Magnesium-activated ATPase

P-gp associated ATPase is vanadate sensitive. A membrane product prepared from baculovirus infected insect cells containing this activity is now commercially available from Gentest Corp. (Woburn, Massachusetts, U.S.). Substrates of P-gp, such as verapamil, have been demonstrated to stimulate this vanadate-sensitive membrane ATPase (123). By determination of inorganic phosphate liberated in the reaction containing a P-gp preparation and a test compound, in the presence and absence of vanadate, one can determine if the test compound is a substrate/inhibitor of P-gp (123,422). Any compound that binds to P-gp would stimulate the magnesium-dependent ATPase, and thus, this method cannot distinguish between a substrate and inhibitor of P-gp. 

The enzymes called ATP phosphohydrolase are widely distributed in the evolutionary chain and in biological systems. In some cases the ATPase is activated either by magnesium (Mg2+ ATPase) or by calcium (Ca2+ ATPase), and in other cases by both calcium and magnesium (Ca2+ Mg2+ ATPase). Another class of ATPase is stimulated by sodium and potassium and is inhibited by ouabain being denominated Na+ K+ ATPase. There are some ATPases that hydrolyze other nucleotides than ATP, however, with a high preference for ATP. 

The microsomal fraction contained a magnesium-dependent ATPase different from all other ATPases. The enzyme was not stimulated by Na or by HCOf, nor was it inhibited by ouabain or SCN". Instead, it was stimulated by K, and it was always described as the K -stimulated ATPase. The microsomes also catalyzed the hydrolysis of / nitrophenylphosphate, and because that activity was easier to measure than ATPase activity, purification of the fragments was often followed by the former rather than by the latter catalytic power. Some thought the catalysis of the two substrates was the property of one enzyme. ... 

Edelfors S, Raven-Jonsen A. 1992. Effect of organic solvents on nervous cell membrane as measured by changes in the calcium magnesium ATPase activity and fluidity of synaptosomal membrane. Pharmacology and Toxicology 70(3) 181-187. 

In binding experiments, the affinity of magnesium ADP to native membranes and to the isolated calcium dependent ATPase was found to be considerably lower than that of magnesium ATP173. On the other hand, from the inhibition of the calcium-dependent ATPase or the activation of calcium release and ATP synthesis apparent affinities for ADP are obtained that are very similar to those of ATP (Fig. 12). The affinity of ADP for the enzyme apparently depends on its functional state. The affinity of ADP for the membranes under conditions of calcium release depends markedly on the pH of the medium. When the medium pH is reduced from 7.0 to 6.0, the affinity drops by a factor of 10. At pH 7.0 the affinity of the membrane for ADP corresponds to the affinity for ATP to the high affinity binding sites in the forward running mode of the pump. In contrast to the complex dependence of the forward reaction on the concentration of ATP, the dependence of the reverse reaction on ADP seems to follow simple Michaelis-Menten kinetics. 

Fig. 7. Influence of magnesium ion concentration on ATPase activity of thrombosthenin at low (left) and high ionic strength (right). ATP, 10 Af temperature, 20°C. From Beltex-Galland and Luscher (1961).

With ATP and Mg++ ions, precipitates of these extra< ts will show superprecipitation, i.e., a comparatively slow contraction, which is abolished by the addition of Salyrgan. ATP sensitivity is about 70 % and the ATPase activity amounts to 0.002-0.003 mole P, per milligram of protein per minute (cf. Table IV). This activity varies with the ionic strength and with the magnesium concentration it too is inhibited by Salyrgan. 

Magnesium is distinguished by the fact that it is required by most ATP-using enzymes. Here, Mg occurs as a complex with ATP, as shown in Figure 10,49. In other words, the true substrate for most ATF-requiring enzymes is not ATP, but the Mg-ATP complex. A deficiency in magnesium is uncommon. When it does occur, the physiological funchon that is most sensitive is neuromuscular activity. In molecular terms, the enzymes involved in neuromuscular activity that appear to be sensitive to Mg deficiency are those involved in the transport of sodium, potassium, and calcium, these enzymes are Na.K-ATPase and the calcium pump (Ca-ATPase). 

The question for the nutritionist and clinician is Which Mg-dependent function is most sensitive to depletion of the body s magnesium and to hypomagnesemia The answer is probably ion transport systems, such as the calcium pump and Np,K-ATPase. The impaired activity of these ion pumps is likely to be responsible for the neuromuscular problems that present with an Mg deficiency. The defects would involve a difficulty in maintaining the normal movements of calcium, sodium, and potassium ions required for nerve conduction and muscle contraction,... 

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