Na+ gradient-dependent transport of hypoxanthine by calf intestinal brush border membrane vesicles

The properties of hypoxanthine transport were investigated in purified brush border membrane vesicles isolated from calf proximal and distal jejunum. Hypoxanthine uptake in the vesicles was stimulated by a transmembrane Na+ gradient and an inside negative potential resulting in a transient accumulation of intra-vesicular hypoxanthine, especially in the proximal jejunum. Na+-dependent hypoxanthine uptake at this site seemed to occur by two saturable transport systems, a high affinity (K-m = 0.33 mumol/l) and a low affinity (K-m = 165 mumol/l) transporter. Guanine, hypoxanthine, thymine and uracil inhibited intravesicular hypoxanthine uptake, whereas adenine and the nucleosides inosine and thymidine were without effect. These findings represent the first demonstration of active Na+ gradient-dependent nucleobase transport in intestinal brush border membrane vesicles.