Carcinogenicity study of trichloroethylene by longterm inhalation in three animal species

Pure trichloroethylene (tri), stabilized by an amine base, was administered by inhalation at 0, 100, and 500 ppm for 6 h/day, 5 days/week, for 18 months to mice, rats and Syrian hamsters of both sexes. No significant increase in tumor formation was observed in any species or dosing group, except in malignant lymphomas, which were increased in female mice in the following incidence rates: 9/29 (controls), 17/30 (100 ppm), and 18/28 (500 ppm). Whether or not this high occurrence of lymphomas, which is peculiar to this strain of mice (NMRI) has any relationship to tri-exposure, cannot be decided upon by the present experiment. It is concluded that from these findings no indication for a carcinogenic potential of pure trichloroethylene can be deduced.

Solid-state NMR approaches to internal dynamics of proteins: from picoseconds to microseconds and seconds

Solid-state nuclear magnetic resonance (NMR) spectroscopy has matured to the point that it is possible to determine the structure of proteins in immobilized states, such as within microcrystals or embedded in membranes. Currently, researchers continue to develop and apply NMR techniques that can deliver site-resolved dynamic information toward the goal of understanding protein function at the atomic scale. As a widely-used, natural approach, researchers have mostly measured longitudinal (T1) relaxation times, which, like in solution-state NMR, are sensitive to picosecond and nanosecond motions, and motionally averaged dipolar couplings, which provide an integral amplitude of all motions with a correlation time of up to a few microseconds. While overall Brownian tumbling in solution mostly precludes access to slower internal dynamics, dedicated solid-state NMR approaches are now emerging as powerful new options. In this Account, we give an overview of the classes of solid-state NMR experiments that have expanded the accessible range correlation times from microseconds to many milliseconds. The measurement of relaxation times in the rotating frame, T1ρ, now allows researchers to access the microsecond range. Using our recent theoretical work, researchers can now quantitatively analyze this data to distinguish relaxation due to chemical-shift anisotropy (CSA) from that due to dipole-dipole couplings. Off-resonance irradiation allows researchers to extend the frequency range of such experiments. We have built multidimensional analogues of T2-type or line shape experiments using variants of the dipolar-chemical shift correlation (DIPSHIFT) experiment that are particularly suited to extract intermediate time scale motions in the millisecond range. In addition, we have continuously improved variants of exchange experiments, mostly relying on the recoupling of anisotropic interactions to address ultraslow motions in the ms to s ranges. The NH dipolar coupling offers a useful probe of local dynamics, especially with proton-depleted samples that suppress the adverse effect of strong proton dipolar couplings. We demonstrate how these techniques have provided a concise picture of the internal dynamics in a popular model system, the SH3 domain of α-spectrin. T1-based methods have shown that large-amplitude bond orientation fluctuations in the picosecond range and slower 10 ns low-amplitude motions coexist in these structures. When we include T1ρ data, we observe that many residues undergo low amplitude motions slower than 100 ns. On the millisecond to second scale, mostly localized but potentially cooperative motions occur. Comparing different exchange experiments, we found that terminal NH2 groups in side chains can even undergo a combination of ultraslow large-angle two-site jumps accompanied by small-angle fluctuations that occur 10 times more quickly.

A solid-state NMR study of the fast and slow dynamics of collagen fibrils at varying hydration levels

We report solid-state NMR investigations of the effect of temperature and hydration on the molecular mobility of collagen isolated from bovine achilles tendon. (13)C cross-polarization magic angle spinning (MAS) experiments were performed on samples at natural abundance, using NMR methods that detect motionally averaged dipolar interactions and chemical shift anisotropies and also slow reorientational processes. Fast motions with correlation times much shorter than 40 micro s scale dipolar couplings and chemical shift anisotropies of the carbon sites in collagen. These motionally averaged anisotropic interactions provide a measure of the amplitudes of the segmental motions expressed by a molecular order parameter. The data reveal that increasing hydration has a much stronger effect on the amplitude of the molecular processes than increasing temperature. In particular, the Cgamma carbons of the hydroxyproline residues exhibit a strong dependence of the amplitude of motion on the hydration level. This could be correlated with the effect of hydration on the hydrogen bonding structure in collagen, for which this residue is known to play a crucial role. The applicability of 1D MAS exchange experiments to investigate motions on the millisecond time-scale is discussed and first results are presented. Slow motions with correlation times of the order of milliseconds have also been detected for hydrated collagen.

