Studies on the Chemical Modification of Monensin. II. : Measurement of Sodium Ion Permeability of Monensylamino Acids Using Sodium-23 Nuclear Magnetic Resonance Spectroscopy

Abstract

A technique to assay Na⁺ ions in cells is presented. Intracellular and extracellular Na⁺ ions in a suspension of guinea pig erythrocytes were conveniently determined by using sodium-23 unclear magnetic resonance (²³Na-NMR), in combination with two anionic shift reagents : Dy(TTHA)^3- and Dy(PPPi)₂^7-. Monensin (1), monensylalanine (2b), monensylserine (2c), and monensylphenylalanine (2d) induced large increases of intracellular Na⁺ ion concentration ([Na_in]), while monensylglycine (2a), monensyltyrosine (2e), monensylaspartic acid (2f), and monensylglutamic acid (2g) showed slight increases. The values of initial increasing rate (V_i) of 2a-g were much smaller than that of 1. This fact was probably due to the lower lipophilicity of 2a-g than 1, because a good correlation was observed between V_i and Rm₅₀ values of 1 and 2a-g. This lower lipophilicity is a consequence of conformational differences between 1 and 2a-g.