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Chapter 6. Magnesium
As the eleventh most abundant mineral in the body, magnesium is one of the more ubiquitous minerals, occurring in every cell and organ, but like calcium it has its largest deposit in bone and muscle. Its multitude of functions comprise a wide range of biological processes, most of which center on its use as an enzyme cofactor targeted to substrates containing phosphate groups.
History and Early Insights
The word magnesium comes from the Greek Magnesia, referring to a district in the area of Thessaly in the southeastern part of central Greece. This same region is credited with the discovery of magnetite and manganese. Magnesium’s importance in biology drew attention when the sulfate salt of magnesium was shown to be the active factor in Epson salts, which at the time had been noted for healing properties. A greater insight into biological significance, however, came when magnesium was shown to be one of the more abundant mineral in sea waters, the caldron of life.
Chemical Properties
Magnesium is an alkaline earth metal sitting just above calcium in the periodic table of elements. It has an atomic number is 12 and atomic weight of 24. Like calcium, its most stable oxidation state is +2 which draws attention to a possible overlap in properties of Mg2+ and Ca2+. Being only two-thirds the size of calcium, however, magnesium ions are distinct from calcium in certain properties, most notably the binding strength to proteins and other biological ligands. Mg2+ behaves mainly as a spherical ion with little tendency to share electron or enter into coordinate covalent bonding. The absence of 3 d electrons makes complexes of Mg2+ colorless.
Solubility considerations
The salts of magnesium for the most part are readily soluble in water. This property allows Mg2+ to exist as a freely diffusible cation inside cells. A mark of Mg2+ solubility is noted by its concentration in sea water (50 mM); fresh water has a magnesium concentration of less than 0.1 mM. The cytosolic concentration of magnesium approaches 1 mM, a concentration that attests to its prevalence inside cells, but because its high concentration within also explains why Mg2+ is unable to form a gradient across the cell membrane.
Biochemical Properties
Most of the attention on magnesium focuses on its role as a cofactor for kinase enzymes, a family of enzymes that use ATP as a source of phosphate in energy-transfer reactions in cells. A familiar example is hexokinase, the enzyme that transfers a phosphate
group from ATP to glucose or other monosaccharides as a first step in the glycolysis pathway. A subclass of kinases, the protein kinases, target proteins as receivers of phosphate groups. The protein kinases are particularly noteworthy since these enzymes are subject to regulation by cell signaling agents and hormones. Their action is testament to varied roles of magnesium at the biochemical level of cell function.
Because of a strongly intense 2+ charge, magnesium ions would be predicted to engage primarily in electronstatic bonding with negative ions where charge is a primary determinant of bonding strength. There are very few instances in biology can engage in covalent bonding. The most notable example is covalently-bound magnesium in the ring structure of chlorophyll. Selection of Mg2+ for this role is uncertain, but its smaller size and lack of redox properties fit with its role in the light-capture property of chlorophyll. Other biochemical processes that rely on magnesium are the synthesis of cholesterol, where Mg2+ ions not only assist in forming the pyrophosphate bond from Mg-ATP in cholesterol precursors, but also in subsequent condensations reaction that give rise to the familiar ring structure of cholesterol. Magnesium is also a prominent in the nucleus and cytosol where it serves as a crosslinking ion that stabilizes DNA and RNA. It is also instrumental to the stability and overall architecture of the cell membrane, a property it shares with calcium and to a lesser extent zinc. Stabilization is brought about by acting as a bridging ion between phosphorylated and carboxylated molecules.
Enzyme Cofactors Table X.1. Non-structural roles of calcium in biology
Passive Functions
Enzyme cofactor (lipases, hydrolases) Blood clotting cascade
Active Functions
Relaxation and constriction of blood vessels Cell aggregation Muscle protein contraction Cellular protein turnover Hormone secretion Nervous impulse transmission Intracellular trafficking Cell signaling Genetic expression Apoptosis Table x.4. Enzymes dependent on calcium for function
Enzyme Location Function
Phospholipase A2 intestine acylphospholipid hydrolysis Lipase intestine triacylglycerol hydrolysis Thermolysin bacteria protein hydrolysis Trypsin duodenum protein digestion
Diet and Bioavailability Absorption efficiency for magnesium is very high, ranging from near 50-90 percent from human milk and formula to around 50 percent for solid food. The high solubility of magnesium in water and the failure to form strong complexes with proteins and other macromolecules in the food source is perhaps one reason. Two notable exceptions are foods high in fiber that contain phytic acid. This combined with diets rich in phosphate are a major deterrent to magnesium absorption. As noted previously, phosphate salts and organic phosphate compounds have a strong attraction for divalent cations such as Ca2+ and Zn2+. Magnesium (Mg2+) does not escape this attraction.
