Effects of additional Mg2+ on the growth,lipid production,and fatty acid composition of Monoraphidium sp. FXY-10 under different culture conditions

The effects of supplementing the culture medium with Mg²⁺ on the growth, lipid production, and fatty acid composition of Monoraphidium sp. FXY-10 were studied under photoautotrophic, heterotrophic, and mixotrophic conditions. Under the photoautotrophic condition, microalgae supplemented with 100 μM Mg²⁺ grew significantly better than the control group and exhibited a secondary growth state. The final cell density was 1.25-fold higher than that of the control group (2.98 g L^?1), and the peak lipid content reached 59.8 % (control group 52.3 %). Culture under the heterotrophic condition did not significantly increase the growth rate, but the experimental group (100 μM Mg²⁺ supplementation) achieved a 37.03 % lipid content compared to 28.47 % by the control group. The lipid productivity of the experimental group (100 μM Mg²⁺ supplementation) was higher, reaching 65.93 mg L^?1 day^?1 compared with 56.10 mg L^?1 day^?1 for the group without additional Mg²⁺. Under the mixotrophic condition, the experimental group achieved a final density of 3.10 g L^?1, which was higher than that of the control group (2.98 g L^?1). There was also no variation in fatty acid composition between the experimental group and the control group. Under the heterotrophic and mixotrophic conditions, the experimental group produced more than 50% saturated fatty and mono-unsaturated fatty acids, and the degree of unsaturation was <137. This result was relatively lower than that of the control.     

Dietary and physiological studies to investigate the relationship between calcium and magnesium signalling in the mammalian myocardium

This study employs both dietary and physiological studies to investigate the relationship between calcium (Ca²⁺) and magnesium (Mg²⁺) signalling in the mammalian myocardium. Rats maintained on a low Mg²⁺ diet (LMD; 39 mg Kg^-1 Mg²⁺ in food) consumed less food and grew more slowly than control rats fed on a control Mg²⁺ diet (CMD; 500 mg Kg^-1 Mg²⁺ in food). The Mg²⁺ contents of the heart and plasma were 85 ± 3% and 34 ± 6.5%, respectively relative to the control group. In contrast, Ca²⁺ contents in the heart and plasma were 177 ± 5% and 95 ± 3%. The levels of potassium (K⁺) was raised in the plasma (129 ± 16%) and slightly decreased in the heart (88 ± 6%) compared to CMD. Similarly, sodium (Na⁺) contents were slightly higher in the heart and lowered in the plasma of low Mg²⁺ diet rats compared to control Mg²⁺ diet rat. Perfusion of the isolated Langendorff's rat heart with a physiological salt solution containing low concentrations (0-0.6 mM) of extracellular magnesium [Mg²⁺]₀ resulted in a small transient increase in the amplitude of contraction compared to control [Mg²⁺]₀ (1.2 mM). In contrast, elevated [Mg²⁺]₀ (2-7.2 mM) caused a marked and progressive decrease in contractile force compared to control. In isolated ventricular myocytes the L-type Ca²⁺ current (I_Ca,L was significantly (p < 0.001) attenuated in cells dialysed with 7.1 mM Mg²⁺ compared to cells dialysed with 2.9 µM Mg²⁺. The results indicate that hypomagnesemia is associated with decrease levels of Mg²⁺ and elevated levels of Ca²⁺ in the heart and moreover, internal Mg²⁺ is able to modulate the Ca²⁺ current through the L-type Ca²⁺ channel which in turn may be involved with the regulation of contractile force in the heart.     

Some characteristics of cotton pyrophosphatase activation by magnesium

The effect of Mg²⁺ ions on inducing pyrophosphatase activity of germinating cotton (Gossypium hirsutum L.) seeds was investigated. The presence of Mg²⁺ ions in the germination medium markedly shortened time for the attainment of the pyrophosphatase maximum activity (T _max). In the absence of Mg²⁺ ions in the nutrient medium, T _max comprised 6.0–6.5 days, whereas in the presence of 3–5 mM Mg²⁺, T _max was decreased to 3–4 days. An increase in the concentration of Mg²⁺ ions in the medium up to 5 mM resulted in an increase in pyrophosphatase activity. The effect of Mg²⁺ ions on the activity of a purified pyrophosphatase preparation isolated from three-day-old cotton seedlings was investigated. Mg²⁺ ions did not affect the rate of attainment of a maximum pyrophosphatase activity, but decreased the value of the Michaelis-Menten constant.     

