Studies on the denaturation of biological macromolecules by chemical carcinogens. 3. Optical rotatory dispersion and light-scattering changes of ovalbumin during denaturation and aggregation by water-soluble carcinogens

Interaction of N-nitrosodiethylamine/bovine serum albumin complexes with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine monolayers at the air-water interface

We report the effect of N-nitrosodiethylamine (NDA) on the interaction between bovine serum albumin (BSA) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine monolayers (DPPC) at the air-water interface. We prepared aqueous solutions of NDA/BSA complexes maintaining a constant concentration of BSA of 1.49 x 10(-9) M and using NDA concentrations to obtain 2000, 4000, 6000, 12,500, and 25,000 NDA/BSA molar ratios. The hysteresis area and the compressional modulus of the compression-expansion cycles performed at different times were dependent on the NDA concentration. The cycles performed demonstrate the stability of the new phase of DPPC/BSA and DPPC/NDA/BSA monolayers. This was achieved probably because the BSA concentration used was lower than the one needed for BSA to inhibit the return of DPPC molecules to the interface. Results of the compressional modulus at the onset of the new phase, obtained around 17 mN/m, 15 min and 1, 3, 5, and 12 h after DPPC deposition, indicated that the 3.0 x 10(-6) M NDA concentration produced a more rigid film, probably due to the higher alpha-helix content of BSA. AFM images were obtained for DPPC/BSA and two DPPC/NDA/BSA complexes. Our images show that 12,500 NDA/BSA molecules were mostly adsorbed in the liquid condensed phase. However, BSA molecules were distributed more homogeneously.

Interfacial Behavior of N-Nitrosodiethylamine/Bovine Serum Albumin Complexes at the Air−Water and the Chloroform−Water Interfaces by Axisymmetric Drop Tensiometry

Interfacial properties of N-nitrosodiethylamine/bovine serum albumin (NDA/BSA) complexes were investigated at the air-water interface. The interfacial behavior at the chloroform-water interface of the interaction product of phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), dissolved in the chloroform phase, and NDA/BSA complex, in the aqueous phase, were also analyzed by using a drop tensiometer. The secondary structure changes of BSA with different NDA concentrations were monitored by circular dichroism spectroscopy at different pH and the NDA/BSA interaction was probed by fluorescence spectroscopy. Different NDA/BSA mixtures were prepared from 0, 7.5 x 10(-5), 2.2 x 10(-4), 3.7 x 10(-4), 5 x 10(-4), 1.6 x 10(-3), and 3.1 x 10(-3) M NDA solutions in order to afford 0, 300/1, 900/1, 1 500/1, 2 000/1, 6 000/1, and 12 500/1 NDA/BSA molar ratios, respectively, in the aqueous solutions. Increments of BSA alpha-helix contents were obtained up to the 2 000/1 NDA/BSA molar ratio, but at ratios beyond this value, the alpha-helix content practically disappeared. These BSA structure changes produced an increment of the surface pressure at the air-water interface, as the alpha-helix content increased with the concentration of NDA. On the contrary, when alpha-helix content decreased, the surface pressure also appeared lower than the one obtained with pure BSA solutions. The interaction of DPPC with NDA/BSA molecules at the chloroform-water interface produced also a small, but measurable, pressure increment with the addition of NDA molecules. Dynamic light scattering measurements of the molecular sizes of NDA/BSA complex at pH 4.6, 7.1, and 8.4 indicated that the size of extended BSA molecules at pH 4.6 increased in a greater proportion with the increment in NDA concentration than at the other studied pH values. Diffusion coefficients calculated from dynamic surface tension values, using a short-term solution of the general adsorption model of Ward and Tordai, also showed differences with pH and the NDA concentration. Both, the storage and loss dilatational elastic modulus were obtained at the air-water and at the chloroform-water interfaces. The interaction of NDA/BSA with DPPC at the chloroform-water produced a less rigid monolayer than the one obtained with pure DPPC (1 x 10(-5) M), indicating a significant penetration of NDA/BSA molecules at the interface. At short times and pH 4.6, the values of the storage elastic modulus were larger and more sensible to the NDA addition than the ones at pH 7.1 and 8.4, probably due to a gel-like network formation at the air-water interface.