Dietary Supplements A typical adult supplement for magnesium is around 100 mg/day with women tending to take more than men. For children, the number is around 25 mg/day. Most of the supplement is MgO, which accounts for nearly 60 percent by weight. The order of occurrence in supplements is MgO> MgCO3> Mg(OH)2 > Mg-citrate >Mg-lactate >MgCl2>MgSO4.
Upper Limit Based on milk alkali syndrome which is a measure of renal insufficiency, there is no upper limit for magnesium in food and water. For magnesium in the form of supplements or fortified foods, the upper limit for children 1-3 years is 65 mg/day; for 4-8, 100 mg/day and for adults 350 mg/day.
Digestion and Absorption
Digestion With the exception of a chlorophyll-magnesium complex in green leafy vegetables, magnesium in foods is generally present as a weak complex with other food molecules. This allows the magnesium to readily dissociate and enter the system as a soluble cation. As with calcium, salivary amylases combined with mastication assist in the release of magnesium ions from glycogen and starch, although a greater portion is liberated by amylases in the proximal intestine. Proteases in the stomach and duodenum also liberate magnesium as does the highly acidic gastric juice environment of the stomach.
Table x.2. Food Sources of Magnesium
Richest: (500 milligrams/1000 grams) Green leafy vegetables Wheat-soy flower Shell fish Middle Milk Banana Lowest White flour (refined)
Absorption On first impression one may consider the absorption of magnesium to mimic calcium. Such is not the case, however. Like calcium, magnesium is absorbed all along the intestinal tract with the ileum and colon, not the jejunum, being more active. Like calcium, magnesium employs both active (energy-dependent) mediated and passive (diffusion-driven) paracellular transport. Active is characterized by saturation with increased intake. Generally the response is curvilinear, suggesting saturation at the higher levels of intake and diffusion at the lower. Diffusion, however, only accounts for 7-10% of the magnesium taken in. This signals involvement of mediating factors that oversee the movement across the membrane and through the cell. Unique to magnesium that was not seen with either sodium or potassium is a lowering of the fractional absorption with increasing amounts in the diet. For example, raising the amount taken in from 7 mg to 36 mg lowered the fractional absorption from 65-75% to 11- 14%. Unlike Na+^ and K+, Mg2+^ absorption is clearly subject to regulation at the absorption stage and mimics calcium. The latter statement is true up to the point where absorption becomes dependent on calcitriol the active form of vitamin D3. Although calcitriol causes a slight increase in the amount of magnesium absorbed, no data to support a calcitriol-dependent mechanism for magnesium absorption.
Even though calcium ions are nearly twice the size of magnesium, calcium still has the potential to impede the movement of magnesium into the absorbing enterocytes. That observation infers a common carrier or entry portal for the two divalent cations. Long term studies, however, dismiss interference between the two, but in the short term the fractional uptake of Mg2+^ is clearly dependent on Ca2+. Nonetheless, a calcium intake must exceed 2, mg/day, i.e., nearly two and one-half times the RDA, to disrupt calcium balance. Phosphorus as phosphate likewise interferes with Mg2+^ absorption. As noted for alkaline earth metal ions in general, this group of metal ions is strongly attracted to phosphate either as the free ion or as a organophosphate complex. Conversely, magnesium inversely affects phosphate and to a lesser extent the absorption of calcium.
Competing ions are not the only deterrent. A major deterrent to absorption is phytic acid from foods rich in fiber in the diet. This compound with its six open phosphate groups has a strong potential to bind magnesium and prevent is transluminal movement.
Interaction with other Minerals
Magnesium/Calcium
Interactions in Muscle Contraction
The series of reactions that are part of the muscle contraction-relaxation cycle are an excellent example of the biological synergism between Mg2+ and Ca2+. Although Mg-ATP bound to the actin-myosin complex provides the energy for the contraction, the contraction event itself is triggered by Ca2+ bound to troponin C or calmodulin. A second role for Mg2+ comes after the contraction, when it is necessary to restore the relaxation state by removing Ca2+. Removal is accomplished by both activating a calcium pump that uses Mg-ATP for energy
Difficulty breathing Metabolic alkalosis Hypokalemia Paralytic ileus Low blood pressure
SUMMARY
Although magnesium bears a close chemical identity with calcium, the two macrominerals are far apart in the roles played in living systems. Magnesium is a companion to phosphate and readily forms complexes with ATP and nucleic acids in general. It is a found in all cells in fairly high amounts but is notable low in blood. Muscle contraction critically depends on the magnesium content.
