Genotoxicity studies with mineral oils; effects of oils on the microbial mutagenicity of precursor mutagens and genotoxic metabolites

In vitro genotoxicity assays are extensively used to predict carcinogenic activity in vivo. The standard microbial mutagenicity assays however often fail to yield positive results with mineral oils which are carcinogenic to mice in long-term skin-cancer studies. A comprehensive programme of studies has therefore investigated the basis of this apparently anomalous behaviour. This investigation has addressed the possible effects of oils on the bioactivation of precursor mutagens and the disposition of mutagenic metabolites by studying the microbial mutagenicity of selected precursor mutagens (benzo[a]pyrene, benzo[a]anthracene, 2-aminoanthracene and 2-naphthylamine) and intrinsically reactive mutagens [+/- )-benzo[a]pyrene-4,5-oxide and (+/-)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene) in the presence and absence of mineral oils. Notably the mutagenicity associated with the deliberate additions of these mutagens or precursor mutagens to oils was readily detected by the microbial assays. The mutagenicity of only one of the precursor mutagens, benzo[a]pyrene, was significantly reduced by the oils, and then only in the standard plate-incorporation assay. Interestingly the degree of suppression appeared to be related to the polycyclic aromatic hydrocarbon content of the oils. In the case of 2-aminoanthracene large enhancements in its mutagenicity were observed in the presence of oils. These latter findings appear to be due to effects of oils on the bioactivation of precursor mutagens rather than on the disposition of their bioactivation products. The mutagenicity of intrinsically reactive mutagens, of a type generated by bioactivation of polycyclic aromatic hydrocarbons, was not significantly reduced in the presence of mineral oils. This indicates that it is unlikely that components in oils trap or facilitate the deactivation of ultimate mutagens whether these pre-exist in the oil or are formed from precursors by bioactivation in the in vitro test system. Viewed overall these results suggest that mineral oils judged to be carcinogenic on the basis of in vivo studies in mouse skin may possess only very weak genotoxic potential. While this potential is likely to be a prerequisite for carcinogenic action, the current results cause attention to be focussed on other factors, e.g. promotion, as potentially important determinants of the carcinogenic potencies of mineral oils in mouse skin.     

Cell injury caused by Trichinella spiralis in the mucosal epithelium of B10A mice

Most of the mucosal epithelium in the anterior small intestine of B10A mice infected with Trichinella spiralis showed no cytopathology. However, isolated foci of damaged cells or dense masses of multinucleate cytoplasm were seen in the crypt-villus junction, or the base of the villi. Cells occupied by the nematode ranged from a nearly normal appearance, showing only compressed nuclei and organelles, to progressive inflation and vesiculation of endoplasmic reticulum, loss of terminal web and hence disoriented and reduced microvilli, and pycnosis of nuclei. Damaged cells and multinucleate cytoplasmic masses may be derived from the cells previously occupied by the nematode that were linked together by fusion of their lateral cell membranes. Damaged cells and multinucleate masses are apparently sloughed from the epithelium at the villus base without migrating up the villi. Eosinophils were seen in the lamina propria, in the mucosal epithelium (usually associated with damaged cells) and in the intestinal lumen (also with damaged cells). As no eosinophils were seen in contact with the nematode, their activities may be related more to the cells killed by the worm than to the worm itself.     

Novel silylated steroids as aromatase inhibitors

Androst-4-en-3-one analogs incorporating a trimethylsilyl or a trimethylsilylmethyl group at C-1, C-2 or C-19 were prepared and evaluated as inhibitors of aromatase. Only 10-[1-hydroxy-2-(trimethylsilyl)ethyl]estr-4-ene-3,17-dione inhibited human placental aromatase. Enzyme kinetic analysis revealed competitive inhibition [apparent dissociation constant (Ki) of 562 +/- 12 nM] associated with marginal time-dependent inhibition.     

Multiple pathways for amino acid transport inMytilus gill

Summary The epidermal tissues of marine mussels can accumulate amino acids from surrounding sea water. In the present study, gill tissue isolated from the California coastal mussel,Mytilus californianus, was used in conjunction with intact, actively pumping mussels to study epidermal transport processes. There appeared to be at least four pathways for this uptake: i) a -neutral pathway which transports taurine; ii) an -acidic pathway specific for substrates such as aspartate; iii) an -neutral pathway having a general specificity for this class of compound, but which also accepts the basic amino acid, lysine; and iv) a second -neutral pathway, also of broad specificity, capable of accepting the imino acid, proline, as a substrate. Replacement of Na in sea water with choline reduced uptake of leucine, taurine, aspartate, and proline by more than 95%, and reduced lysine uptake by 75%, suggesting that Na-independent pathways play no significant role in epidermal transport in the gill. Isolated gill tissue was used to estimate the maximum transport capacities (J _max's) of the pathways, which ranged from approximately 5 to 25 mol/(g·hr). Apparent Michaelis constants (K _t ^*'s) of the epidermal transporters were estimated using a convection-diffusion model introduced previously (Wright and Secomb, Am J Physiol 247:R346–R355, 1984). TheseK _t ^*'s ranged from 1 to 5 M. The characteristics of the epidermal transporters are such that they can play a significant role in both animal nutrition and in the reacumulation of endogenous amino acids lost from surface cells through passive diffusion.Abbreviation ASW artificial sea water     

