between whole body osteoporosis and jawbone osteoporosis, and also the relationship between bone density from whole body and jawbone have been recognized. As Mg is related to the bone density of whole body and the weakness of alveolar bone, it deeply connects to the denture treatment and the convalescence after implant treatment. It related to avoid losing teeth during the suppression of acid solubility of enamel, too. For the importance to control the absorption of alveolar bone which can support the teeth, it is necessary to have more consideration on Mg in the relationship between whole body's bone and alveolar bone.

[Role of magnesium in nerve tissue]

The role of magnesium on nerve tissue was discussed. Two main topics of "magnesium and neural activity" and "magnesium-therapy and brain neurons" were described together with introducing our research on rat cultured neurons of cortex and hippocampus.

Magnesium, a drug of diverse use

Magnesium has evolved as a drug with diverse clinical applications. Mg++ is an important caution and its homeostasis is very important for normal body functioning. The physiological role of Mg is due to its calcium channel blocking properties at smooth muscle, skeletal muscle and conduction system levels. The analgesic properties are due to NMDA receptor blocking action. Mg++ is beneficial in acute Myocardial Infarction, protection during open heart surgery and treatment and prevention of heart rhythm disturbances. Mg has an established role in the management of preeclampsia and eclampsia. Magnesium prevents or controls convulsions by blocking neuromuscular transmission and decreasing the release of acetylcholine at the motor nerve terminals. The use of MgSO4 in treating tetanus and acute asthma is established. In conclusion, Mg is a cost effective, widely used drug with multidisciplinary applications. Its majority of physiological effects are attributed to calcium channel blocking properties.

Paying the piper: the cost of Ca2+ pumping during the mating call of toadfish

Superfast fibres of toadfish swimbladder muscle generate a series of superfast Ca(2+) transients, a necessity for high-frequency calling. How is this accomplished with a relatively low rate of Ca(2+) pumping by the sarcoplasmic reticulum (SR)? We hypothesized that there may not be complete Ca(2+) saturation and desaturation of the troponin Ca(2+) regulatory sites with each twitch during calling. To test this, we determined the number of regulatory sites by measuring the concentration of troponin C (TNC) molecules, 33.8 μmol per kg wet weight. We then estimated how much SR Ca(2+) is released per twitch by measuring the recovery oxygen consumption in the presence of a crossbridge blocker, N-benzyl-p-toluene sulphonamide (BTS). The results agreed closely with SR release estimates obtained with a kinetic model used to analyse Ca(2+) transient measurements. We found that 235 μmol of Ca(2+) per kg muscle is released with the first twitch of an 80 Hz stimulus (15(o)C). Release per twitch declines dramatically thereafter such that by the 10th twitch release is only 48 μmol kg(-1) (well below the concentration of TNC Ca(2+) regulatory sites, 67.6 μmol kg(-1)). The ATP usage per twitch by the myosin crossbridges remains essentially constant at ∼25 μmol kg(-1) throughout the stimulus period. Hence, for the first twitch, ∼80% of the energy goes into pumping Ca(2+) (which uses 1 ATP per 2 Ca(2+) ions pumped), but by the 10th and subsequent twitches the proportion is ∼50%. Even though by the 10th stimulus the Ca(2+) release per twitch has dropped 5-fold, the Ca(2+) remaining in the SR has declined by only ∼18%; hence dwindling SR Ca(2+) content is not responsible for the drop. Rather, inactivation of the Ca(2+) release channel by myoplasmic Ca(2+) likely explains this reduction. If inactivation did not occur, the SR would run out of Ca(2+) well before the end of even a 40-twitch call. Hence, inactivation of the Ca(2+) release channel plays a critical role in swimbladder muscle during normal in vivo function.

