PubMed Health⌕ Search

Biomedical subjects

R C Jackson

Publications and source records attributed to R C Jackson.

At least 19 recordsLinked to original sources
PubMed1979

Tryptic dissection and reconstitution of translocation activity for nascent presecretory proteins across microsomal membranes.

The ability of microsomal membranes to translocate nascent presecretory proteins across their lipid bilayer into the intravesicular space was investigated by using trypsin as a proteolytic probe. We found that under defined conditions trypsin is able to dissect the translocation activity of microsomal membranes into components that can be separated into two fractions, one soluble and the other membrane bound. The trypsinized membrane fraction has lost its translocation activity. Addition of the trypsin-generated soluble fraction, however, results in reconstitution of translocation activity. These results are compatible with the notion proposed in the signal hypothesis that the translocation activity of the microsomal membrane resides in transmembrane protein(s). We propose that trypsin effects solubilization from the membrane of cytosol-exposed domain(s) involved in recognition of the signal sequence or ribosome or both, leaving behind membrane-integrated domain(s) that provide the environment for the passage of the nascent chain across the membrane. Signal peptidase activity was unaffected by trypsinization of microsomal vesicles consistent with a localization of the active site of this enzyme on the cisternal side of the vesicles.

Animals↗
PubMed1978

Adenosine deaminase and adenosine kinase in rat hepatomas and kidney tumours.

Adenosine deaminase and adenosine kinase have been measured in rat liver, 12 transplantable hepatomas, regenerating, foetal and neonatal liver, adult and neonatal rat kidney and 2 transplantable kidney tumours. Adenosine, deaminase activity, relative to the normal liver value, was elevated 2-4 fold in hepatomas of rapid growth rate, was in the normal range in more slowly growing hepatomas and in regernerating liver, and was low in foetal and neonatal liver. Adenosine kinase activity was decreased, relative to rat liver values, in all the hepatomas; activity of this enzyme gave a negative correlation with tumour growth rate. Kinetic properties of the two enzymes were examined in partially purified preparations. Adenosine deaminases from both liver and rapidly growing hepatoma 3924A were subject to weak product inhibition by inosine. Adenosine kinase from liver and hepatoma 3924A was inhibited by the reaction products ADP and AMP, and the enzyme was also subject to excess substrate inhibition by concentrations of ATP in excess of 1 mM. In rat hepatoma cell lines growing in culture, the toxicity of adenosine correlated inversely with the ratio of adenosine deaminase activity to adenosine kinase activity. Chromatographic measurements showed that hepatoma cells incorporated less extracellular adenosine into their adenine nucleotide pools than did isolated liver cells. These results indicate that increased adenosine deaminase activity and decreased adenosine kinase activity may confer a selective advantage upon the cancer cell.

Adenosine↗
PubMed1977

Partial purification, properties and regulation of inosine 5'phosphate dehydrogenase in normal and malignant rat tissues.

IMP dehydrogenase (EC 1.2.1.14) was purified 180-fold from rat liver and from the transplantable rat hepatoma 3924A. The enzymes from the two sources were apparently identical; they exhibited hyperbolic saturation kinetics and an ordered, sequential mechanism, and were subject to inhibition by a number of purine nucleotides. Km values for the substrates, IMP and NAD+, were 12 and 24 micrometer respectively. IMP dehydrogenase activity in a spectrum of rat hepatomas was increased, relative to normal liver, by 2.5--13-fold; these increases correlated with tumour growth rate. Activity in two rat kidney tumours was increased 3-fold relative to that in normal renal cortex; control of activity of this enzyme is apparently altered in neoplastic cells. After partial hepatectomy, IMP dehydrogenase activity began to rise 6 h after operation, reaching a peak of 580% of normal activity by 18 h. Activity in neonatal liver, however, was only slightly higher than that in the adult. Organ-distribution studies showed highest enzyme activities in spleen and thymus. In livers of rats starved for 3 days, where all enzymes, except those involved in gluconeogenesis, showed decreased activity IMP dehydrogenase activity was increased; this change was accompanied by a rise in hepatic GTP concentrations. It is concluded that IMP dehydrogenase is a key enzyme in the regulation of GTP production, and thus involved in regulation of nucleic acid biosynthesis. The increased activity of IMP dehydrogenase in liver of starved rats may be related to the requirements for GTP for gluconeogenesis.

Animals↗
PubMed1977

Post-translational cleavage of presecretory proteins with an extract of rough microsomes from dog pancreas containing signal peptidase activity.

The protease(s) responsible for removing the amino-terminal extension of nascent presecretory proteins (signal peptidase) has been extracted from rough microsomes of dog pancreas with the detergent sodium deoxycholate. Preprolactin and pre-growth-hormone, prepared by in vitro translation of bovine pituitary RNA in the wheat germ system, were used to assay signal peptidase in the extract. When added to the wheat germ system during translation, the extract reduced the size of preprolactin and pre-growth-hormone to that of prolactin and growth hormone, respectively. Post-translational addition of the extract also reduced the size of preprolactin and pre-growth-hormone to that of the authentic hormones. The prolactin produced by post-translational cleavage of radiolabeled preprolactin has been shown, by partial amino-terminal sequence analysis, to have the correct amino terminus. This post-translational assay has permitted the investigation of the subcellular localization of the enzyme. Sodium deoxycholate extracts of rough microsomes were active, whereas extracts of smooth microsomes were inactive. However, without detergent treatment, neither rough nor smooth microsomes were capable of cleaving preprolactin in the post-translational assay. From this we conclude that the signal peptidase activity is confined to the rough endoplasmic reticulum and is latent. Finally, we have detected two small peptides which we believe could be the signal peptides generated by the endoproteolytic cleavage of preprolactin and pre-growth-hormone by signal peptidase.

Animals↗
PubMed1976

On the identity of nuclear membrane and non-histone nuclear proteins.

The fate of plasma and nuclear membrane polypeptides in preparations of acidic chromosomal protein from chicken erythrocytes has been investigated. It is shown that detergent extraction procedures (Nonidet P-40, Triton X-100, and saponin), commonly employed in the preparation of acidic chromosomal protein, cannot be relied upon to remove plasma and nuclear membrane polypeptides. These polypeptides persist in nuclear and chromatin preparations and subsequently fractionate as acidic chromosomal protein. In fact, the polypeptides in a preparation of erythrocyte acidic chromosomal protein are shown by gel electrophoresis in dodecyl sulfate to be almost identical to those in a preparation of erythrocyte nuclear membrane. The implication of these results for the preparation of acidic chromosomal protein is dicussed.

Animals↗