Direct measurement of intracellular free magnesium in frog skeletal muscle using magnesium-selective microelectrodes

Mg²⁺-selective microelectrodes have been used to measure the intracellular free Mg²⁺ concentration in frog skeletal muscle fibers. Glass capillaries with a tip diameter of less than 0.4 μm were backfilled with the Mg²⁺ sensor, ETH 1117. In the absence of interfering ions, they gave Nernstian responses between 1 and 10 mM free Mg²⁺. In the presence of an ionic environment resembling the myoplasm, the microelectrode response was sub Nernstian (18–24 mV) but still useful. The electrodes were calibrated before and after muscle-fiber impalements. In quiescent fibers from sartorius muscle (Rana pipiens), with resting membrane potentials not less than ?82 mV, the intracellular free Mg²⁺ concentration was 3.8±0.41 (S.E.) mM (n=58) at 22°C. No significant change in the intracellular free Mg²⁺ was observed following extensive (approx. 6 h) incubation in Mg²⁺-free media. Increasing the external concentration of magnesium from 4 to 20 mM (approx. 15 min) produced a slow and small enhancement (1.8 mM) of [Mg²⁺]_i, which was fully reverted when the divalent cation was removed from the bathing solution. No change in ionic magnesium resting concentration was observed when the muscle fibers were treated either with caffeine 3 mM or with Na⁺-free solutions. In depolarized muscle fibers (?23±2.7 mV) treated with 100 mM K⁺, the myoplasmic [Mg²⁺] was 3.7±0.45 (S.E.) mM, n=6, immediately after the spontaneous relaxation of the contracture. Similar determinations in muscle fibers during stimulation at low frequency (5 Hz), and after fatigue development, showed no changes in the concentration of free cytosolic Mg²⁺. These results point out that [Mg²⁺]_i is not modified under these three different experimental conditions.     

Manganese Supplementation Reduces High Glucose-induced Monocyte Adhesion to Endothelial Cells and Endothelial Dysfunction in Zucker Diabetic Fatty Rats

Endothelial dysfunction is a hallmark of increased vascular inflammation, dyslipidemia, and the development of atherosclerosis in diabetes. Previous studies have reported lower levels of Mn²⁺ in the plasma and lymphocytes of diabetic patients and in the heart and aortic tissue of patients with atherosclerosis. This study examines the hypothesis that Mn²⁺ supplementation can reduce the markers/risk factors of endothelial dysfunction in type 2 diabetes. Human umbilical vein endothelial cells (HUVECs) were cultured with or without Mn²⁺ supplementation and then exposed to high glucose (HG, 25 mm) to mimic diabetic conditions. Mn²⁺ supplementation caused a reduction in monocyte adhesion to HUVECs treated with HG or MCP-1. Mn²⁺ also inhibited ROS levels, MCP-1 secretion, and ICAM-1 up-regulation in HUVECs treated with HG. Silencing studies using siRNA against MnSOD showed that similar results were observed in MnSOD knockdown HUVECs following Mn²⁺ supplementation, suggesting that the effect of manganese on monocyte adhesion to endothelial cells is mediated by ROS and ICAM-1, but not MnSOD. To validate the relevance of our findings in vivo, Zucker diabetic fatty rats were gavaged daily with water (placebo) or MnCl₂ (16 mg/kg of body weight) for 7 weeks. When compared with placebo, Mn²⁺-supplemented rats showed lower blood levels of ICAM-1 (17%, p < 0.04), cholesterol (25%, p < 0.05), and MCP-1 (28%, p = 0.25). These in vitro and in vivo studies demonstrate that Mn²⁺ supplementation can down-regulate ICAM-1 expression and ROS independently of MnSOD, leading to a decrease in monocyte adhesion to endothelial cells, and therefore can lower the risk of endothelial dysfunction in diabetes.     

