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Biomedical subjects

R C Jackson

Publications and source records attributed to R C Jackson.

At least 91 records · Page 5Linked to original sources
PubMed1980

Secretion requires a cytoplasmically disposed sulphydryl of the RER membrane.

The capacity of the rough endoplasmic reticulum (RER) membrane of eukaryotic cells in translocate nascent presecretory proteins from the cytosol to the intracisternal space is preserved on cell fractionation and can be assayed in vitro. Two attempts to characterize this translocation activity have been reported. Warren and Dobberstein reported that microsomal membranes can be depleted of their translocation activity by extraction with a solution of high ionic strength (500 mM KCl) and that activity can be restored to the depleted membranes by re-addition of the salt extract. On the other hand, Walter et al. reported that KCl extraction of the microsomal membrane does not result in complete depletion of its translocation activity. However, mild trypsinization of the microsomal membrane released a tryptic fragment(s) from the membrane which, when recombined with a tryptically inactivated membrane fraction, restored translocation activity. We now show that both the trypsin and the KCl extracted factors, but not the membrane-integrated remainder of the translocation apparatus, contain at least one sulphydryl group that is essential for activity.

Biological Transport↗
PubMed1980

Kinetic simulation of anticancer drug interactions.

A model is described that simulates the biochemical pathways of folate and nucleotide metabolism involved in DNA precursor biosynthesis. Examples are given of use of the model to study various aspects of the biochemical pharmacology of antitumour drugs. Modelling may be done in two ways: detailed simulation of all variables may be conducted for short time periods (less than 6 h of real time); alternatively, by assuming that concentrations of rapidly interconvertible metabolites remain close to steady-state proportions, time periods of several days may be modelled, facilitating simulation of cell growth in presence of drugs. Experiments designed to test predictions for these various types of study are described.

Animals↗
PubMed1980

Biochemical approaches to enhancement of antitumor drug selectivity: selective protection of cells from 6-thioguanine and 6-mercaptopurine by adenosine.

The cytotoxicity of 6-thioguanine and 6-mercaptopurine to cultured lymphoblasts and fibroblasts was strongly antagonized by pretreatment of the cells with 100 microM adenosine. Administration of adenosine 2 hours after the antipurine agent did not cause antagonism. In two rat hepatoma cell lines, adenosine pretreatment did not protect cells from the antipurines. Treatment of lymphoblasts or fibroblasts with 100 microM adenosine gave increases up to 150% in cellular ATP and ADP and decreases greater than 80% in UTP and UDP. In the hepatoma lines, adenine nucleotides did not increase by greater than 45%, and uridine nucleotides did not decrease by greater than 40% following adenosine treatment. The selective protection of the normal cells from 6-thioguanine and 6-mercaptopurine was probably the consequence of phosphoribosylpyrophosphate (PRPP) depletion, since adenosine pretreatment decreased PRPP pools by greater than 90% in the normal cells but by only 30% in the malignant hepatoma cells. In the absence of PRPP the antipurines would not be metabolically activated. The selectivity of the adenosine and antipurine combinations was probably attributable to the low activity of adenosine kinase and high activities of adenosine deaminase and PRPP synthetase characteristic of malignant hepatomas.

Adenosine↗
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↗