Carcinogen activation by human liver enzymes in the Ames mutagenicity test.

Liver post-mitochondrial supernatants derived from 10 individuals were used as the source of metabolic activation for carcinogens in the Ames quantitative mutagenicity test using Salmonella typhimurium TA 100. The liver samples were obtained from brain-dead donors and autopsy cases. The ability of human enzymes to activate aromatic amines ranged from the undetectable to highly active for 2-acetylaminofluorene. None of the samples exhibited any ability to activate benzidine. A generally low activity was observed in the capability of human enzymes to activate the polynuclear aromatic hydrocarbons, 3-methylcholanthrene and benzo(a)pyrene. Most samples were positive for activating 4-nitrobiphenyl. However, the highest mutagenic activity in the presence of human enzymes was consistently observed for aflatoxin B1 and sterigmatocystin. These results indicated that (a) human enzyme systems, like rodent systems, are more effective in inducing mutagenic activity from mycotoxins than aromatic amines and polynuclear aromatic hydrocarbons, and (b) samples derived from different individuals exhibited considerable variation in the ability to activate carcinogens belonging to a same class of compound.     

Identification of a highly conserved domain in the EcoRII methyltransferase which can be photolabeled with S-adenosyl-L-[methyl-3H]methionine. Evidence for UV-induced transmethylation of cysteine 186

DNA methyltransferases can be photolabeled with S-adenosyl-L-methionine (AdoMet). Specific incorporation of radioactivity has been demonstrated after photolabeling with either [methyl-3H]AdoMet or [35S]AdoMet (Som, S., and Friedman, S. (1990) J. Biol. Chem. 265, 4278-4283). The labeling is believed to occur at the AdoMet binding site. With the purpose of localizing the site responsible for [methyl-3H]AdoMet photolabeling, we cleaved the labeled EcoRII methyltransferase by chemical and enzymatic reactions and isolated the radiolabeled peptides by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and high pressure liquid chromatography. The labeled peptides were identified by amino-terminal sequencing. A common region was localized which accounted for 65-70% of the total label. This region includes a highly conserved core sequence present in all DNA (cytosine 5)-methyltransferases. One such fragment was digested further with chymotrypsin, and amino acid analysis of the resulting 3H-labeled peptide was consistent with the sequence Ala-Gly-Phe-Pro-(Cys)-Gln-Pro-Phe-Ser-Leu. However, the cysteine residue was not recovered as carboxymethylcysteine. The Pro-Cys bond was found to be protected from cleavage at cysteine residues after cyanylation. These results suggest that the cysteine residue is modified by the labeling reaction. The chymotryptic fragment was hydrolyzed enzymatically to single amino acids, and the labeled amino acid was identified as S-methylcysteine by thin layer chromatography. These results indicate that the cysteine residue is located at or close to the AdoMet binding site of EcoRII methyltransferase.     

Selective metal binding to Cys-78 within endonuclease V causes an inhibition of catalytic activities without altering nontarget and target DNA binding

T4 endonuclease V is a pyrimidine dimer-specific DNA repair enzyme which has been previously shown not to require metal ions for either of its two catalytic activities or its DNA binding function by virtue of its ability to function in the presence of metal-chelating agents. However, we have investigated whether the single cysteine within the enzyme was able to bind metal salts and influence the various activities of this repair enzyme. A series of metals (Hg2+, Ag+, Cu+) were shown to inactivate both endonuclease Vs pyrimidine dimer-specific DNA glycosylase activity and the subsequent apurinic nicking activity. The binding of metal to endonuclease V did not interfere with nontarget DNA scanning or pyrimidine dimer-specific binding. The Cys-78 codon within the endonuclease V gene was changed by oligonucleotide site-directed mutagenesis to Thr-78 and Ser-78 in order to determine whether the native cysteine was directly involved in the enzyme's DNA catalytic activities and whether the cysteine was primarily responsible for the metal binding. The mutant enzymes were able to confer enhanced ultraviolet light (UV) resistance to DNA repair-deficient Escherichia coli at levels equal to that conferred by the wild type enzyme. The C78T mutant enzyme was purified to homogeneity and shown to be catalytically active on pyrimidine dimer-containing DNA. The catalytic activities of the C78T mutant enzyme were demonstrated to be unaffected by the addition of Hg2+ or Ag+ at concentrations 1000-fold greater than that required to inhibit the wild type enzyme. These data suggest that the cysteine is not required for enzyme activity but that the binding of certain metals to that amino acid block DNA incision by either preventing a conformational change in the enzyme after it has bound to a pyrimidine dimer or sterically interfering with the active site residue's accessibility to the pyrimidine dimer.     

Differing macrophage and lymphocyte roles in resistance to Legionella pneumophila infection.