Daily magnesium supplementation on serum and urinary magnesium changes in rats during prolonged restriction of motor activity

The objective of this investigation was to determine whether a plentiful magnesium (Mg²⁺) supplementation might be used to normalize or prevent Mg deficiency. This is manifested by increased rather than decreased serum Mg²⁺ concentration as is observed during prolonged hospitalization, which is developed during prolonged hypokinesia (HK) (decreased motor activity). Eighty male Wistar rats with an initial body weight of 370–390 g were used to perform the studies: They were equally divided into four groups:

1. Unsupplemented control animals (UCA);
2. Supplemented control animals (SCA);
3. Unsupplemented hypokinetic animals (UHA); and
4. Supplemented hypokinetic animals (SHA).

For the simulation of the hypokinetic effect, the hypokinetic animals were kept in small individual cages made of wood, which restricted their movements in all directions without hindering food and water intake. The control and hypokinetic supplemental animals receive 0.9 mg/mL Mg sulfate daily with their drinking water. Prior to and during the experimental period, urinary excretions of Mg, calcium, and phosphate along with their concentrations in serum, water intake, and urine excretion, and body weight were determined in the control and hypokinetic animals. In the supplemental and unsupplemental hypokinetic rats, urinary excretions and serum concentrations of electrolytes increased significantly, whereas serum concentration and urinary excretion thereof remained unchanged in the supplemented and unsupplemented control animals. It was concluded that a daily intake of large amounts of Mg supplementation cannot be used to prevent or normalize Mg deficiency in rats during prolonged exposure to HK.     

Leaf magnesium alters photosynthetic response to low water potentials in sunflower

We grew sunflower (Helianthus annuus L.) plants in nutrient solutions having nutritionally adequate but low or high Mg²⁺ concentrations and determined whether photosynthesis was effected as leaf water potentials (ψ_w) decreased. Leaf Mg contents were 3- to 4-fold higher in the plants grown in high Mg²⁺ concentrations (10 millimolar) than in those grown in low concentrations (0.25 millimolar). These contents were sufficient to support maximum growth, plant dry weight, and photosynthesis, and the plants appeared normal. As low ψ_w developed, photosynthesis was inhibited but moreso in high Mg leaves than in low Mg leaves. The effect was particularly apparent under conditions of light- and CO₂-saturation, indicating that the chloroplast capacity to fix CO₂ was altered. The differential inhibition observed in leaves of differing Mg contents was not observed in leaves having differing K contents, suggesting that the effect may have been specific for Mg. Because Mg²⁺ inhibits photophosphorylation and coupling factor activities at concentrations likely to occur as leaves dehydrate, Mg may play a role in the inhibition of chloroplast reactions at low ψ_w, especially in leaves such as sunflower that markedly decrease in water content as ψ_w decreases.     

Absorption of magnesium and chloride by excised corn root

Absorption characteristics of Mg²⁺ and Cl⁻ were investigated with 5-day-old excised corn (Zea mays) roots. Uptake from both 0.5 and 10 milliequivalents per liter MgCl₂ solutions occurred at steady state rates for the first 6 hours. Inhibition by dinitrophenol and low temperatures established that absorption during this period was metabolically mediated in the absence and presence of Ca²⁺. Absorption isotherms indicated dual mechanisms of Mg²⁺ and Cl⁻ absorption from solutions above 1 milliequivalent per liter. The effect of H⁺ on absorption of Mg²⁺ and Cl⁻ was typical of that generally reported for other plant roots and other ions. In the physiological pH range, Ca²⁺ greatly suppressed the rate of Mg²⁺ absorption but had little effect on Cl⁻. The influence of Ca²⁺ on Mg²⁺ appeared to be noncompetitive and independent of its effect on membrane permeability.     

