Development and localization of the thymidine phosphorylating systems in brain.

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 degrees 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.