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Biomedical subjects

J Y Chan

Publications and source records attributed to J Y Chan.

At least 163 records · Page 9Linked to original sources
PubMed1981

Differential inhibition of rat liver DNA polymerases in vitro by direct-acting carcinogens and the protective effect of a thiol reducing agent.

The direct-acting carcinogens acetoxyacetylaminofluorene, methylnitrosourea, and N-methyl-N'-nitro-N-nitrosoguanidine were tested for their ability to inhibit rat liver DNA polymerase-alpha, -beta, and -gamma activity in vitro. DNA polymerase-alpha was the most sensitive, polymerase-beta was the most resistant, and polymerase-gamma exhibited an intermediate response. When the reactions were reassayed in the presence and absence of dithiothreitol, a thiol reducing agent, it was shown that the inhibition by carcinogens was generally reversible with increasing dithiothreitol, except that polymerase-beta recovered only 80-90% of control values. These and binding data suggest that DNA polymerase-beta, the putative repair enzyme, is highly resistant to carcinogen damage. This resistance may contribute to the retention of normal function and fidelity of the repair enzyme during carcinogen exposure in vivo and to a normal cellular repair.

Acetoxyacetylaminofluorene↗
PubMed1981

Further characterization of isolated presumptive euchromatin fractions of mouse hepatoma cells utilizing a cDNA probe complementary to the homologous poly(A)+ mRNA.

Poly(A)+ mRNA from mouse hepatoma ascites cell cytoplasm is characterized by three frequency classes: an abundant frequency class of a limited number of different nucleotide sequences, a less abundant frequency class of a larger number of different nucleotide sequences, and a rare frequency class containing a high number of different nucleotide sequences. [3H]cDNA synthesized on this poly(A)+ mRNA template hybridizes with some of the DNAs of the putative transcribable euchromatin fraction at a significantly faster rate than with total DNA if residual contaminating RNA is not removed. Following NaOH incubation to remove such RNA, the cDNA probe hybridized with essentially the same rate to the euchromatin fractions and total DNA. Nick translation of the nuclease-sensitive sequences of chromatin demonstrated that, even with limited nuclease digestion, the excised sequences rapidly converted to small oligonucleotides. The nick-translatable, small chromatin segments showed no enrichment for transcribable sequences. Chromatin segments, which distribute to the 50S-70S glycerol gradient fractions and which satisfy several of the presumptive criteria for enrichment for transcribable sequences, therefore show no enrichment for sequences complementary to the cDNA for poly(A)+ mRNA.

Animals↗
PubMed1980

Decreased fidelity in one subspecies of DNA polymerase-alpha that appears during N-2-acetylaminofluorene hepatocarcinogenesis.

During the exposure of rat livers to the hepatocarcinogen 2-acetylaminofluorene (N-2-fluorenylacetamide), it was demonstrated that the cytoplasmic DNA polymerase-alpha (E.C. 2.7.7.7) was strikingly error-prone when compared to that of normal liver (Chan, J.Y.H. and Becker, F.F. (1979) Proc. Natl. Acad. Sci. U.S.A. 76, 814-818). The fidelity of polymerization of these enzymes was assayed by determining the incorporation of noncomplementary deoxyribonucleotide triphosphates (misincorporation) on a poly(dA-dT) template. To identify the mechanism of infidelity, we modified and extended our purificaton scheme. As a result, a subspecies of polymerase-alpha 1 was identified and separated from the normal component, polymerase-alpha 2. Polymerase-alpha 1 activity eluted from a phosphocellulose column at 0.07-0.12 M NaCl, while polymerase-alpha 2 eluted at 0.15-0.2 M NaCl. Polymerase-alpha 2 demonstrated normal fidelity throughout the various steps of purification while polymerase-alpha 1, despite being purified some 10 250-fold, continued to demonstrate a severe degree of infidelity.