Nitrate absorption and assimilation in ryegrass as influenced by calcium and magnesium

The absorption and assimilation patterns of ¹⁵NO₃⁻ supplied as the Ca²⁺ and Mg²⁺ salts to intact ryegrass (Lolium perenne) seedlings were compared. No statistically significant effect of ambient cation on the amounts of ¹⁵NO₃⁻ absorbed was observed in the initial six hours, but during the subsequent six hours, absorption from Ca(¹⁵NO₃)₂ exceeded that from Mg (¹⁵NO₃)₂.     

Use of cytosolic metabolite patterns to estimate free magnesium in normoxic myocardium

Cytosolic free magnesium (Mg_f) is considered relatively constant. To test this concept, Mg_f was estimated during hyperkalemic ventricular akinesis, normal and maximum adrenergic stimulation, and sulfate loading of the normoxic perfused guinea-pig heart. The Mg_f estimates utilized a new sliding scale derived from the Mg²⁺-dependence of glyceraldehyde-3-phosphate dehydrogenase/phosphoglycerate kinase (GAPDH/PGK). The pseudo constant K_GAPDH′·K_PGK′ was measured as ([creatine phosphate][3-phosphoglycerate][lactate]K_LDH/([creatine][P_i[glyceraldehyde 3-phosphate][pyruvate]K_CK), which varied with magnesium due to K_CK (CK, LDH = creatine kinase, lactate dehydrogenase). However, the correct magnesium dependencies of the true constants K_GAPDH·K_PGK and K_CK were taken from the literature. The [Mg²⁺] at which pseudo K_GAPDH′·K_PGK′ equalled true K_GAPDH·K_PGK was the best estimate of Mg_f. Mg_f fell to ≈0.13 mM in hyperkalemic arrest from a control of ≈0.6 mM, rising to ≈0.85 mM only during maximum adrenergic stress. Mg_f increased further to ≈1.3 mM during sulfate loading which induced ATP catabolism. Mg_f and ATP were reciprocally related. Thus; (1) myocardial free [Mg²⁺] judged from GAPDH/PGK mass-action relations changed appreciably only under extreme physiological states; (2) ATP was a major chelator of Mg²⁺ in perfused myocardium, i.e., acute ATP pool size reduction may be associated with increments in Mg_f.     

The role of magnesium in regulating CCK-8-evoked secretory responses in the exocrine rat pancreas

This study investigates the effect of magnesium (Mg²⁺) on the secretory responses and the mobilization of calcium (Ca²⁺) and Mg²⁺ evoked by cholecystokinin-octapeptide (CCK-8) in the exocrine rat pancreas. In the isolated intact perfused pancreas CCK-8 (10^–10 M) produced marked increases in juice flow and total protein output in zero and normal (1.1 mM) extracellular Mg²⁺ [Mg²⁺]_o compared to a much reduced secretory response in elevated (5 mM and 10 mM) [Mg²⁺]_o Similar effects of perturbation of [Mg²⁺]_o on amylase secretion and ⁴⁵Ca²⁺ uptake (influx) were obtained in isolated pancreatic segments. In pancreatic acinar cells loaded with the fluorescent bioprobe fura-2 acetomethylester (AM), CCK-8 evoked marked increases in cytosolic free Ca²⁺ concentration [Ca²⁺]_i in zero and normal [Mg²⁺]_o compared to a much reduced response in elevated [Mg²⁺]_o Pretreatment of acinar cells with either dibutyryl cyclic AMP (DB₂ cAMP) or forskolin had no effect on the CCK-8 induced changes in [Ca²⁺]_i. In magfura-2-loaded acinar cells CCK-8 (10^–8 M) stimulated an initial transient rise in intracellular free Mg²⁺ concentration [Mg²⁺]_i followed by a more prolonged and sustained decrease. This response was abolished when sodium Na⁺ was replaced with N-methyl-D-glucamine (NMDG). Incubation of acinar cells with 10 mM Mg²⁺ resulted in an elevation in [Mg²⁺]_i. Upon stimulation with CCK-8, [Mg²⁺]_i. decreased only slightly compared with the response obtained in normal [Mg²⁺]_o. CCK-8 caused a net efflux of Mg²⁺ in pancreatic segments; this effect was abolished when extracellular sodium [Na⁺]_o was replaced with either NMDG or choline. The results indicate that Mg²⁺ can regulate CCK-8-evoked secretory responses in the exocrine pancreas possibly via Ca²⁺ mobilization. Moreover, the movement of Mg²⁺ in pancreatic acinar cells is dependent upon extracellular Na⁺.     

