Magnetic susceptibility to measure total protein concentration from NMR metabolite spectra: Demonstration on blood plasma.

PurposeAccurate metabolite and protein quantification in blood plasma and other body fluids from one single NMR measurement, allowing for improved quantitative metabolic profiling and better assessment of metabolite-protein interactions. Theory and MethodsThe total protein concentration is derived from the common chemical-shift changescaused by protein-induced bulk magnetic susceptibility (BMS)measured on well-accessible and exchange-free metabolite resonances. These BMS shifts are simply obtained by external referencing with respect to 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid, sodium salt in a coaxial insert. ResultsBased on blood-plasma data from five volunteers, the estimated accuracy of the BMS method is5% with respect and comparable to the 3.8% error of the standard colorimetric, Biuret, method. Valine, alanine, glucose, leucine, and lactate display no exchange-induced shift changes. Their well-accessible signals act as reliable probes for pure protein-induced BMS. The slopes and intercepts of their chemical-shift change versus protein concentration were derived from metabolite mixtures with (fatted) human and bovine albumin acting as blood-plasma mimics. ConclusionThe BMS method, demonstrated on blood plasma, can also be used on other samples containing sufficient protein (> 10 g/L). Also, it allows measurement of the presence and sign of exchange-induced chemical-shift changes. Magn Reson Med 73:459-468, 2015. (c) 2014 Wiley Periodicals, Inc.