The role of magnesium in hypertension and cardiovascular disease

Magnesium intake of 500 mg/d to 1000 mg/d may reduce blood pressure (BP) as much as 5.6/2.8 mm Hg. However, clinical studies have a wide range of BP reduction, with some showing no change in BP. The combination of increased intake of magnesium and potassium coupled with reduced sodium intake is more effective in reducing BP than single mineral intake and is often as effective as one antihypertensive drug in treating hypertension. Reducing intracellular sodium and calcium while increasing intracellular magnesium and potassium improves BP response. Magnesium also increases the effectiveness of all antihypertensive drug classes. It remains to be conclusively proven that cardiovascular disease such as coronary heart disease, ischemic stroke, and cardiac arrhythmias can be prevented or treated with magnesium intake. Preliminary evidence suggests that insulin sensitivity, hyperglycemia, diabetes mellitus, left ventricular hypertrophy, and dyslipidemia may be improved with increased magnesium intake. Various genetic defects in magnesium transport are associated with hypertension and possibly with cardiovascular disease. Oral magnesium acts as a natural calcium channel blocker, increases nitric oxide, improves endothelial dysfunction, and induces direct and indirect vasodilation.

Analysis of the role of Mg²⁺ on conformational change and target recognition by ciliate Euplotes octocarinatus centrin

The binding of Mg(2+) with the Euplotes octocarinatus centrin (EoCen) and the effect of Mg(2+) on the binding of EoCen with the peptide melittin were examined by spectroscopic methods. In this study, it was found that Mg(2+) may bind with Ca(2+)-binding sites, at least partly, on EoCen, which displays ∼10-fold weaker affinity than Ca(2+). In the presence of Mg(2+), Ca(2+)-saturated EoCen undergoes significant conformational changes resulting in decreased exposure of hydrophobic surfaces on the protein. Additionally, excess Mg(2+) did not change the stoichiometry, but rather reduced the affinity of EoCen to melittin. The Mg(2+)-dependent decrease in the affinities of EoCen to melittin is an intrinsic property of Mg(2+), rather than a nonspecific ionic effect. The inhibitory effect of Mg(2+) on the formation of complexes between EoCen and melittin may contribute to the specificity of EoCen in target activation in response to cellular Ca(2+) concentration fluctuations.

Relation between serum magnesium level and migraine attacks

OBJECTIVE

The determination of serum magnesium levels in migraine.

METHODS

In a case control study performed between January 2007 and December 2007 at Tabriz University of Medical Sciences, Tabriz, Iran, 140 migraine patients were enrolled and their level of serum magnesium was determined and the results were compared with 140 healthy people who did not have any headache, kidney, or gastrointestinal disorders, and no consumption of magnesium complements.

RESULTS

Migraine patients (22 male, 118 female) with a mean age of 33.82+/-10.31 and 140 healthy people (26 male, 114 female) with a mean age of (34.19+/-9.95) were enrolled. Forty patients had aura and 100 patients did not have aura. The average serum magnesium level in the patient group (26.14+/-4.3) was significantly lower than the control (31.09+/-4.32) group (p=0.000). There was no significant difference between the mean level of serum magnesium in patients with migraine with aura and without aura, however, there was a significant linear relationship between the amount of serum magnesium and the frequency of headache.

CONCLUSION

Serum magnesium in migraine patients was significantly lower than the normal population and related to the frequency of migraine attacks, supporting the use of magnesium in prevention and treatment of migraine.

Insulin secretion is decreased in non-diabetic individuals with hypomagnesaemia

BACKGROUND

Magnesium modulates insulin-mediated glucose uptake but data regarding its role in insulin secretion are scarce; therefore, in this study we determined whether decreased serum magnesium levels are associated with the impairment of insulin secretion in non-diabetic individuals.

METHODS

A total of 182 apparently healthy subjects, men and non-pregnant women, 18-65 years of age, were enrolled in a population-based cross-sectional study and allocated to groups with hypomagnesaemia and normomagnesaemia. The groups in the study were subsequently stratified according to glucose status: normal glucose tolerance, impaired fasting glucose, and impaired glucose tolerance. Insulin secretion was evaluated by the first and second phases of insulin secretion.

