Changes in polysomes of the developing rat brain.

Abstract

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. The 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. The 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. The 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. The 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. The 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. Polysomal 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.