Light-induced slow changes in chlorophyll a fluorescence in isolated chloroplasts: Effects of magnesium and phenazine methosulfate

We have investigated the possible relationships between the cation-induced and phenazine methosulfate (PMS)-induced fluorescence changes and their relation to light induced conformational changes of the thylakoid membrane.1. In isolated chloroplasts, PMS markedly lowers the quantum yield of chlorophyll a fluorescence (φ_f) when added either in the presence or the absence of dichloro-phenyldimethylurea (DCMU). In contrast, Mg²⁺ causes an increase in φ_f. However, these effects are absent in isolated chloroplasts fixed with glutaraldehyde that retain (to a large extent) the ability to pump protons, suggesting that structural alteration of the membrane—not the pH changes—is required for the observed changes in φ_f. The PMS triggered decrease in φ_f is not accompanied by any changes in the emission (spectral) characteristics of the two pigment systems, whereas room temperature emission spectra with Mg²⁺ and Ca²⁺ show that there is a relative increase of System II to System I fluorescence.2. Washing isolated chloroplasts with 0.75 mM EDTA eliminates (to a large extent) the PMS-induced quenching and Mg²⁺-induced increase of φ_f, and these effects are not recovered by the further addition of dicyclohexyl carbodiimide. It is known that washing with EDTA removes the coupling factor, and thus, it seems that the coupling factor is (indirectly) involved in conformational change of thylakoid membranes leading to fluorescence yield changes.3. In purified pigment System II particles, neither PMS nor Mg²⁺ causes any change in φ_f. Our data, taken together with those of the others, suggest that a structural modification of the thylakoid membranes (not macroscopic volume changes of the chloroplasts) containing both Photosystems I and II is necessary for the PMS-induced quenching and Mg²⁺-induced increase of φ_f. These two effects can be explained with the assumption that the PMS effect is due to an increase in the rate of internal conversion (k_h), whereas the Mg²⁺ effect is due to a decrease in the rate of energy transfer (k_t), between the two photosystems.4. From the relative ratio of φ_f with DCMU and DCMU plus Mg²⁺, we have calculated k_t (the rate constant of energy transfer between Photosystems II and I to be 4.2·10⁸ s^?1, and φ_t (quantum yield of this transfer) to be 0.12.     

Independent effects of magnesium ions on energy-transfer processes in chloroplasts

The effect of removal of Mg²⁺ on the fluorescence properties of LHCP-PS-II has been examined by different methods: (a) by titration with the artificial quenchers of chlorophyll fluorescence, m-dinitrobenzene and DBMIB (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone); (b) as a function of wavelengths absorbed preferentially by LHCP, compared with wavelengths relatively enriched in PS II absorbed light; (c) by measurement of the fluorescence induction parameters as a function of the Mg²⁺ concentration or the excitation wavelength (i.e., light absorbed preferentially by LHCP or relatively enriched in PS II absorbed wavelengths). The following conclusions are drawn. (a) In the presence of magnesium ions, energy-transfer coupling between LHCP and PS II is tight, which argues against the idea of a weakly coupled population of LHCP molecules. (b) On lowering the Mg²⁺ concentration of a chloroplast suspension: (1) the increased spillover of energy to PS-I involves virtually all LHCP-PS-II entities and not just a part, which is strongly quenched; (2) there is a decrease in LHCP-PS-II energy-transfer coupling and this occurs only at low Mg²⁺ concentrations (below 0.5 mM). This process therefore seems distinct from the spillover interaction; (3) the rate constant for energy transfer to PS-II reaction centers decreases and this seems independent of the decreased LHCP-PS-II energy coupling.     

