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Biomedical subjects

M J Clemens

Publications and source records attributed to M J Clemens.

At least 109 records · Page 6Linked to original sources
PubMed1978

The effect of dietary protein deficiency on albumin synthesis and on the concentration of active albumin messenger ribonucleic acid in rat liver.

In rats fed on a protein-deficient diet, albumin synthesis as a percentage of total liver protein synthesis falls from the normal value of approx. 15% to about 8%. We have extracted total cytoplasmic RNA from individual rat livers and measured the concentration of active albumin mRNA by translation in a reticulocyte lysate system from which the endogenous mRNA had been removed [Pelham & Jackson (1976) Eur. J. Biochem. 67, 247-256]. In this messenger-dependent system it is possible to measure the synthesis of albumin as a proportion of the overall protein synthesis promoted by the addition of the hepatic RNA. The results show that the concentration of translatable albumin mRNA in samples of total cytoplasmic RNA from livers of protein-deficient rats is decreased markedly. These findings suggest that dietary protein supply affects selectively the synthesis and/or functional stability of albumin mRNA in rat liver.

Albumins↗
PubMed1976

Functional relationships between a reticulocyte polypeptide-chain-initiation factor (IF-MP) and the translational inhibitor involved in regulation of protein synthesis by haemin.

The rate of initiation of protein synthesis in rabbit reticulocyte lysates is regulated by a translational inhibitor protein which is activated in the absence of added haemin. The effects of this inhibitor on amino acid incorporation are overcome by the protein synthesis initiation factor IF-MP which binds Met-tRNAf in a ternary complex with GTP and which can transfer this complex to small ribosomal subunits. Addition of this factor to haemin-deficient lysates prevents loss of polysomes and regenerates polysomes from 80-S single ribosomes, thus confirming an effect at the level of polypeptide initiation. The ability of the initiation factor to overcome the effects of various concentrations of the translational inhibitor suggests that the inhibitor inactivates the factor catalytically rather than stoichiometrically. In a system in vitro consisting of salt-washed 40-S ribosomal subunits, initiator Met-tRNAf and GTP, the initiation factor IF-MP transfers Met-tRNAf to the subunits in the absence of any other factor or mRNA. Equilibrium buoyant density gradient analysis in CsCl shows that formaldehyde-fixed subunits carrying Met-tRNAf bound under these conditions have a buoyant density approximately 0.02 g/cm3 lower than the bulk of salt-washed subunits, suggesting that approximately 100000 daltons of additional protein are associated with these subunits. This is in marked contrast to the amounts of protein bound to subunits incubated with Met-tRNAf and GTP in the presence of a crude ribosomal salt-wash fraction. The translational inhibitor has no effect on formation of the ternary complex IF-MP-Met-tRNAf-GTP but does impair the factor-catalysed transfer of Met-tRNAf to washed subunits. The possible mechanisms of action of the inhibitor on polypeptide chain initiation are reviewed in the light of these results.

Animals↗
PubMed1976

Morphological and biochemical changes in the hepatic endoplasmic reticulum and golgi apparatus of male Xenopus laevis after induction of egg-yolk protein synthesis by oestradiol-17 beta.

This report describes morphological and biochemical changes accompanying oestrogen induced synthesis of the egg-yolk protein precursor, vitellogenin, in male Xenopus liver. Extensive proliferation of the rough and smooth endoplasmic reticulum and the Golgi apparatus occurs between 3 and 9 days after administration of oestradiol-17 beta. Subcellular fractionation showed that microsomal fractions have an increased number of ribosomes available for protein synthesis, hormone treatment enhances the in vitro protein synthetic capacity per unit of RNA; both in microsome and ribosome preparations. Polypeptides synthesized in vitro by ribosome preparations show an enrichment in serine content after hormone treatment and an increased proportion of ribosomes can be immunoprecipitated by antibodies directed against vitellogenin. Our data are consistent with the proposal that vitellogenin is synthesized on the ribosomes of the rough endoplasmic reticulum and processed and packaged for secretion in the smooth endoplasmic reticulum and Golgi apparatus. Response to hormonal induction of vitellogenin involves an early phase in which membrane proliferation occurs in order to increase the cellular capacity to synthesize, process and secrete large quantities of egg-yolk protein precursor.

