The determination of magnesium in human blood plasma by 31P magnetic resonance spectroscopy using a macrocyclic reporter ligand

Jurriaan Huskens, Mike Main, Craig R. Malloy, A. Dean Sherry

Research output: Contribution to journalArticlepeer-review

11 Scopus citations


The ligand 1,4,7-triazacyclononane-1,4,7-tris(methylene methylphosphinic acid), NOTMP, was used to measure free Mg(II) levels in blood plasma by 31P MRS. Separate resonances were observed for the free ligand and the Mg(II) complex and the ratio of their resonance areas was used to evaluate the free, ionized Mg(II) concentration, [Mg](free). The Ca(II) and the Zn(II) complexes gave rise to separate resonances in the 31P spectrum in an aqueous sample. In human blood plasma samples, however, these resonances were never observed thus excluding the interference of these metal ions. Heparin, up to 150 units/ml, had no influence on the Mg-NOTMP equilibrium. The 31P MRS methodology was applied to twenty human blood plasma samples. Total Mg(II) ([Mg](total)), as measured by atomic absorption spectroscopy, averaged 0.85 ± 0.12 mM while free ionized Mg(II) ([Mg](free)) measured by 31P MRS was 0.66 ± 0.09 mM. The 31P MRS method gave inherently larger values for free ionized Mg(II) than that reported by ion-selective electrodes (ISE). This was traced to a redistribution of existing plasma Mg(II) species after the addition of about 2 mM of NOTMP. Calculations using existing thermodynamic data show that the ionized Mg(II) concentration (iMg) and the concentration of Mg(II) weakly complexed to small anions (Mg(comp)) both drop after the addition of NOTMP, with Mg(comp) dropping to negligible levels. Thus, the 31P MRS method appears to be less sensitive to variations in the concentration of weakly binding anions (bicarbonate, carbonate, chloride, lactate, phosphate, etc.) than the ISE method. Our data indicates that the difference between Mg(total) as measured by atomic absorption spectroscopy, and Mg(free), as measured by 31P MRS, provides an direct estimate of the protein bound Mg(II) fraction.

Original languageEnglish (US)
Pages (from-to)434-444
Number of pages11
JournalBiochimica et Biophysica Acta - General Subjects
Issue number3
StatePublished - Oct 20 1997


  • 1,4,7-Triazacyclononane
  • Blood plasma
  • Magnesium
  • P magnetic resonance

ASJC Scopus subject areas

  • Biophysics
  • Biochemistry
  • Molecular Biology


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