High concentration magnesium inhibits extracellular matrix calcification and protects articular cartilage via Erk/autophagy pathway

The significant cytopathological changes of osteoarthritis are chondrocyte hypertrophy, proteoglycan loss, extracellular matrix (ECM) calcification, and terminally, the replacement of cartilage by bone. Meanwhile, magnesium ion (Mg²⁺), as the second most abundant divalent cation in the human body, has been proved to inhibit the ECM calcification of hBMSCs (human bone marrow stromal cells), hVSMCs (Human vascular smooth muscle cells), and TDSCs (tendon-derived stem cells) in vitro studies. The ATDC5 cell line, which holds chondrocyte characteristics, was used in this study as an in vitro subject. We found that Mg²⁺ can efficiently suppress the ECM calcification and downregulate both hypertrophy and matrix metalloproteinase-related genes. Meanwhile, Mg²⁺ inhibits the formation of autophagy by inhibiting Erk phosphorylation signaling and lowers the expression of LC3, and eventually effectively reduces the formation of ECM calcification in vitro. In this study, we also used destabilization of the medial meniscus (DMM)-induced osteoarthritis (OA) animal model to further confirm the protective effect of Mg²⁺ on articular cartilage. Compared with the control group (saline-injected), continuous intra-articular magnesium chloride (MgCl₂) injection can significantly alleviate the severity of cartilage calcification in OA animal model. Immunofluorescence staining also revealed that saline-injected DMM group had a higher positive rate of LC3 expression in cartilage chondrocytes, compared with MgCl₂-injected DMM group. In general, Mg²⁺ can significantly downregulate the hypertrophic gene Runx2, MMP13, and Col10α1, upregulate the chondrogenic genes Sox9 and Col1α1, inhibit the Erk phosphorylation signaling, reduce the expression of autophagy protein LC3, and effectively inhibit the ECM calcification of ATDC5. In vivo study also proved that intra-articular injection of Mg²⁺ protected knee cartilage by inhibiting the autophagy formation.     

Protective role of magnesium in cardiovascular diseases: A review

A considerable number of experimental, epidemiological and clinical studies are now available which point to an important role of Mg²⁺ in the etiology of cardiovascular pathology. In human subjects, hypomagnesemia is often associated with an imbalance of electrolytes such as Na⁺, K⁺ and Ca²⁺. Abnormal dietary deficiency of Mg²⁺ as well as abnormalities in Mg²⁺ metabolism play important roles in different types of heart diseases such as ischemic heart disease, congestive heart failure, sudden cardiac death, atheroscelerosis, a number of cardiac arrhythmias and ventricular complications in diabetes mellitus. Mg²⁺ deficiency results in progressive vasoconstriction of the coronary vessels leading to a marked reduction in oxygen and nutrient delivery to the cardiac myocytes.Numerous experimental and clinical data have suggested that Mg²⁺ deficiency can induce elevation of intracellular Ca²⁺ concentrations, formation of oxygen radicals, proinflammatory agents and growth factors and changes in membrane permeability and transport processes in cardiac cells. The opposing effects of Mg²⁺ and Ca²⁺ on myocardial contractility may be due to the competition between Mg²⁺ and Ca²⁺ for the same binding sites on key myocardial contractile proteins such as troponin C, myosin and actin.Stimulants, for example, catecholamines can evoke marked Mg²⁺ efflux which appears to be associated with a concomitant increase in the force of contraction of the heart. It has been suggested that Mg²⁺ efflux may be linked to the Ca²⁺ signalling pathway. Depletion of Mg²⁺ by alcohol in cardiac cells causes an increase in intracellular Ca²⁺, leading to coronary artery vasospasm, arrhythmias, ischemic damage and cardiac failure. Hypomagnesemia is commonly associated with hypokalemia and occurs in patients with hypertension or myocardial infarction as well as in chronic alcoholism.The inability of the senescent myocardium to respond to ischemic stress could be due to several reasons. Mg²⁺ supplemented K⁺ cardioplegia modulates Ca²⁺ accumulation and is directly involved in the mechanisms leading to enhanced post ischemic functional recovery in the aged myocardium following ischemia. While many of these mechanisms remain controversial and in some cases speculative, the beneficial effects related to consequences of Mg²⁺ supplementation are apparent. Further research are needed for the incorporation of these findings toward the development of novel myocardial protective role of Mg²⁺ to reduce morbidity and mortality of patients suffering from a variety of cardiac diseases.     

