Development and Localization of the Thymidine Phosphorylating Systems in Brain

Abstract: The development of the thymidine phosphorylating systems was studied in various regions of brain. Brain slices from cerebellum, brain stem, and forebrain of rabbits 2, 7, 14, 30, 90, 500, and 2500 days of age were incubated for various times in artificial CSF containing 3 nM-³H]thymidine at 37°C under 95% O₂-5% CO₂. When slices from all brain regions of 2-day-old rabbits were incubated in ³H]thymidine for 30 min, tissue-to-medium ratios of ³H were between 2 and 4 and declined with age, and the percentages of the total ³H in perchloric acid homogenates of brain slices as ³H]DNA were 26–29%, declining to low levels with age. However, at all ages and in all regions studied, 41 -88% of the ³H within the slices was phosphorylated. After homogenization and subcellular fractionation of the brain slices incubated in ³H]thymidine for 30 min, the highest percentage of ³H]thymidine phosphates plus ³H]DNA was present in the nuclear (crude and purified) and mitochondrial fractions of all brain regions. The ³H]DNA content in the nuclear and mitochondrial fractions declined with age, but the percentage of ³H]thymidine phosphates did not. Thymidine phosphates were synthesized from thymidine in all brain regions tested throughout the entire life span.