Molecular mechanism of 1,1-dichloroethylene toxicity: excreted metabolites reveal different pathways of reactive intermediates

The excretion and biotransformation of [14C] 1,1-dichloroethylene (vinylidene chloride, VDC) after administration of a single oral dose has been investigated in female rats. Seventy-two hours after a dose of 0.5, 5.0, and 50.0 mg/kg, 1.26, 9.70, 16.47%, respectively, are exhaled as unchanged VDC, and 13.64, 11.35, 6.13% as 14CO2. The main pathway of elimination is through renal excretion with 43.55, 53.88, 42.11% of the administered radioactivity. Through the biliary system, 15.74, 14.54, 7.65% of the activity are eliminated. The isolation of the main metabolites of VDC from 24 h urine is accomplished through the combined application of solvent extraction, ion exchange chromatography and thin layer chromatography. Then gas chromatography and mass spectrometry are used for their identification. Three metabolites have been identified: thiodiglycolic acid, N-acetyl-S-(2-carboxymethyl)cysteine and methyl-thio-acetylaminoethanol. In addition, three smaller unidentified radioactive peaks have been found. Thiodiglycolic acid is the main metabolite in VDC metabolism. The simultaneous formation of an ethanolamine- and a cysteine-conjugation product points to different reaction pathways of the postulated intermediate reactive epoxide; ethanolamine probably originates from membrane lipids, which react with VDC-epoxide and/or its derivatives. This pathway could explain, in part, the parenchyma damaging effect of VDC.

Direct observation of millisecond to second motions in proteins by dipolar CODEX NMR spectroscopy

We present a site-resolved study of slow (ms to s) motions in a protein in the solid (microcrystalline) state performed with the use of a modified version of the centerband-only detection of exchange (CODEX) NMR experiment. CODEX was originally based on measuring changes in molecular orientation by means of the chemical shift anisotropy (CSA) tensor, and in our modification, angular reorientations of internuclear vectors are observed. The experiment was applied to the study of slow (15)N-(1)H motions of the SH3 domain of chicken alpha-spectrin. The protein was perdeuterated with partial back-exchange of protons at labile sites. This allowed indirect (proton) detection of (15)N nuclei and thus a significant enhancement of sensitivity. The diluted proton system also made negligible proton-driven spin diffusion between (15)N nuclei, which interferes with the molecular exchange (motion) and hampers the acquisition of dynamic parameters. The experiment has shown that approximately half of the peaks in the 2D (15)N-(1)H correlation spectrum exhibit exchange in a different extent. The correlation time of the slow motion for most peaks is 1 to 3 s. This is the first NMR study of the internal dynamics of proteins in the solid state on the millisecond to second time scale with site-specific spectral resolution that provides both time-scale and geometry information about molecular motions.

Mercapturic acid formation is an activation and intermediary step in the metabolism of hexachlorobutadiene

