Gl. Mendz, FACTORS INFLUENCING THE MEASUREMENT OF CALCIUM CONCENTRATIONS IN BIOLOGICAL-FLUIDS BY F-19 NMR-SPECTROSCOPY, Australian Journal of Chemistry, 47(4), 1994, pp. 677-692
Fluorinated derivatives of fluorescent calcium probes have been recent
ly used to measure free calcium concentrations in biological fluids by
F-19 n.m.r. spectroscopy. To measure calcium levels a method utilizes
the change in chemical shift experienced by the fluorine resonance of
the calcium chelator amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacet
ic acid upon complexation of cations. The procedure assumes that (A) t
he fluorinated probe Is in equilibrium between the calcium complex and
the free acid form; (B) the chemical exchange between different sites
has similar effects on the intensities of the observable resonances;
and (c) the presence of macromolecules affects equally the intensities
of the resonances arising from different complexes. Experiments desig
ned to test these assumptions were carried out in plasma and in model
systems containing mixtures of cations and macrormolecules. The result
s indicated that in biological fluids the calcium probe is involved in
a multisite equilibrium in which 'invisible' (very broad) peaks affec
t the equilibrium intensities of the 'observable' peaks. Binding to ma
cromolecules affected differently the intensities of the resonances ar
ising from different complexes in the spectrum of the chelator, and in
fluenced significantly the calculated free calcium concentrations.