Transport of carnosine by mouse intestinal brush-border membrane vesicles

The characteristics of carnosine (β-alanyl-l-histidine) transport have been studied using purified brush-border membrane vesicles from mouse small intestine. Uptake curves did not exhibit any overshoot phenomena, and were similar under Na⁺, K⁺ or choline⁺ gradient conditions (extravesicular > intravesicular). However, uptake of histidine showed an overshoot phenomenon in the presence of a Na⁺-gradient. There was no detectable hydrolysis of carnosine during 15 min of incubation with membrane vesicles under conditions used for transport experiments. Analysis of intravesicular contents further showed the complete absence of the constituent free amino acids of carnosine, and indicates that intact carnosine is transported. Studies on the effect of concentration on peptide uptake revealed that transport occurred by a saturable process conforming to Michaelis-Menten kinetics with a K_m of 9.6 ± 1.4 mM and a V_max of 2.9 ± 0.2 nmol / mg protein per 0.4 min. Uptake of carnosine was inhibited by both di- and tripeptides with a maximum inhibition of 68% by glycyl-l-leucyltyrosine. These results clearly demonstrate that carnosine is transported intact by a carrier-mediated, Na⁺-independent process.