14C-hexachlorobutadiene (HCBD), a mutagenic and nephrocarcinogenic pollutant, was administered by oral gavage of 100 mg/kg to female rats, and the radioactivity in 24 hr urine pooled. The average amount of radioactivity recovered in urine was 5.4% of the total 14C-activity ingested. Solvent extraction, high performance liquid chromatography (HPLC), radio gas chromatography and gas chromatography/mass spectrometry were used for separation and identification of metabolites. After solvent extraction and HPLC four fractions were separated containing 1%, 5%, 15% and 80% of radioactivity. In the 80% fraction one metabolite was identified after derivatization and comparison with the authentic compound as the mercapturic acid of HCBD (N-acetyl-S-1,1,2,3,4-pentachlorobutadienyl)-L-cysteine). The mercapturic acid accounts for 10% of the urinary 14C-activity. In a first attempt the mutagenic potential of the mercapturic acid was determined on Salmonella typhimurium TA 100 with and without metabolic activating S9 mix. In the presence of S9 mix the mercapturic acid exerts a strong mutagenic effect which proved to be about 80 times higher than that of HCBD. The results identify the formation of the mercapturic acid via direct glutathione conjugation as an activating and intermediary step in the metabolism of hexachlorobutadiene.

Carcinogenicity of dichloroacetylene: an inhalation study

Dichloroacetylene (DCA) is a by-product of the synthesis of certain chlorinated, aliphatic hydrocarbons (e.g., vinylidene chloride). In a long-term carcinogenicity inhalation study, groups of 30 male and 30 female rats and mice were exposed to DCA vapour under the following conditions: mice, group I: 9 p.p.m. 6 h/day, 1 day/week for 12 months; group II: 2 p.p.m. 6 h/day, 1 day/week for 18 months; group III: 2 p.p.m. 6 h/day, 2 days/week for 18 months; rats: 14 p.p.m. 6 h/day, 2 days/week for 18 months; controls: under identical conditions except for addition of DCA to breathing air. The most important result is a striking increase in the formation of kidney cystadenomas of the proximal tubuli in all DCA exposed animals. The median latency time of this tumor varied widely in both mice and rats. Renal cystic adenocarcinomas were found in statistically significant numbers in male mice. In addition to kidney tumors, the development of cystadenomas of the Harderian gland in mice and of liver cholangiomas in rats were highly significant findings. DCA failed to induce liver tumors in mice. Our results confirm that DCA possesses a high carcinogenic potential, pronounced organotropic properties and distinct species differences referring to the number and nature of the induced tumors. The risk associated with handling chlorinated aliphatic hydrocarbons is essentially increased by the decomposition product DCA.

Induction of morphological transformation and unscheduled DNA synthesis in Syrian hamster embryo fibroblasts by hexachlorobutadiene and its putative metabolite pentachlorobutenoic acid

Hexachloro-1,3-butadiene (HCBD) is a well known environmental carcinogen. The genotoxic properties of HCBD and its monooxidation product pentachloro-3-butenoic acid (PCBA) were investigated by comparative induction of unscheduled DNA synthesis (UDS) and morphological transformation in the same cell system (Syrian hamster embryo fibroblasts). HCBD and PCBA induce unscheduled DNA synthesis both in the presence and absence of an exogenous metabolizing system. The lowest effective dose for UDS induction is smaller for PCBA (1 microgram/ml) than for HCBD (2 micrograms/ml). The intensity of UDS induction is increased about 3-fold for both compounds after metabolic activation. HCBD and PCBA induce morphological transformation. The lowest effective dose for transformation differs considerably between PCBA (0.8 micrograms/ml) and HCBD (10 micrograms/ml). The results are indicative of a genotoxic mechanism for the carcinogenic actions of HCBD and PCBA.

A benzylic linker promotes methyltransferase catalyzed norbornene transfer for rapid bioorthogonal tetrazine ligation

Site-specific alkylation of complex biomolecules is critical for late-stage product diversification as well as post-synthetic labeling and manipulation of proteins and nucleic acids. Promiscuous methyltransferases in combination with analogs of S-adenosyl-l-methionine (AdoMet) can functionalize all major classes of biomolecules. We show that benzylic moieties are transferred by Ecm1 with higher catalytic efficiency than the natural AdoMet. A relative specificity of up to 80% is achieved when a norbornene moiety is placed in para-position, enabling for the first time enzymatic norbornene transfer to specific positions in DNA and RNA- even in cell lysate. Subsequent tetrazine ligation of the stable norbornene moiety is fast, efficient, biocompatible and - in combination with an appropriate tetrazine - fluorogenic.

