The characteristics of nucleobase transport and metabolism by the perfusedsheep choroid plexus

The uptake of nucleobases was investigated across the basolateral membrane of the sheep choroid plexus perfused in situ. The maximal uptake (U-max) fo r hypoxanthine and adenine, was 35.51+/-1.50% and 30.71+/-0.49% and for gua nine, thymine and uracil was 12.00+/-0.53%, 13.07+/-0.48% and 12.30+/-0.55% , respectively with a negligible backflux, except for that of thymine (35.1 1+/-5.37% of the U-max). HPLC analysis revealed that the purine nucleobase hypoxanthine and the pyrimidine nucleobase thymine can pass intact through the choroid plexus and enter the cerebrospinal fluid CSF so the lack of bac kflux for hypoxanthine was not a result of metabolic trapping in the cell. Competition studies revealed that hypoxanthine. adenine and thymine shared the same transport system, while guanine and uracil were transported by a s eparate mechanism and that nucleosides can partially share the same transpo rter. HPLC analysis of sheep CSF collected in vivo revealed only two nucleo bases were present adenine and hypoxanthine; with an R-CSP/Plasma 0.19+/-0. 02 and 3.43+/-0.20, respectively. Xanthine and urate, the final products of purine catabolism. could not be detected in the CSF even in truce amounts. These results suggest that the activity of xanthine oxidase in the brain o f the sheep is very low so the metabolic degradation of purines is carried out only as far as hypoxanthine which then accumulates in the CSF. In concl usion, the presence of saturable transport systems for nucleobases at the b asolateral membrane of the choroidal epithelium was demonstrated. which cou ld be important for the distribution of the salvageable nucleobases, adenin e: and hypoxanthine in the central nervous system. (C) 2001 Elsevier Scienc e B.V. All rights reserved.