Magnesium ions are essential in mediating processes such as enzymatic reaction, muscular contraction, hormonal secretion, and DNA synthesis. The divalent magnesium cation, although in the same group as calcium, is a smaller atom. It is therefore not surprising that truncated versions of calcium indicators work well for magnesium. BAPTA-based indicators have a selectivity of about 105, for Ca2+ over Mg2+. Trimming the binding site by roughly half greatly reduces affinity for Ca2+, but still allows binding of the much smaller Mg2+. Thus, APTRA (Figure 7.1), approximately half BAPTA, is a non-fluorescent Mg2+ indicator, and the structural basis for all our other magnesium indicators. In view of this design, we offer Mg2+ versions of almost all our Ca2+ indicator versions.
These Mg2+ indicators have also gained popularity as very low affinity indicators. The practical problem with their use is one of distinguishing the contribution of Mg2+ in Ca2+ measurements. We therefore offer special low-affinity Ca2+ indicators that are not based on APTRA and that are highly selective for binding Ca2+ over Mg2+ (link to low affinity)
Long wavelength magnesium indicators
TEFLabs offers the long-wavelength FLUO-2 Mg (Figure 7.2), Asante Magnesium RedTM, and Asante Magnesium GreenTM. Fluo-2 Mg is available as the K+ Salt or the acetoxymethyl (AM) ester.
UV Magnesium Indicators
TEFLabs offers the UV-excitable Mg2+ indicators that correspond to Fura-2 (Fura-2 Mg, Figure 7.3) and Indo-1 (Indo-1 Mg, Figure. 7.4). Fura-2 Mg was first invented by Raju et al as “Furaptra”.
Table 7.2 below illustrates the properties of Fura-2 Mg and Indo-1 Mg.
Figure 7.1 APTRA
Figure 7.2 Fluo-2 Mg
Figure 7.3 Fura-2 Mg
Figure 7.4 Indo-1 Mg