DNase I sensitivity of nuclear DNA measured by flow cytometry

The DNase I digestion kinetics of DNA in isolated nuclei (from HeLa or murine mammary carcinoma, 67 cells) were assayed flow cytometrically by measuring the changes in ethidium bromide (EtBr) fluorescence following various digestion time intervals. The DNase I digestion curve was characterized by an initial 25-30% increase in fluorescence upon addition of the enzyme, a rapid reduction in fluorescence to approximately 50-55% in 30 minutes, and a limit digest of 45-50% beyond 45 minutes. Throughout digestion, the DNA histogram retained its characteristic bimodal shape, showing that histogram rearrangement was not responsible for the changes in EtBr fluorescence. Irradiation with 5 X 10(6) rads (137Cs-gamma-rays) or exposure to 50 mM EDTA caused an increase in EtBr fluorescence similar to that caused by DNase I, suggesting that DNA nicking and/or chromatin loosening were responsible for this increase. Residual DNA assayed by the solubilization of 14C-TdR (thymidine)-labeled DNA indicated a similar kinetic pattern without the initial increase. However, at the limit digest, the fraction of DNA remaining trichloroacetic acid (TCA) insoluble (10%) was smaller than that measured by loss of EtBr fluorescence (50% of initial, 40% of maximum). Part of this difference was due to the presence of TCA soluble DNA trapped within the nuclear matrix (15-20%). This trapped DNA was released when the digested nuclei were exposed to 0.5-1.0 M NaCl just prior to EtBr staining. Exposure of HeLa cells to three agents that are believed to cause changes in chromatin structure resulted in alterations in the DNase I digestion kinetics measured flow cytometrically.(ABSTRACT TRUNCATED AT 250 WORDS)     

Commercial freezing of bovine embryos in straws

Bovine embryos were frozen commercially in clear double length $\text{1}\text{2}$ cc French straws with the wick and powder plug in the center of the straw. One-half of the double length straw serves as a handle and contains a color coded $\text{1}\text{4}$ cc straw around which an adhesive backed label has been applied. After plunging into liquid nitrogen, straws are transferred into goblets on canes while under liquid nitrogen. The straws are stored in the liquid phase of a nitrogen tank and canes containing straws are not transferred from one container to another unless the goblet containing the straws is full of liquid nitrogen.Embryos held for longer than 4 hours after collection prior to freezing showed a steady decline in pregnancy rate related to the length of time held prior to freezing. The percentage of embryos thawed and then evaluated as being transferrable was related to the quality of the embryos prior to freeze (Grade 1–93.6%, Grade 2–87.0%, Grade 3–63.8%). There was no statistical difference in pregnancy rates obtained from prefreeze Grade 1 embryos when comparing advanced blastocysts (45.2%), blastocysts (38.7%), early blastoclyst (43.1%) and advanced morula (41.6%).     

The structure of the lactose permease derived from Raman spectroscopy and prediction methods.

The secondary structure of the lactose permease of Escherichia coli reconstituted in lipid membranes was determined by Raman spectroscopy. The alpha-helix content is approximately 70%, the beta-strand content below 10% and beta-turns contribute 15%. About 1/3 of the residues in alpha-helices and most other residues are exposed to water. Employing a method for structural prediction which accounts for amphipathic helices, 10 membrane-spanning helices are predicted which are either hydrophobic or amphipathic. They are expected to form an outer ring of helices in the membrane. The interior of the ring would be made of residues which are predominantly hydrophilic and, evoking the analogy to sugar-binding proteins, suited to provide the sugar binding site.     

The lactose/H+ carrier of Escherichia coli: lac YUN mutation decreases the rate of active transport and mimics an energy-uncoupled phenotype.

The Escherichia coli K12 strain X71-54 carries the lac YUN allele, coding for a lactose/H+ carrier defective in the accumulation of a number of galactosides [Wilson, Kusch & Kashket (1970) Biochem. Biophys. Res. Commun. 40, 1409-1414]. Previous studies proposed that the lower accumulation in the mutant be due to a faulty coupling of H+ and galactoside fluxes via the carrier. Immunochemical characterization of the carriers in membranes from mutant and parent strains with an antibody directed against the C-terminal decapeptide of the wild-type carrier leads to the conclusion that the mutant carrier is similar to the wild-type in terms of apparent Mr, C-terminal sequence, and level of incorporation into the membrane. The pH-dependence of galactoside transport was compared in the mutant and the parent. At pH 8.0-9.0, mutant and parent behave similarly with respect to the accumulation of beta-D-galactosyl 1-thio-beta-D-galactoside and to the ability to grow on the carrier substrate melibiose. At pH 6.0, both the maximal velocity for active transport and the level of accumulation of beta-D-galactosyl-1-thio-beta-D-galactoside are lower in the mutant. The mutant also is unable to grow on melibiose at pH 5.5. However, at pH 6.0 and low galactoside concentrations, the symport stoichiometry is 0.90 H+ per galactoside in the mutant as compared with 1.07 in the parent. These observations suggest that symport is normal in the mutant and that the lower rate of transport in the mutant is responsible for the phenotype. At higher galactoside concentrations, accumulation is determined not only thermodynamically but also kinetically, contrary to a simple interpretation of the chemiosmotic theory. Therefore lower rates of active transport can mimic the effect of uncoupling H+ and galactoside symport. Examination of countertransport in poisoned cells at pH 6.0 reveals that the rate constants for the reorientation of the loaded and unloaded carrier are altered in the mutant. The reorientation of the unloaded carrier is slower in the mutant. However, the reorientation of the galactoside-H+-carrier complex is slower for substrates like melibiose, but faster for substrates like lactose. These findings suggest that lactose-like and melibiose-like substrates interact with the carrier in slightly different ways.