Excretion pattern and metabolism of hexachlorobutadiene in rats. Evidence for metabolic activation by conjugation reactions

Excretion, covalent binding and metabolism of hexachloro-1,3-butadiene (HCBD), a nephrotoxic and nephrocarcinogenic compound, have been studied in female rats. Seventy-two hours after administration of a single oral dose of 1 mg/kg [14C]HCBD, 5.3% of the dose were exhaled as unchanged HCBD and 76.3% were metabolized and excreted in urine and feces or exhaled as 14CO2. After a 50 mg/kg dose of [14C]HCBD, the amount of exhaled parent compound was nearly unchanged at 5.4%. At the higher dose the gastro-intestinal absorption of HCBD appeared to be saturated with the result that unchanged HCBD constituted the major portion of the 69% radioactivity eliminated. Covalent binding to proteins in kidney and liver agreed well with the organ-specific toxicity of HCBD: binding was higher in the kidney, independent of the dose. It increased significantly when the rats were pretreated with phenobarbital, an inducer of monooxygenases; it decreased when the inhibitor piperonyl butoxide was given. Urinary radioactivity in 24 hr urine was separated by column chromatography into four fractions. High performance liquid chromatography, radio gas chromatography and gas chromatography/mass spectrometry were used for further separation and identification. Two major metabolites were identified as pentachlorobutadiene methylthio ether and pentachlorobutadiene carboxymethylthio ether. Their formation is plausibly explained via glutathione conjugation, which appears to be the first step in HCBD metabolism. The mechanism of the conjugation at the olefinic double bond of HCBD is explained by an addition-elimination reaction. This pathway, which appears to lead to a destabilization of the HCBD molecule, could explain the distinct nephrotoxic effects of HCBD.

Expanding the frequency range of the solid-state T1rho experiment for heteronuclear dipolar relaxation

Solid-state spin-lattice relaxation in the rotating frame permits the investigation of dynamic processes with correlation times in the range of microseconds. The relaxation process in organic solids is driven by the fluctuation of the local magnetic field due to the dipole-dipole interaction of the probe nuclei (13C,15N) with 1H in close proximity. However, its effect is often hidden by a competing relaxation process due to the contact between the rotating frame 13C/15N Zeeman and 1H dipolar reservoirs. In most cases the latter process becomes superior for the commonly applied low and moderate spin-lock fields and practically does not provide information about the molecular dynamics. To suppress this undesired process and to expand the dynamic range of T1rho experiments, we present two approaches. The first one uses a resonance offset of the frequency of the spin-lock irradiation, which leads to a significant enhancement of the effective spin-lock frequency without the application of destructive high transmitter powers. We derive the theory and demonstrate the applicability of the method on various model compounds. The second approach utilizes heteronuclear 1H decoupling during the 13C/15N spin-lock irradiation which disrupts the contact between the 13C/15N Zeeman and 1H dipolar reservoirs. We demonstrate the method and discuss the results qualitatively.

Neurotoxic effects of dichloroacetylene

In inhalation tests involving lethal as well as sublethal doses of dias examined in rabbits by means ofhistologic and neurofunctional methods. Histologic examination revealed chromatolysis, disintegration of Nissl bodies, and cell shrinkage in proportion to the doses employed in the nuclei of the unpaired brain stem, in the sensory cortical regions and especially in the sensory cranial nerve nuclei. The sensory trigeminal nucleus was involved most severely, followed in decreasing intensity 0y the facial and oculomotor nerves and the motor trigeminal nucleus; the least changes were observed in the acoustic nerve. By testing the protopathic sensitivity of the facial skin it was possible for the first time to produce evidence of a sensory loss in the distribution of the trigeminal nerve by animal experimentation. The neuropathological and functional deficits observed may explain the cranial nerve lesions in human DCA intoxication; however, these lesions seem to be less distinct in experimental animals.