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Journal of Dairy Science Vol. 84 No. 3 614-619
© 2001 by American Dairy Science Association ®
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Active Intestinal Absorption of Nucleosides by Na+-Dependent Transport across the Brush Border Membrane in Cows

E. Scharrer 1 and B. Grenacher 1

1 Institute of Veterinary Physiology, University of Zurich, Winterthurerstr. 260, CH-8057 Zurich, Switzerland

Transport of 3H-labeled nucleosides across the bovine intestinal brush border membrane (BBM) was characterized with BBM vesicles (BBMV) isolated from midjejunum of cows because large amounts of nucleic acids are digested in the small intestine of ruminants. Two Na+-dependent electrogenic nucleoside transporters with overlapping substrate specificity were shown to be present in the jejunal BBM, one for pyrimidine nucleosides and one for purine nucleosides. As indicated by inhibitory studies, thymidine seemed to be a specific substrate for the pyrimidine nucleoside transporter, while this applied to guanosine and deoxyguanosine for the purine nucleoside transporter. Uridine and adenosine appear to have an affinity to both transporters. This also applies to deoxyadenosine and deoxyuridine. Nucleobases (uracil, hypoxanthine) did not affect transport of nucleosides. The kinetic constants (Km and Vmax) for Na-dependent thymidine and guanosine transport were 29 and 24 µmol/L and 78 and 51 pmol (mg protein)–1 s–1, respectively. These values are much higher than those reported for monogastric species.

Key Words: digestion of nucleic acids • absorption of nucleosides • brush border membrane • Na+-dependent nucleoside transport

Submitted on July 19, 2000
Accepted on October 27, 2000




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