2-Acetylaminofluorene↗
PubMed1979

Decreased fidelity of DNA polymerase activity during N-2-fluorenylacetamide hepatocarcinogenesis.

alpha and beta DNA polymerases (DNA nucleotidyltransferase; deoxynucleosidetriphosphate:DNA deoxynucleotidyltransferase, EC 2.7.7.7) were isolated from nuclear and cytoplasmic fractions of rat livers exposed to a carcinogenic regimen with the hepatocarcinogen N-2-fluorenylacetamide and from 24-hr regenerating liver. The fidelity of polymerization of these enzymes was compared by determining the incorporation of noncomplementary deoxyribonucleoside triphosphates (misincorporation) on a poly(dA-dT).poly(dA-dT) template, with MnCl2 and MgCl2 as divalent cations. Our initial studies indicate that the cytoplasmic alpha polymerases from carcinogen-exposed rat livers were strikingly error-prone whereas the nuclear and cytoplasmic beta polymerases retained their fidelity throughout the feeding cycles. The misincorporation was significantly accentuated by MnCl2 compared with that obtained with MgCl2 as divalent cation. The products were sensitive to pancreatic DNase I digestion, indicating that the noncomplementary bases had been incorporated by the polymerization process. Nuclear alpha polymerase showed some degree of infidelity but less than that of cytoplasmic alpha polymerase.

2-Acetylaminofluorene↗
PubMed1978

Surface activation of factor XII (Hageman factor)--critical role of high molecular weight kininogen and another potentiator).

When factor XII was adsorbed to kaolin it slowly became activated and converted prekallikrein to kallikrein. In the presence of HMW-kininogen the rate of activation of factor XII and consequently that of prekallikrein was markedly enhanced. The enhancing effect of HMW-kininogen was a dose-dependent phenomenon. In order to enhance the activation of factor XII on a surface the HMW-kininogen molecule had to be intact. Cleavage of HMW-kininogen by kallikrein decreased the enhancing effect of HMW-kininogen, there being an inverse relation between the bradykinin-generated and the capacity to enhance factor XII activation. Another 'potentiator' of factor XII activation was isolated from proteins adsorbed to aluminum hydroxide. This potentiator further increased the activation of factor XII, also in a dose-dependent fashion. It was postulated that factor XII is slowly converted into its active form by exposure to negatively charged surfaces; that this process is enhanced by kallikrein and further accelerated by HMW-kininogen and the 'potentiator'; and that these enhancing substances probably act by opening active sites on the factor XII molecule.

Factor XII↗
PubMed1977

Endogenous DNA polymerase activity in fractionated rat lever chromatin.

Chromatin isolated from adult rat liver was fractionated into template active and inactive components by controlled shearing and glycerol gradient centrifugation. The fractionated chromatin was assayed for DNA-dependent DNA polymerase (DNA mucleotidyl transferase EC 2.7.7.7) activity with and without exogenous activated DNA serving as template. With endogenous chromatin as template, it was found that 90% of the endogenous chromatin bound DNA polymerase activity was located in the transcriptionally active fraction of chromatin, while the distribution of DNA polymerase assayed with exogenous activated DNA was found to be 65% in the transcriptionally active and 35% in the inactive fractions. However, when DNA polymerase was solubilized from these fractions by salt extraction, enzyme activity was found to be equally distributed, suggesting that the difference in endogenous DNA polymerase activity observed between eu- and heterochromatin is due to the restricted template found in repressed fractions.

Animals↗
PubMed1976

Antigenic relationships in calf thymus and human leukemic cell terminal deoxynucleotidyl transferase.

Antibody to purified terminal deoxynucleotidyl transferase (nucleosidetriphosphate : DNA deoxy-nucleotidylexotransferase, E.C. 2.7.7.31) from calf thymus was prepared in rabbits using terminal deoxynucleotidyl transferase crosslinked to bovine serum albumin. These antibodies, partially purified by 60% ammonium sulfate precipitation and Sephadex G-200 column chromatography, produced one precipitation band with calf thymus terminal deoxynucleotidyl transferase on immunodiffusion. This antibody preparation also inhibited the in vitro activity of terminal deoxynucleotidyl transferase from calf thymus, acute leukemic lymphoblasts and Molt-4 cells but not that of DNA polymerases alpha, beta and psi from these cells

Animals↗