Active Transport of Foreign Pyrimidines across the Intestinal Epithelium

Abstract

THE small intestine of mammals possesses at least three independent active transport processes for the absorption of organic compounds: one for monosaccharides like glucose and galactose¹; one for L-amino-acids²; and one for pyrimidines like uracil and thymine³. These specialized absorption processes have a number of characteristics in common which distinguish them from the process of simple diffusion: the substrate is transferred against a concentration gradient; the transport systems become saturated at high concentrations of substrate; and one substrate depresses the passage of another by competing with it for the transport process.