TY - JOUR
T1 - The determination of magnesium in human blood plasma by 31P magnetic resonance spectroscopy using a macrocyclic reporter ligand
AU - Huskens, Jurriaan
AU - Main, Mike
AU - Malloy, Craig R.
AU - Sherry, A. Dean
N1 - Funding Information:
This research was supported in part by grants from the Robert A. Welch Foundation (AT-584) and the NIH Biotechnology Research Program (P41-RR02584).
PY - 1997/10/20
Y1 - 1997/10/20
N2 - 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.
AB - 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.
KW - 1,4,7-Triazacyclononane
KW - Blood plasma
KW - Magnesium
KW - P magnetic resonance
UR - http://www.scopus.com/inward/record.url?scp=0030663501&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=0030663501&partnerID=8YFLogxK
U2 - 10.1016/S0304-4165(97)00054-8
DO - 10.1016/S0304-4165(97)00054-8
M3 - Article
C2 - 9367171
AN - SCOPUS:0030663501
SN - 0304-4165
VL - 1336
SP - 434
EP - 444
JO - Biochimica et Biophysica Acta - General Subjects
JF - Biochimica et Biophysica Acta - General Subjects
IS - 3
ER -