RESULTS

The Spearman coefficient between serum magnesium and the first and second phases of insulin secretion showed a significant positive correlation in the overall (r = 0.265, p < 0.0005; r = 0.541, p < 0.0005), normal glucose tolerance (r = 0.369, p = 0.001; r = 0.618, p < 0.0005), impaired fasting glucose (r = 0.320, p = 0.02; r = 0.449, p = 0.001), and impaired glucose tolerance (r = 0.129, p = 0.37; r = 0.522, p < 0.0005) groups. The multivariate linear regression analysis showed a significant association between serum magnesium levels and the first and second phases of insulin secretion: for the entire groups [B = 75.2; 95% confidence interval (CI) 27.6-122.7; B = 25.4; 95% CI 16.4-34.3], normal glucose tolerance (B = 129.6, 95% CI 38.1-221.1; B = 40.3, 95% CI 23.7-56.8), impaired fasting glucose (B = 75.2, 95% CI 27.6-122.7; B = 15.1, 95% CI 4.2-30.2), and impaired glucose tolerance (B = 57.4, 95% CI 23.5-138.3; B = 25.4, 95% CI 16.4-34.3) groups.

CONCLUSIONS

Our results show that hypomagnesaemia is associated with the decrease of the first and second phases of insulin secretion in non-diabetic subjects with hypomagnesaemia.

Neonatal diabetes caused by mutations in sulfonylurea receptor 1: interplay between expression and Mg-nucleotide gating defects of ATP-sensitive potassium channels

CONTEXT

ATP-sensitive potassium (KATP) channels regulate insulin secretion by coupling glucose metabolism to β-cell membrane potential. Gain-of-function mutations in the sulfonylurea receptor 1 (SUR1) or Kir6.2 channel subunit underlie neonatal diabetes.

OBJECTIVE

The objective of the study was to determine the mechanisms by which two SUR1 mutations, E208K and V324M, associated with transient neonatal diabetes affect KATP channel function.

DESIGN

E208K or V324M mutant SUR1 was coexpressed with Kir6.2 in COS cells, and expression and gating properties of the resulting channels were assessed biochemically and electrophysiologically.

RESULTS

Both E208K and V324M augment channel response to MgADP stimulation without altering sensitivity to ATP4- or sulfonylureas. Surprisingly, whereas E208K causes only a small increase in MgADP response consistent with the mild transient diabetes phenotype, V324M causes a severe activating gating defect. Unlike E208K, V324M also impairs channel expression at the cell surface, which is expected to dampen its functional impact on β-cells. When either mutation was combined with a mutation in the second nucleotide binding domain of SUR1 previously shown to abolish Mg-nucleotide response, the activating effect of E208K and V324M was also abolished. Moreover, combination of E208K and V324M results in channels with Mg-nucleotide sensitivity greater than that seen in individual mutations alone.

CONCLUSION

The results demonstrate that E208K and V324M, located in distinct domains of SUR1, enhance transduction of Mg-nucleotide stimulation from the SUR1 nucleotide binding folds to Kir6.2. Furthermore, they suggest that diabetes severity is determined by interplay between effects of a mutation on channel expression and channel gating.

[Nutrition and bone health. Magnesium and bone]

Magnesium is related to a number of biological enzymatic reactions such as catalytic role for the reaction of kinases in ATP production. On the other hand, magnesium is one of the essential minerals for bone formation. In the magnesium-deficient rats, apparent bone loss caused by increase in bone resorption and decrease in bone formation was observed. Although, epidemiological studies suggest that magnesium deficiency is one of the risk factor for osteoporosis, a relationship between magnesium intake and bone mineral density is not clear. This may be due to the differences in the population, decrease in sex hormone secretion, and the possibility that magnesium-deficiency is also accompanied with another nutrient insufficiency, e.g., calcium.

