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Notes: —In growing rat brain, the specific activity of DNA at 12 h after the subcutaneous injection of [3H]thymidine underwent a sharp rise during the first 6 days of life, dropping just as precipitously by 15 days, thereafter continuing to decrease with increasing age.When [3H]thymidine was given to 6-day-old rats, a considerable amount was taken up immediately into the brain. Thymidine taken up into the acid-soluble fraction was readily phosphorylated to its nucleotides, thymidine mono-, di-, and triphosphate (TMP, TDP and TTP) within only 30 min following injection. The highest specific activity was found in TTP. The incorporation of of [3H]thymidine into DNA took place over a longer period of time after injection.

Notes: Abstract— Rat brain polysomes were prepared from a deoxycholate-treated postmito-chondrial supernatant in the presence of 2% bentonite and 1 mg/ml of yeast RNA to prevent partial degradation during preparation.〈list xml:id="l1" style="custom"〉1The polysomal preparations had an absorption maximum at 260 mμ and an absorption minimum at 235 mμ. The ratio of absorption maximum to minimum and the RNA to protein ratio were 1·58 and 1·06 respectively in 6-day-old rat brain polysomes. The sedimentation patterns showed six distinct peaks with sedimentation coefficients of 235S, 185S, 173S, 135S, 100S and 80S, indicating that these preparations have the characteristics of pure heavy polysomes.〈list xml:id="l2" style="custom"〉2The rate of [14C]phenylalanine incorporation into brain polysomal protein was maximal at approximately 10 days of age and decreased thereafter. A similar progressive reduction with increasing age was found in the stimulation of phenylalanine incorporation by the addition of 60 μg/tube of polyuridylic acid. However, the incorporation of phenylalanine into young rat brain polysomes was usually greater even with the addition of polyuridylic acid than in the older animals.〈list xml:id="l3" style="custom"〉3The comparative studies on sucrose density gradient centrifugation of polysomes between young and adult rat brains showed a considerable decrease of heavy polysomes in the older animals.〈list xml:id="l4" style="custom"〉4The effect of various factors on the stability of brain polysomes from both ages has been studied. The rates of RNA, protein and acid-soluble phosphorus release from polysomes of the adult rat brains were usually greater in the presence of high salt concentration, ethylenediaminetetra-acetic acid and urea than those from the corresponding preparations of younger animals. On the basis of evidence obtained from the above results it suggested that the adult brain polysomes were more unstable than those of younger animals.〈list xml:id="l5" style="custom"〉5The amount of polysomal RNA linearly increased up to the first 20 days after birth and then levelled off. The ratio of G + C/A + U of polysomal RNA was less in the young rat brains, falling to 1·30 as compared to 1·50 in older animals. The differences were statistically significant at less than a 1% level of confidence.〈list xml:id="l6" style="custom"〉6Polysomal preparations also contained RNase, phosphomonoesterase, phospho-diesterase and 5′-nucleotidase activities which cannot be washed off. The specific activities of these enzymes were generally higher in young rat brains than those in the adult.

Notes: Abstract— Thymidine kinase (ATP: thymidine-5′-phosphotransferase EC 2.7.1.21) of the supernatant fraction from 6-day-old rat brain possessed a pH optimum of 8.0 and required the presence of 5mM-ATP and 2.5 mM-MgCl2 for maximum activity. The activity was completely inhibited by addition of 1.8 mM-TTP. The enzyme activity was lost if the same supernatant fraction was refrozen and thawed. Km was 2.8 × 10−6 M for [6-3H]thymidine.Following subcellular fractionation of rat brain, the greatest proportion and highest specific activity of thymidine kinase was found in the supernatant fraction. Thymidine kinase activities reached a maximum at 6 days of age and then dropped sharply during maturation. Comparative studies of thymidine kinase activities of cerebrum, cerebellum and the remainder of the brain during growth indicated that the activity in the cerebellum was usually higher than those in the cerebrum and the remainder, and the biggest differences obtained at 6 days after birth corresponded with the peak in cerebellar activity.

Notes: Little work has been done on the chemical and physical properties of brain nuclear DNA. Recently Du Buy, Mattern and Riley (1966) described the heat denaturation of mouse brain nuclear DNA, which apparently differs from the related mitochondrial DNA and the various protozoal DNA's tested. For such studies, it is essential to prepare the nuclear DNA from brain tissues in a native and highly polymerized state. In this brief communication preparation procedures of the native DNA from isolated brain nuclei are described, and some physical and chemical properties are given.