Aragonite crystallization in a Christmas-tree model microfluidic device with the addition of magnesium ions

Aragonite is an important dimorph of calcium carbonate, industrially and biologically. However, aragonite is so thermodynamically unstable that it is difficult to understand its formation mechanism. A continuous microfluidic system was employed, in which crystallization was induced only by diffusion in a micron-scale channel. Calcium carbonate (CaCO₃) formed by liquid-liquid reaction and magnesium ions (Mg²⁺) were used as additives. To assess the influence of Mg²⁺ concentration, the Mg²⁺/Ca²⁺ molar ratio was set to 1, 3, and 5. Laminar streams flowed in the detection channel with different concentration gradients. The initial crystallization time (t_I.C) increased exponentially and the density of crystals decreased as the Mg²⁺ ion concentration increased. Following transformation of all particles into snowman or sphere shapes, they became spinose sphere-shaped crystals, which was the final form in this study.     

Analysis of chlorophyll fluorescence induction curves in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea as a function of magnesium concentration and NADPH-activated light-harvesting chlorophyll ab-protein phosphorylation

The effect of Mg²⁺ concentration and phosphorylation of light-harvesting chlorophyll $\text{a}\text{b-}\text{protein}$ on various chlorophyll fluorescence induction parameters of isolated pea thylakoids has been studied. (1) Lowering the Mg²⁺ concentration from 3 to 0.4 mM decreases only the variable fluorescence (F_v) and the area above the induction curve while at the same time increasing the slow exponential component of the rise (β_max). (2) A further decrease in Mg²⁺ concentration from 0.4 to 0 mM decreases the initial (F₀) fluorescence level such that the ratio $\text{F}{}_{\mathrm{<mtext>v</mtext>}}\text{F}{}_{\mathrm{<mtext>m</mtext>}}$ increases slightly as does the area above the induction curve and β_max. (3) Thylakoid membranes, phosphorylated at 5 mM Mg²⁺, show an equal decrease in F_v and F₀, no change in the area above the induction curve and an increase in β_max. At 2 mM Mg²⁺, however, phosphorylation induced a more extensive quenching of F_v so that the $\text{F}{}_{\mathrm{<mtext>v</mtext>}}\text{F}{}_{\mathrm{<mtext>m</mtext>}}$ ratio was lowered and the area above the induction curve decreased while β_max increased. (4) When phosphorylated membranes were subsequently suspended in an Mg²⁺-free medium the effect on F₀ due to phosphorylation was found to be additive to that due to the absence of Mg²⁺. The effect of membrane phosphorylation on fluorescence is discussed in relation to the control of excitation energy distribution and shows that different mechanisms operate depending on the background Mg²⁺ levels. At high Mg²⁺ the phosphorylation seems to affect the absorption cross-section of Photosystem II while at lower Mg²⁺ levels there is an additional effect of increased spillover from Photosystem II to I.     