Similar to guinea pig macrophages and human monocytes, macrophages from the peritoneal cavity of thioglycolate pretreated A/J mice are permissive for growth of Legionella pneumophila. In contrast, macrophages from BDF1 mice are not permissive for L. pneumophila. Lymphocytes from A/J and BDF1 mice proliferated in response to Legionella Ag but guinea pig lymphocytes did not. Also, splenocyte cultures from A/J mice treated with either Con A or Legionella vaccine produced supernatants which induced A/J macrophages to restrict Legionella growth, but guinea pig splenocyte culture supernatants obtained after stimulation with L. pneumophila vaccine did not induce Legionella growth restriction activity by guinea pig macrophages. Murine rIFN-gamma but not rIFN-alpha markedly inhibited growth of Legionella in A/J mouse macrophages and monoclonal anti-IFN-gamma antibody neutralized the anti-Legionella activity of culture supernatants from A/J mouse splenocytes responding to Legionella Ag. From these data, IFN-gamma appears to be an important factor in anti-Legionella activity of Ag-activated mouse splenocyte culture supernatants. Cyclosporin A, when given to either A/J or BDF1 mice, reduced the proliferation responses of splenocytes to T cell mitogens and also decreased the IFN production of A/J spleen cells to Legionella Ag. In addition, drug treatment decreased the resistance of A/J mice to Legionella infection as shown by an increase in the number of viable bacteria in the liver. However, injection of drug treated mice with lymphokine-rich splenocyte culture supernatant reconstituted the resistance of these animals. These results suggest an important role for lymphocyte activation and lymphokine production in the resistance of A/J mice to Legionella infection. The greater resistance of BDF1 mice, however, may result from nonpermissive macrophages and responsive lymphocytes. In the case of guinea pigs, susceptibility to Legionella infections may result from both the permissive nature of the macrophages and the relatively unresponsive nature of the lymphocytes in these animals.     

Waardenburg syndrome (WS): the analysis of a single family with a WS1 mutation showing linkage to RFLP markers on human chromosome 2q.

Waardenburg syndrome type I (WS1; MIM 19350) is caused by a pleiotropic, autosomal dominant mutation with variable penetrance and expressivity. Of individuals with this mutation, 20%-25% are hearing impaired. A multilocus linkage analysis of RFLP data from a single WS1 family with 11 affected individuals indicates that the WS1 mutation in this family is linked to the following four marker loci located on the long arm of chromosome 2: ALPP (alkaline phosphatase, placental), FN1 (fibronectin 1), D2S3 (a unique-copy DNA segment), and COL6A3 (collagen VI, alpha 3). For the RFLP marker loci, a multilocus linkage analysis using MLINK produced a peak lod (Z) of 3.23 for the following linkage relationships and recombination fractions (theta i): (ALPP----.000----FN1)----.122----D2S3----.267----CO L6A3. A similar analysis produced a Z of 6.67 for the following linkage relationships and theta i values among the markers and WS1: (FN1----.000----WS1----.000----ALPP)----.123----D2S 3----.246----COL6A3. The data confirm the conclusion of Foy et al. that at least some WS1 mutations map to chromosome 2q.     

Deletion-mutant epidermal growth factor receptor in human gliomas: effects of type II mutation on receptor function.

Malignant human glioma D-298 MG amplifies a rearranged epidermal growth factor receptor (EGFR) gene (c-erbB proto-oncogene), resulting in an in-frame deletion of 83 amino acids in domain IV of the extracellular domain of the EGFR. EGF and transforming growth factor-a (TGF-a) bound to the mutant EGFR with high affinity and enhanced the intrinsic mutant EGFR kinase activity. The mutant EGFR was capable of transducing EGF-stimulated glioma cell proliferation and invasiveness in an in vitro three-dimensional spheroid model. The deletion-mutant EGFR in D-298 MG is capable of being activated by growth factor; this suggests that overexpression of this mutant EGFR protein rather than structural alteration may be the more significant biologic event.     

Transposition of the gene cluster CYC1-OSM1-RAD7 in yeast

A mutant of the yeast Saccharomyces cerevisiae contains an increased amount of iso-1-cytochrome c because two copies of a segment, denoted COR, were transposed to a new position on chromosome VII, while the original COR region was retained at the normal position on chromosome X; this COR segment encompasses the CYC1, OSM1 and RAD7 loci which determine, respectively, iso-1-cytochrome c, osmotic sensitivity and ultraviolet light sensitivity. The analysis of genomic DNA with cloned probes indicates that the length of the COR segment is approximately 12,000 base-pairs. We suggest that certain normal strains of yeast, which possibly may contain reiterated sequences, can produce extended transpositions similar to prokaryotes.     

Dissociation of interferon effects on murine leukemia virus and encephalomyocarditis virus replication in mouse cells.

Two subclones of Swiss mouse cells infected with Moloney murine leukemia virus (M-MuLV) were tested for their response to interferon (IFN). Whereas M-MuLV production in the two subclones was inhibited to the same extent, one of the subclones was significantly more sensitive to IFN when the antiviral effect was measured by replication of encephalomyocarditis (EMC) virus. The same subclone was also more sensitive to the anticellular activities of IFN. Additionally, NIH 3T3 cells infected with M-MuLV were completely resistant to IFN actions when EMC virus replication or the anticellular activities were tested. However, under the same conditions, M-MuLV production was completely inhibited by IFN. These results indicate that IFN may affect cell growth functions and EMC replication through mechanisms different from those by which MuLV production is inhibited.