The protein components of subcellular structures involved in protein synthesis

Downie, Eileen D (1962) The protein components of subcellular structures involved in protein synthesis. PhD thesis, University of Glasgow.

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A nitrogen-containing material which is removed by lipid solvents from liver tissue residues has been investigated chemically and biologically. Certain organic solvents commonly used to extract lipids from trichloracetic acid-treated residues of liver solubilise more than 20% of the total acid-insoluble nitrogen. Only a small part of this nitrogen is contributed by phospholipid, and little if any by nucleic acid. Results obtained by means of amino-nitrogen and peptide-bound nitrogen estimations, and of paper chromatography, Indicate that the nitrogenous material of the solvent extract consists mainly of protein or polypeptide. This ''lipid protein" can be separated from the phosphorus-containing substances of the solvent extract by washing with Co2 volumes of water. The "lipid protein" occurs in all the cell fractions examined (mitochondria, microsomes and cell sap), but is especially abundant in the mitochondria and larger microsomes, fractions which are rich in cell membranes. The distribution of the "lipid protein" follows that of the phospholipid and it is probably a component of the cytoplasmic membranes. Chemical treatment of microsomal pellets prepared by differential centrifugation in sucrose solution confirms the suggestion that the "lipid protein" is of membranous origin. The microsomal fraction corresponds to the endoplasmic reticulum of the intact cell, and fractionation of the microsomes by means of a variety of reagents indicates that the "lipid protein" of microsomes is derived from their membranous component rather than the ribosomal granules. Protein deficiency for 11 days decreases the amount of "lipid protein" in the liver cell, but the other constituents estimated (non-lipid protein, RM and phospholipid) also fall by the same proportion. Thus protein deficiency appears to lead to the loss of cell structures as whole units rather than the specific loss of any one chemical constituent. The regrowth of endoplasmic reticulum membranes which follows the feeding of a protein meal to fasted rats is accompanied by an increase in the amount of "lipid protein" which thus seems to vary in parallel with changes in the amount of membranous material in the cell. The lipid solvent extract of the acid-insoluble residue of liver from rats injected 5-60 minutes previously with 14C-leucine contains considerable amounts of radioactivity, which are probably not due to contamination with free leucine. The uptake in the solvent extract, relative to that in the non-lipid protein, is stimulated by the feeding of a protein meal to rats fasted overnight. This effect is not seen in rats which have been depleted of protein for 6 days; the endoplasmic reticulum may be modified by protein depletion so that it is no longer able to respond to the protein meal. The response to the meal observed in the adequately nourished rats is seen in all the cytoplasmic fractions. The uptake is highest in the microsomes, and especially in the lipid solvent extract of their membranous component. The activity reaches a peak 20-30; minutes after the injection of the radioactive leucine. The evidence therefore suggests that certain organic solvents are capable of removing from the trichloacetic acid-treated residue of liver a specified nitrogenous material which is free from phosphorus and is probably protein or polypeptide in nature. This ''lipid protein'' is probably derived from the membranes of the endoplasmic reticulum is specifically sensitive to, starvation and to re-feeding with protein, and is associated with a highly radioactive fraction in the microsomal membrane 5-30 minutes after the intravenous injection of 14C-leucine.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: H N Munro
Keywords: Molecular biology
Date of Award: 1962
Depositing User: Enlighten Team
Unique ID: glathesis:1962-73313
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jun 2019 08:56
Last Modified: 14 Jun 2019 08:56

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