Intracellular pH measurements by phosphorus-31 nuclear magnetic resonance. Influence of factors other than pH on phosphorus-31 chemical shifts

Abstract

Titration curves plotting chemical shift vs. pH for inorganic phosphate and glucose 6-phosphate in solutions of various composition are presented. Physiological concentrations of K+ (0.1 M) and Mg2+ (5 mM) are shown to significantly shift the titration curve. The Mg2+ effect can be partly or completely reversed by addition of sufficient quantities of adenosine triphosphate or organic acids. The acidic protein bovine serum albumin and soluble maize root tip protein have no noticeable effect on the titration curves, whereas the basic protein protamine exerts a profound effect. The results clearly indicate that knowledge of intracellular ionic strength and free Mg2+ concentrations in the sample are required if the determination of intracellular pH by 31P NMR is to be considered accurate within +/- 0.05-01 pH unit.