Kinetics and mechanisms of peptide aggregation. I: Aggregation of a cholecystokinin analogue

Aggregation kinetics for a tetrapeptide analogue of cholecystokinin (A-71623) have been studied by quasi-elastic light scattering. Aggregation kinetics were quantitated with a kinetic model, described herein, which was modified for quasi-elastic light scattering data. The model predicts that the average molecular weight of peptide aggregates increases in a linear fashion with time. Data generated for A-71623 were consistent with the model presented under conditions of varied ethanol concentration at a fixed peptide concentration, as well as varied A-71623 concentration at fixed ethanol concentration. Although not the primary thrust of this study, experimental design permitted some understanding of the effect of environmental changes on the apparent aggregation kinetics of A-71623. These studies suggest A-71623 aggregation may be partially mediated by hydrophobic bonding.

Studies on the metabolism of dimethylnitrosamine in vitro by rat-liver preparations. I. Comparison with mixed-function oxidase enzymes

1. The metabolism of dimethylnitrosamine (DMN) to formaldehyde by rat-liver preparations has been studied at substrate concn. of 0.5, 5 and 50 mM and compared with mixed-function oxidase enzyme activities. 2. The microsomal metabolism of low (0.5 and 5 mM) and high (50 mM) substrate concn. of DMN was differentially affected by acetone addition or KI treatment. 3. A series of heterocyclic compounds related to pyrazole were potent inhibitors of metabolism of 0.5 and 5 mM DMN at concn. which had little effect on mixed-function oxidase activities. In contrast, purine addition slightly stimulated the metabolism of low but not high concn. of DMN. 4. The results are consistent with the suggestion that multiple enzymic pathway(s) are involved in hepatic DMN metabolism and that some of these pathway(s) may be independent of cytochrome P-450.

Effects of dioxane on RNA synthesis in the rat liver

The effect of dioxane on transcription was investigated. On intravenous administration of the lower dose (10 mg/rat) both RNA polymerase A and B activities were initially depressed but recovered after 1 h. A secondary effect on RNA polymerase A was observed but the enzyme activity recovered at 24 h when it was about 35% above control values whereas RNA polymerase B activity had achieved control values. With a higher dose (100 mg/rat) the pattern was similar but the suppressive effects were more pronounced - RNA polymerase B activity never attained control values during the times studied whereas there was no increase in RNA polymerase A activity at 24 h.

Protein synthesis inhibition induced by dimethylnitrosamine and diethylnitrosamine on isolated rat hepatocytes

Time- and dose-dependent protein synthesis inhibition takes place following exposure to high doses of dimethylnitrosamine (DMN) or diethylnitrosamine (DENA) in isolated rat hepatocytes. The ability of DENA to depress protein synthesis is 5-fold higher than that of DMN. Cells inhibited by 60 min exposure to DMN or DENA, and then incubated in a nitrosamine-free medium, regain their initial rate of protein synthesis. This recovery is faster and more complete for DENA-treated cells.

Studies of the denaturation and partial renaturation of ovalbumin

1. The denaturation of ovalbumin by the reagents sodium dodecyl sulphate and guanidinium chloride was investigated, by following the changes in sedimentation velocity, optical rotatory dispersion and viscosity as a function of denaturant concentration. 2. With sodium dodecyl sulphate both the optical-rotatory-dispersion parameters a(0) and b(0) become more negative, the sedimentation coefficient decreases and the viscosity increases; significant differences in the denaturation profiles are observed. The change in each parameter is indicative of only limited denaturation. 3. With guanidinium chloride the transition occurs over the concentration range 1-4m: more extensive changes occur in all the physical parameters than with sodium dodecyl sulphate. The values of a(0) and b(0) are indicative of complete denaturation. Reduction by mercaptoethanol produces only minor further changes. 4. Renaturation was attempted from both denaturants, the removal of reagent being accomplished reversibly by controlled slow dialysis. Partial renaturation was observed, but aggregated or insoluble material was produced in both cases at relatively low concentrations of denaturant. Similar behaviour was observed with fully reduced protein in guanidinium chloride-mercaptoethanol; complete renaturation could not be brought about even at very low protein concentrations.

Induction of carcinomas in the nasal cavity of rats by dioxane

Six out of 120 rats fed dioxane in drinking water at levels of 0.75, 1.0, 1.4 or 1.8% developed carcinomas in the nasal cavity. Spontaneous tumors at this tissue localization have not been reported to occur in laboratory animals. The carcinomas were pre-eminently of epidermoid type with few adenocarcinomatous areas and epithelial papillomas. Four rats with carcinoma of the nasal cavity had hepatocellular carcinoma in addition.