Intracellular free zinc and zinc buffering in human red blood cells

Abstract

Zn²⁺ has been allowed to equilibrate across the red cell membrane using two agents that increase membrane permeability to this ion: the ionophore A23187 and the specific carrier ethylmaltol. Extracellular free Zn²⁺ was controlled with EGTA (1,2-di(2-aminoethoxy)ethane-NNN′N′tetra-acetic acid)) buffers, except in the case of ethylmaltol, which itself acts as a buffer. Measurement of cellular zinc content at different levels of free Zn²⁺ facilitated the study of intracellular Zn²⁺ binding. It was also possible to estimate intracellular free Zn²⁺ concentration in untreated cells using a “null-point” technique. Intracellular zinc was found to consist of an inexchangeable component of about 129 μmol/10¹³ cells and an exchangeable component of 6.7±1.5 μmol/10¹³ cells, with a free concentration of about 2.4×10⁻¹¹ m. The main component of Zn²⁺ buffering is hemoglobin, with a dissociation constant of about 2×10⁻⁸ m.