APPARENT MG2-ADENOSINE 5-TRIPHOSPHATE DISSOCIATION-CONSTANT MEASURED WITH MG2+ MACROELECTRODES UNDER CONDITIONS PERTINENT TO P-31 NMR IONIZED MAGNESIUM DETERMINATIONS()

Using Mg2+ macroelectrodes based on the sensor ETH 7025 and accurate M g2+-EDTA buffer solutions, the apparent Mg2+-ATP dissociation constant (K-app) was measured at 25 and 37 degrees C in background solutions m imicking the cationic intracellular milieu of muscle cells. The mean /- SD (in mu M) at 25 degrees C was 157.0 +/- 13 (n = 4), 127.5 +/- 12 .0 (n = 11), 101.0 +/- 9.0 (n = 4) and at 37 degrees C was 106.6 +/- 9 .6 (n = 4), 87.4 +/- 4.9 (n = 4), 78.1 +/- 2.0 (n = 4) at pH values of 6.7, 7.2, and 7.7, respectively. The dependence of K-app at 25 degree s C on the ionic strength was also measured, the mean +/- SD (mu M) be ing 61.9 +/- 2.2 (n = 3), 127.5 +/- 12 (n = 11), and 243.0 +/- 11.8 (n = 3) at ionic strengths of 0.087, 0.156 (normal background), and 0.3 M, respectively. These values are larger than the K-app values most co mmonly used in the literature (87.4 mu M compared to 38 mu M at pH 7.2 and 37 degrees C) to estimate the [Mg2+](i) in P-31 NMR experiments, attributed to the difficulties in setting the [Mg2+](i) without the us e of Mg2+ buffer solutions. If these new values are used, the literatu re values for [Mg2+](i) estimated by P-31 NMR increase by a factor of around 1.5, making them similar to values obtained by direct Mg2+ micr oelectrode measurements. (C) 1997 Academic Press.