Ionized magnesium concentration in axoplasm of dialyzed squid axons.

Magnesium ion is a significant constituent of cell cytoplasm and participates in many intracellular enzymatic reactions. Although total cell magnesium can be rather easily determined by atomic absorption spectroscopy, the free magnesium is more difficult to measure and is much less than the total because of magnesium binding to cellular organelles or ionic constituents. Moreover, since magnesium is strongly bound to the phosphonucleotides involved in energy metabolism the concentration of the free form might be expected to vary with the state of metabolism of the cell, as for example might occur following metabolic poisoning. Despite the ubiquitous participation of magnesium in cellular reactions, very little is known about concentration of the ionized form under physiological conditions. This report describes the application of a dual wavelength miclospectrometry to permit nondestructive measurement of ionized magnesium in single, isolated squid giant axons. The formation of the magnesium complex of the dye, Eriochrome Blue SE, [345-chloro-2-hydroxyphenyl)azo] -4$dihydroxy-2,7-naphthalene disulfonic acid] is accompanied by a significant shift in spectrum compared with the unbound dye. This dye also binds calcium, but it is possible by appropriate selection of wavelengths, to obtain difference spectra which are almost insensitive to changes in ionized calcium [ 11.