Reconstitution of the energy-linked transhydrogenase activity in membranes from a mutant strain of Escherichia coli K12 lacking magnesium ion- or calcium ion-stimulated adenosine triphosphatase

1. We have isolated a mutant of Escherichia coli K12 (strain AN295) that forms de-repressed amounts of Mg²⁺,Ca²⁺-stimulated adenosine triphosphatase. 2. The Mg²⁺,Ca²⁺-stimulated triphosphatase activity was separated from membrane preparations from strain AN295 by extraction with 5mm-Tris–HCl buffer containing EDTA and dithiothreitol, resulting in a loss of the ATP-dependent transhydrogenase activity. The non-energy-linked transhydrogenase activity remained in the membrane residue. 3. The solubilized Mg²⁺,Ca²⁺-stimulated adenosine triphosphatase activity from strain AN295 was partially purified by repeated gel filtration. The addition of the purified Mg²⁺,Ca²⁺-stimulated adenosine triphosphatase to the membrane residue from strain AN295 reactivated the ATP-dependent transhydrogenase activity. 4. Strain AN296, lacking Mg²⁺,Ca²⁺-stimulated adenosine triphosphatase activity, was derived by transducing the mutant allele, uncA401, into strain AN295. The ATP-dependent transhydrogenase activity was lost but the non-energy linked transhydrogenase was retained. 5. The ATP-dependent transhydrogenase activity in membrane preparations from strain AN296 (uncA⁻) could not be re-activated by the purified Mg²⁺,Ca²⁺-stimulated adenosine triphosphatase from strain AN295. However, after extraction by 5mm-Tris–HCl buffer containing EDTA and dithiothreitol, the ATP-dependent transhydrogenase activity could be re-activated by the addition of the purified Mg²⁺,Ca²⁺-stimulated adenosine triphosphatase from strain AN295 to the membrane residue from strain AN296 (uncA⁻).     

Effect of photosynthetic intermediates on the magnesium inhibition of oxygen evolution by barley chloroplasts

Millimolar concentrations of Mg²⁺ inhibited CO₂-dependent O₂ evolution by barley (Hordeum vulgare L.) chloroplasts and also prevented the activation of NADP-glyceraldehyde-3-phosphate dehydrogenase, ribulose-5-phosphate kinase, and fructose-1,6-diphosphatase by light in intact chloroplasts. When added in the dark, 3-phosphoglycerate prevented the inhibition of O₂ evolution by Mg²⁺ and reduced the Mg²⁺ inhibition of enzyme activation by light. Fructose 1,6-diphosphate and ribulose 5-phosphate also prevented the inhibition of O₂ evolution by Mg²⁺ whereas glucose 1-phosphate, glucose 6-phosphate, ribulose 1,5-diphosphate, and citrate had no effect. Phosphoenolpyruvate gave an intermediate response. Metabolites that prevented the Mg²⁺ inhibition of O₂ evolution shortened the lag phase of CO₂-dependent O₂ evolution in the absence of M²⁺. Loading chloroplasts in the dark with 3-phosphoglycerate reduced both the lag phase of O₂ evolution and the inhibition of O₂ evolution by Mg²⁺. The results suggested that Mg²⁺ inhibition was lessened either by external metabolites that compete with inorganic phosphate for transport into the chloroplast or by a high concentration of internal metabolites.     

Regulation of electron transport in Photosystem-II fragments by magnesium ions

In Photosystem-II reaction-center particles (TSF-IIa) fractionated from spinach chloroplasts by Triton X-100 treatment, divalent cations appear to regulate electron-transport reactions. Oxidation of cytochrome b-559 after illumination of the particles was accelerated by the presence of Mg²⁺, whereas photoreduction of 2,6-dichlorophenolindophenol (DCIP) by diphenyl carbazide was inhibited, both at a half-effective concentration of Mg²⁺ of approx. 0.1 mM.The site of regulation was shown to be on the oxidizing side of Photosystem II, near P-680, based on the effects of actinic-light intensity and nature of the electron donors on DCIP photoreduction. Mg²⁺ was effective in quenching chlorophyll fluorescence in TSF-IIa particles, but the quenching was sensitive to the presence of 3(3,4-dichloropheny)-1,1-dimethylurea. In the reactioncenter (core) complex of Photosystem II, where the light-harvesting chlorophyll-protein complex is absent, there seems to be no regulation by Mg²⁺ on excitation-energy distribution.