Factors influencing the uptake of cyanocobalam,in (vitamin B 12) by ehrlich ascites carcinoma cells

Abstract

The uptake in vitro of cyanocobalamin by Ehrlich ascites cells in the presence of ascitic fluid is a biphasic reaction consisting of a rapid “primary” uptake followed by a slower “secondary” uptake. The primary phase is completely reversible, equilibrium being attained almost instantaneously. The secondary phase is a relatively slow process requiring approx. 3 h to reach completion. Primary uptake is not appreciably affected by variations in temperature, pH and cellular energy, whereas secondary uptake is optimal at pH 7.4, and exhibits a high degree of sensitivity to variations in temperature and to the metabolic inhibitors 2,4-dinitrophenol, sodium cyanide and iodoacetate. On the basis of kinetic experiments, primary uptake appears to be the uptake-limiting step of the overall cyanocobalamin-uptake process. It is suggested that primary uptake represents a physico-chemical reaction between extracellular-bound cyanocobalamin and receptors on the cell membrane, and that secondary uptake involves the transport of cyanocobalamin from these receptors into the cell.