Kalzium ist nicht alles

Elevation of cerebral Mg2+ with a novel orally delivered ionophore, magnesium threonate, enhances cognition in young and old rats over a 12-24 day treatment interval, as outlined in a paper by Slutsky et al. in this issue of Neuron. Despite both Mg2+ and Zn2+ blocking the NMDA receptor channel, sustained extracellular Mg2+ elevation mimics sustained synaptic Zn2+ concentrations by increasing hippocampal NR2B expression and bouton density.

[Effect of magnesium-free on glucocorticoid receptor expression in primary cultured cortical neurons of fetal rats in vitro]

OBJECTIVE

To study the changes of glucocorticoid receptor (GR) expression in embryonic rat cortical neurons exposed to transient Mg(2+)-free treatment.

METHODS

Six days after rat cortical neuronal cultures, two groups were created based on the medium to which were transiently exposed. The control group was exposed to a physiological solution (PS), and the Mg(2+)-free group was exposed to the same medium as the control group except for the removal of magnesium. The expression of GR mRNA and protein was determined by real-time PCR and immunocytochemistry staining 1, 7 and 12 days after transient Mg(2+)-free treatment.

RESULTS

Compared to the control group, the Mg(2+)-free group displayed the significantly less accumulated optical density (AOD) of GR immunoreactivity 12 days after transient Mg(2+)-free treatment (p<0.05). On the contrary, GR mRNA expression increased significantly 1 and 7 days after transient Mg(2+)-free treatment in the Mg(2+)-free group (p<0.05).

CONCLUSIONS

GR expression is modified following Mg-free-induced injury in cultured developing neurons in rats.

Hair tissue mineral analysis and metabolic syndrome

Deficiency of minerals causes functional abnormality of enzymes, frequently resulting in metabolic disturbance. We investigated possible relationship between minerals and metabolic syndrome by analysis of hair tissue minerals. We selected 848 subjects older than 20 years of age at Ajou University Hospital from May 2004 to February 2007. We excluded the subjects who had cancers, steroid and thyroid medication, and incomplete record from the study. Finally, 343 subjects were eligible. We performed cross-sectional analysis for the relationship between minerals and metabolic syndrome. The contents of calcium, magnesium, and copper in the metabolic syndrome group were significantly lower than those of the normal group, whereas the amounts of sodium, potassium, and mercury in the metabolic syndrome group were significantly higher than those of the normal group. By dividing the subjects into quartile with the level of calcium, magnesium, and mercury concentrations, we carried out logistic regression analysis to study the subjects and found that the subjects in the third quartile of calcium and magnesium concentrations had significantly lower odds ratio (OR) of the metabolic syndrome compared with that of the lowest quartile group [OR = 0.30, confidence interval (CI) = 0.10-0.89; OR = 0.189, CI = 0.063-0.566] and that the subjects in the highest mercury quartile had significantly higher OR of the metabolic syndrome compared with that of the lowest mercury quartile group (OR = 7.35, CI = 1.73-31.1). As part of the metabolic syndrome, the optimal calcium and magnesium concentrations in hair tissue may reflect decreased risk of metabolic syndrome, whereas high mercury concentration in hair tissue may indicate increased risk of metabolic syndrome.

Therapeutic uses of magnesium

Magnesium is an essential mineral for optimal metabolic function. Research has shown that the mineral content of magnesium in food sources is declining, and that magnesium depletion has been detected in persons with some chronic diseases. This has led to an increased awareness of proper magnesium intake and its potential therapeutic role in a number of medical conditions. Studies have shown the effectiveness of magnesium in eclampsia and preeclampsia, arrhythmia, severe asthma, and migraine. Other areas that have shown promising results include lowering the risk of metabolic syndrome, improving glucose and insulin metabolism, relieving symptoms of dysmenorrhea, and alleviating leg cramps in women who are pregnant. The use of magnesium for constipation and dyspepsia are accepted as standard care despite limited evidence. Although it is safe in selected patients at appropriate dosages, magnesium may cause adverse effects or death at high dosages. Because magnesium is excreted renally, it should be used with caution in patients with kidney disease. Food sources of magnesium include green leafy vegetables, nuts, legumes, and whole grains.