Studies on the short-term effects of feeding amino acid mixtures on RNA and protein metabolism in rat liver

Henderson, A. R (1970) Studies on the short-term effects of feeding amino acid mixtures on RNA and protein metabolism in rat liver. PhD thesis, University of Glasgow.

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The investigations of Lagerstedt, Stowell and Stenram indicated that the rat liver nucleolus was extremely sensitive to starvation and to changes in the protein content of the diet. One of the features of the nucleolar response was the rapidity with which the nucleolus enlarged after feeding a high protein diet to a previously starved animal. Stenram demonstrated that this was due to an increase in dry matter and Rl'TA content. Electron microscopic studies of the cytoplasm of rat liver have also shov/n that there is a rapid regeneration of endoplasmic reticulum and ribosomes when a fasted animal was fed a high protein diet, and this response was somewhat 'reduced if the diet had a low protein content. The use of a tube fed amino acid mixture made it possible to take the study of these events further. Fleck et al. showed that if the amino acid mixture lacked the essential amino acid tryptophan, then the cytoplasmic polyribosomes broke down to smaller, less efficient forms and that this breakdown could be reversed by feeding a complete amino acid mixture. They also demonstrated that these cytoplasmic events could not be prevented by pretreatment of the animals with Actinomycin D and thus they must be independent of the nucleus. Nonetheless the studies of Stenram and others had indicated that feeding induced nucleolar changes, and as feeding stimulates ribosome formation (which is now known to occur in the nucleolus) studies were made of nuclear function followings the feeding of amino acid mixtures which were either nutritionally complete or which were lacking tryptophan. The 3 vivo incorporation of radioactive precursors into subsequently isolated nuclear RNA was examined initially. Preliminary studies were made on MA separation by continuous agarase electrophoresis devices and by isokinetic sucrose density gradient analysis. During the course of the latter study a new isokinetic gradient maker was devised. Finally, the investigation was carried out by linear sucrose density gradient analysis and the results indicated that the absence of tryptophan from the amino acid mixture decreased the incorporation of isotope into nuclear EUA within 30 min. of feeding. It was not possible to assume that the observed decrease indicated a decreased synthesis of ribosomal MA because nuclear RNA is known to consist of two main types - ribosomal type MA (synthesised in the nucleolus) and heterogeneous ENA (synthesised elsewhere in the nuclear) and while there is considerably more ribosomal MA present in ENA extracted from nuclei, it cannot be stated with certainty that any observed changes are due only to changes in ribosomal type ENA. A second objection is that nothing is known about any changes induced by diet in the ENA precursor pool size and therefore in any changes which would consequently occur in the precursor pool specific activity. Thus further evidence had to be obtained for the effect of tryptophan omission on ribosomal ENA synthesis. It is thought that the nuclear enzyme 2+MA dependent Mg activated RNA polymerase is dissolved in ribosomal HMA synthesis and it is largely located in the nucleolus. Thus, determinations of the activity of this enzyme may indicate differences in ribosomal ENA synthes following the different diets. (Abstract shortened by ProQuest.).

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: R MS Smellie
Keywords: Physiology
Date of Award: 1970
Depositing User: Enlighten Team
Unique ID: glathesis:1970-72892
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 11 Jun 2019 11:06
Last Modified: 11 Jun 2019 11:06

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