Magnesium in biology
Magnesium is an essential element in biological systems. Magnesium occurs typically as the Mg2+ ion. It is an essential mineral nutrient (i.e., element) for life[1][2][3][4] and is present in every cell type in every organism. For example, adenosine triphosphate (ATP), the main source of energy in cells, must bind to a magnesium ion in order to be biologically active. What is called ATP is often actually Mg-ATP.[5] As such, magnesium plays a role in the stability of all polyphosphate compounds in the cells, including those associated with the synthesis of DNA and RNA.
Over 300 enzymes require the presence of magnesium ions for their catalytic action, including all enzymes utilizing or synthesizing ATP, or those that use other nucleotides to synthesize DNA and RNA.[6]
In plants, magnesium is necessary for synthesis of chlorophyll and photosynthesis.
Measuring magnesium in biological samples[edit]
By radioactive isotopes[edit]
The use of radioactive tracer elements in ion uptake assays allows the calculation of km, Ki and Vmax and determines the initial change in the ion content of the cells. 28Mg decays by the emission of a high-energy beta or gamma particle, which can be measured using a scintillation counter. However, the radioactive half-life of 28Mg, the most stable of the radioactive magnesium isotopes, is only 21 hours. This severely restricts the experiments involving the nuclide. Also, since 1990, no facility has routinely produced 28Mg, and the price per mCi is now predicted to be approximately US$30,000.[66] The chemical nature of Mg2+ is such that it is closely approximated by few other cations.[67] However, Co2+, Mn2+ and Ni2+ have been used successfully to mimic the properties of Mg2+ in some enzyme reactions, and radioactive forms of these elements have been employed successfully in cation transport studies. The difficulty of using metal ion replacement in the study of enzyme function is that the relationship between the enzyme activities with the replacement ion compared to the original is very difficult to ascertain.[67]
By fluorescent indicators[edit]
A number of chelators of divalent cations have different fluorescence spectra in the bound and unbound states.[68] Chelators for Ca2+ are well established, have high affinity for the cation, and low interference from other ions. Mg2+ chelators lag behind and the major fluorescence dye for Mg2+ (mag-fura 2[69]) actually has a higher affinity for Ca2+.[70] This limits the application of this dye to cell types where the resting level of Ca2+ is < 1 μM and does not vary with the experimental conditions under which Mg2+ is to be measured. Recently, Otten et al. (2001) have described work into a new class of compounds that may prove more useful, having significantly better binding affinities for Mg2+.[71] The use of the fluorescent dyes is limited to measuring the free Mg2+. If the ion concentration is buffered by the cell by chelation or removal to subcellular compartments, the measured rate of uptake will give only minimum values of km and Vmax.
By electrophysiology[edit]
First, ion-specific microelectrodes can be used to measure the internal free ion concentration of cells and organelles. The major advantages are that readings can be made from cells over relatively long periods of time, and that unlike dyes very little extra ion buffering capacity is added to the cells.[72]
Second, the technique of two-electrode voltage-clamp allows the direct measurement of the ion flux across the membrane of a cell.[73] The membrane is held at an electric potential and the responding current is measured. All ions passing across the membrane contribute to the measured current.
Third, the technique of patch-clamp uses isolated sections of natural or artificial membrane in much the same manner as voltage-clamp but without the secondary effects of a cellular system. Under ideal conditions the conductance of individual channels can be quantified. This methodology gives the most direct measurement of the action of ion channels.[73]
By absorption spectroscopy[edit]
Flame atomic absorption spectroscopy (AAS) determines the total magnesium content of a biological sample.[68] This method is destructive; biological samples must be broken down in concentrated acids to avoid clogging the fine nebulising apparatus. Beyond this, the only limitation is that samples must be in a volume of approximately 2 mL and at a concentration range of 0.1 – 0.4 μmol/L for optimum accuracy. As this technique cannot distinguish between Mg2+ already present in the cell and that taken up during the experiment, only content not uptaken can be quantified.
Inductively coupled plasma (ICP) using either the mass spectrometry (MS) or atomic emission spectroscopy (AES) modifications also allows the determination of the total ion content of biological samples.[74] These techniques are more sensitive than flame AAS and are capable of measuring the quantities of multiple ions simultaneously. However, they are also significantly more expensive.