Animals↗
PubMed1976

Interferon-induced inhibition of protein synthesis in L-cell extracts: an ATP-dependent step in the activation of an inhibitor by double-stranded RNA.

The translation of encephalomyocarditis virion RNA in extracts from interferon-treated L-cells is inhibited by the addition of double-stranded RNA (dsRNA) at 400 ng/ml. A similar inhibition in response to dsRNA is seen in control cell extracts supplemented with small amounts of a postribosomal supernatant fraction from interferon-treated cells (interferon cell sap): Neither interferon cell sap nor dsRNA alone is inhibitory in control systems. The inhibition is much reduced if translation is carried out at low ATP concentrations.Conversely, the inhibitory capacity of the interferon cell sap is increased 100-fold if it is preincubated with dsRNA and ATP prior to its addition to the protein-synthesizing system. After this preincubation all detectable dsRNA can be removed without any diminution of the inhibitory activity of the cell sap. These results are compatible with a two-step model for the inhibition in which a pre-inhibitor is activated by dsRNA, the activated inhibitor then interacting with the protein synthesis system to inhibit translation.

Adenosine Triphosphate↗
PubMed1975

Sequential changes in the protein synthetic activity of male Xenopus laevis liver following induction of egg-yolk proteins by Estradiol-17 beta.

Administration of a single injection of estradiol-17 beta to adult male Xenopus laevis induces the synthesis and secretion by the liver of the egg-yolk protein complex vitellogenin. Explants taken from livers of hormone-treated animals and maintained in organ culture continue to produce this protein at a rate which is dependent on the length of time between estrogen injection of the donor and preparation of the explants. Organ culture, therefore, can be used to quantitate the vitellogenic response and to study its time course in detail. An increase in the rate of incorporation of radioactive amino acids into secreted protein is first detectable in explants taken from animals 12 hours after estradiol injection. The response becomes increasingly pronounced as the period after hormone treatment of the donors is prolonged, and it is maximal in explants taken from frogs 12 days after estrogen administration. At this time production of vitellogenin can account for as much as 90 per cent of total protein synthesis in the liver. Labeling of intracellular protein with [14C]serine is stimulated 2-fold and incorporation into secreted protein increased 36-fold under conditions where the rate or extent of uptake of radioactive precursor into the acid-soluble pool does not change. After a lag period of 2 to 4 hours the secretion of vitellogenin in culture continues at a constant rate for up to 3 days. At least part of the stability of yolk protein synthesis in culture is attributable to long lived messenger RNA species. The major component of vitellogenin labeled wither in vivo or in culture has a molecular weight of approximately 180,000 as shown by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Material labeled with [3H]serine gives a similar distribution of radioactivity on gel electrophoresis as that labeled with [35S]methionine, with no peak corresponding to the molecular weight of phosvitin. It is suggested that phosvitin is not a primary product of translation but is synthesized as part of the large subunit of vitellogenin.

Animals↗
PubMed1975

Inhibition of protein synthesis in rabbit reticulocyte lysates by double-stranded RNA and oxidized glutathione: indirect mode of action on polypeptide chain initiation.

In the presence of added double-stranded RNA or oxidized glutathione, protein synthesis in heminsupplemented reticulocyte lysates declines abruptly after 8-12 min of incubation at 30 degrees. The kinetics of amino-acid incorporation are very similar to those seen when lysates incorporation are very similar to those seen when lysates are incubated in the absence of added hemin. The inhibitory effects of double-stranded RNA (dsRNA) and oxidized glutathione (GSSG) are partially overcome by a homogeneous initiation factor, IF-MP, which also stimulates protein synthesis in hemin-deficient lysates. This factor is involved in the binding of Met-tRNAfmet to 40S ribosomal subunits during protein chain initiation. However, neither dsRNA alone nor GSSG alone significantly inhibits formation of [40S subunit-Met-tRNAf] complexes induced in reticulocyte lysates by dsRNA or GSSG involves one or more components present in the lysates but absent from the fractionated in vitro system. Such components may be related to the translational inhibitor that is active in hemin-deficient lysates.

Animals↗