Metabolism of Deoxyuridine in Rabbit Brain

Abstract

The metabolism of [3H]deoxyuridine by rabbit brain was investigated in vitro and in vivo. In vitro, brain slices from various regions of brain and from all age groups accumulated [3H]deoxyuridine from artificial CSF. Within the slices, a portion of the accumulated [3H]deoxyuridine was metabolized to [3H]deoxyuridine phosphate, with subsequent conversion to [3H]thymidine phosphate, and ultimately [3H]DNA. The percentage of the [3H]deoxyuridine phosphorylated and subsequently converted into [3H]DNA was highest at birth and declined to adult levels in 3-month-old rabbits. Thymidine, when added to the incubation medium with the [3H]deoxyuridine, was approximately 10 times as potent as unlabeled deoxyuridine in inhibiting the intracellular phosphorylation and conversion of [3H]deoxyuridine to [3H]thymidine phosphate in brain slices. In vivo, 2.5 h after intraventricular injection of [3H]deoxyuridine, over 90% of the [3H]deoxyuridine was cleared from the central nervous system at all ages. However, in both newborn and 3-month-old rabbits, approximately 40 and 12%, respectively, of the 3H remaining in brain was phosphorylated and converted to [3H]thymidine phosphates; and 11 and 4%, respectively, of the 3H remaining in brain was converted to [3H]DNA. These results show that both immature and mature rabbit brain is able to incorporate deoxyuridine into DNA. Thus, all the enzymes involved in this conversion, including thymidylate synthetase (EC 2.1.1.45), are present and active in brain throughout life.

Area, Age and Gender Dependence of the Nucleoside System in the Brain: a Review of Current Literature

“…Similarly to the purine (deoxy)nucleotides, pyrimidine (deoxy)nucleotides are synthesized in salvage reactions in the brain [87][88][89][90][91][92][93][94][95][96][97][98]. Urd and Cyd are salvaged by cytidine deaminase (CDA) and uridine-cytidine kinase (UCK) [94,97,99].…”

The Origin of Deoxynucleosides in Brain: Implications for the Study of Neurogenesis and Stem Cell Therapy

“…Some of the dU in brain was converted into ( 3 H)thymidine by TS and subsequently incorporated into DNA as shown by specific methods. These and other experiments unequivocally showed that RR, TS and DHFR were present and active not only in developing but also in adult and old rabbit brains (10,11,33). In fact, 2% of the ( 3 H) in brain cortex (i.e., the tissue above the lateral ventricle) was covalently incorporated into ( 3 H)DNA in adult rabbits after the intraventricular injection of tracer ( 3 H)dU (33).…”

“…(There may also be ENT2 on both the apical and basal sides of the CP (22).) Therefore, it appears that the rapid clearance of tracer dT, dU and dC from rabbit CSF into blood is due, in large part, to the apically located CNT3 transport system in CP (24,32,33). Unlike the equilibrative systems that are one-half saturated at õ0.2Y0.5 mM dT, the CP system is halfsaturated at 5 to 20 mM for dN as noted above (28Y30).…”

“…Moreover, we showed that tracer [6-( 3 H)dU], injected intraventricularly, was rapidly transported out of CSF into blood and brain (33). Some of the dU in brain was converted into ( 3 H)thymidine by TS and subsequently incorporated into DNA as shown by specific methods.…”

Quantification of efflux into the blood and brain of intraventricularly perfused [3H]thymidine in the anaesthetized rabbit

“…However, our research group has identified a saturable uptake system at the BBB, which would appear to be a major pathway for the entry of thymidine into the brain (Williams & Segal, 1996). In addition, saturable transport mechanism(s) for nucleosides out of the CSF are known to exist (Spector, 1980a(Spector, , 1985Suleiman & Spector, 1982;Spector & Huntoon, 1983;Wu, Hui & Giacomini, 1993). It was thus decided to investigate the exchanges of thymidine between CSF, blood and brain in more detail using ventriculo-cisternal perfusion in the intact animal, rather than employing the isolated plexus, to determine whether the CSF could consitute a source for these deoxyribonucleosides for the brain.…”