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L Lim

Publications and source records attributed to L Lim.

At least 253 records · Page 14Linked to original sources
PubMed1976

The metabolism of high-molecular-weight ribonucleic acid including polyadenylated species, in the developing rat brain.

High-molecular-weight RNA was isolated from rat brain at various times after the intracranial administration of [32P]Pi. The synthesis of 28S and 18S rRNA could be detected within 1h of the injection of the radioactive precursor and appeared to be more pronounced, relative to other high-molecular-weight RNA, in the brains of older rats compared with those of newborn rats. Polyadenylated RNA, representing most mRNA and their precursors, was isolated by chromatography on oligo(dT)-cellulose. The contribution of this polyadenylated RNA to total RNA synthesis was investigated in the cerebral cortex and the phylogenetically older brain stem at different stages in the development of the rats by using a 5h period of labelling as an arbitrary index of transcription. In the brain stem the proportion of labelled polyadenylated RNA comprised 27-30% of the total RNA. The corresponding values for the cortex decreased from 34% in newborn rats to 23% in 40-150-day-old rats. These data indicated that proportionately more polyadenylated RNA is synthesized in the cortex of the newborn than in the adult rat and that there is a progressive decrease in the synthesis of polyadenylated RNA relative to rRNA during development.

Animals↗
PubMed1976

Nucleo-cytoplasmic relationships of high-molecular-weight ribonucleic acid, including polyadenylated species, in the developing rat brain.

The metabolism of high-molecular-weight RNA in the nuclear and cytoplasmic fractions of newborn and adult rat brain was investigated after the intracranial administration of [32P]Pi. In young brain, a considerable proportion of the newly synthesized radioactive RNA is transferred to the cytoplasm, in contrast with the adult brain, where there appears to be a high intranuclear turnover. Electrophoretic analysis of the newly synthesized RNA showed that processing of the rRNA precursor to yield the 28S and 18S rRNA may be more rapid in the adult than in the young, although most of the adult rRNA in the nucleus is not transferred to the cytoplasm. In young brain, processing is probably tightly coupled to transport of rRNA into the cytoplasm, so that 28S and 18S rRNA are not subjected to possible degradation within the nucleus. Polyadenylated RNA turns over in concert with high-molecular-weight RNA in the nuclei of the adult rat brain. In the cytoplasm the polyadenylated RNA has a higher turnover rate relative to rRNA. In the young brain the polyadenylated RNA is transferred to the cytoplasm along with rRNA, although polyadenylated RNA is transported into the cytoplasm at a faster rate. The nuclear and cytoplasmic polyadenylated RNA species of young brain are larger than their corresponding adult counterparts. These results suggest that there are considerable changes in the regulation of the nucleo-cytoplasmic relationship of rRNA and polyadenylated RNA during the transition of the brain from a developing replicative phase to an adult differentiated and non-dividing state.

Animals↗
PubMed1967

Microsomal components in relation to amino acid incorporation by preparations from the developing rat brain.

1. After incorporation of [(14)C]valine in vitro, cerebral microsomes were separated into membrane-bound and free ribosomes by sucrose-density-gradient centrifugation. 2. In preparations from both 4-day-old and adult rats, free and bound ribosomes incorporated [(14)C]valine. Free ribosomes could be found as polysomes, which were highly active. 3. Microsomes labelled with [(14)C]valine in vitro were fractionated after deoxycholate treatment into a preliminary sediment, sedimented at 105000g (5min.), and ribonucleoprotein particles, sedimented at 150000g (70min.), to determine the role of membrane-bound ribosomes. In the adult the ribonucleoprotein particles retained most of the radioactivity, whereas in the young the preliminary sediment was as highly labelled as the ribonucleoprotein particles. 4. The labelled preliminary sediment from young preparations was both ribonuclease- and deoxycholate-resistant, and the nature of this material is discussed in terms of a possible structural component of microsomal membranes.

Animals↗
PubMed1966

Amino acid incorporation by preparations from the developing rat brain.

1. A study has been made of the ability of cerebral microsome-cell sap systems, taken from rats at various ages, to incorporate [(14)C]valine. 2. The systems appear to follow Michaelis-Menten kinetics when cell sap is considered as substrate, microsomes as enzyme and total counts/mg. of microsomal protein as a measure of reaction velocity. 3. Reproducible ;affinity constant' values are obtained, and the system from 4-day-old rats has a higher V(max.) and affinity constant than the system from adult rats. 4. It is suggested the the amino acid-incorporating ability of different systems may be compared by this means.

Amino Acids↗