Formation of cytosine glycol and 5,6-dihydroxycytosine in deoxyribonucleic acid on treatment with osmium tetroxide

OsO4 selectively forms thymine glycol lesions in DNA. In the past, OsO4-treated DNA has been used as a substrate in studies of DNA repair utilizing base-excision repair enzymes such as DNA glycosylases. There is, however, no information available on the chemical identity of other OsO4-induced base lesions in DNA. A complete knowledge of such DNA lesions may be of importance for repair studies. Using a methodology developed recently for characterization of oxidative base damage in DNA, we provide evidence for the formation of cytosine glycol and 5,6-dihydroxycytosine moieties, in addition to thymine glycol, in DNA on treatment with OsO4. For this purpose, samples of OsO4-treated DNA were hydrolysed with formic acid, then trimethylsilylated and analysed by capillary gas chromatography-mass spectrometry. In addition to thymine glycol, 5-hydroxyuracil (isobarbituric acid), 5-hydroxycytosine and 5,6-dihydroxyuracil (isodialuric acid or dialuric acid) were identified in OsO4-treated DNA. It is suggested that 5-hydroxyuracil was formed by formic acid-induced deamination and dehydration of cytosine glycol, which was the actual oxidation product of the cytosine moiety in DNA. 5-Hydroxycytosine obviously resulted from dehydration of cytosine glycol, and 5,6-dihydroxyuracil from deamination of 5,6-dihydroxycytosine. This scheme was supported by the presence of 5-hydroxyuracil, uracil glycol and 5,6-dihydroxyuracil in OsO4-treated cytosine. Treatment of OsO4-treated cytosine with formic acid caused the complete conversion of uracil glycol into 5-hydroxyuracil. The implications of these findings relative to studies of DNA repair are discussed.

Mitochondrial granules: are they reliable markers for heavy metal cations?

Mitochondrial granules have been used as markers for heavy metal cations, but since such granules can also be found in tissues in the absence of such cations, an attempt was made to define conditions under which these different granules might be visualised. The tissue used was the smooth muscle of the central ear artery of the rabbit. In all studies, the presence or absence of mitochondrial granules was determined by several observers, using coded specimens so that the previous treatment of the specimens was not at the time known to the observers. Paired tissues were exposed for 30 or 90 minutes at 20, 30 or 39 degrees C to an incubate containing either 10 mmol/l barium or a control barium-free solution. After fixation in osmium tetroxide, there was no difference between the two groups; in both cases granules appeared more frequently the longer the time and the higher the temperature of incubation. In a further series where glutaraldehyde was the fixative, granules were identified in 23 out of 41 tissues incubated with barium, but in only 1 out of 41 control tissues (P less than 0.001). Electron probe microanalysis showed that granules in osmium-fixed tissues contained osmium as the main element, whereas granules in glutaraldehyde-fixed tissues which had been incubated with barium showed barium as the predominant cation. Thus mitochondrial granules can be reliable markers for heavy metal cations, but only under carefully controlled conditions.

Modification of ColE1 DNA with osmium tetroxide generates positively supercoiled molecules

Covalent binding of osmium tetroxide to negatively supercoiled DNA in vitro initially induces its relaxation, accompanied by a formation of a single denaturation "bubble" per molecule. Binding of further osmium results in DNA overwinding and the appearance of positive supercoils as demonstrated by gel electrophoresis and electron microscopy.

Ultrastructural demonstration of calcitonin in osmium-fixed human medullary carcinoma of thyroid by the protein A-colloidal gold technique

In two medullary carcinomas of the thyroid gland two types of secretory granules were found electron microscopically in the cytoplasm of the tumour cells. The sizes of the granules in one case ranged 103-345 nm in diameter; they were round in shape, and they co-existed in the same tumour cell. They could not, therefore, be distinctively subdivided into two types. In another case, secretory granules in the cytoplasm closely resemble EC granule in morphology. Using the protein A-colloidal gold (PAG) technique the content of secretory granules could be identified as calcitonin irrespective of their sizes or morphology. Immunoreactivity at the ultrastructural level was fairly well preserved even in the osmium-fixed tumour cells. The labelling index, expressed as a mean number of gold particles per unit square area of the secretory granule, was higher in the non-osmium-fixed tumour cells than in the osmium-fixed. Non-osmium-fixed tumour cells embedded either in epoxy or methacryl resin were almost equally labelled with gold particles. The result indicates that the PAG method is practicable to demonstrate the ultrastructural localization of calcitonin even in the osmium-fixed, epoxy resin embedded material.

[Demonstration of lipids by imidazole-buffered osmium tetroxide in the odontoblasts and cells of the enamel organ in the rat incisor]

Tissue post-fixation with imidazole buffered osmium tetroxide allows localization of lipids rich in unsaturated fatty acids (Angermüller and Fahimi, 1982). In odontoblasts, a positive reaction is observed in mitochondria, in membranous structures and in vesicles containing an osmiophilic and amorphous material. In odontoblastic processes, tubulo-vesicular structures and electron-dense coated vesicles seemed to be engaged in endocytosis and transfer processes towards lysosomial structures of the cell body. In the enamel organ, lipidic structures are observed in the stellate reticulum and the stratum intermedium. The plasma membrane of secretory ameloblasts as well as the content of some lysosomial structures are well contrasted. However the distal membranes of the Tomes processes showed weakly stained portions alternating with electron dense segments. Extracellular lipidic globules, sometimes associated with plasma membranes were also seen. These observations demonstrated the alternating weak and strong staining reactions in unsaturated fatty acids of plasma membrane areas of the cell processes, which contrasts with the continuous staining of the plasma membrane of the cell bodies. These observations could be related to functional differences.

Repair of the mouse synovial membrane after chemical synovectomy with osmium tetroxide

Repair of the synovium of the mouse knee joint following intra-articular injection with osmium tetroxide has been studied by light and electron microscopy. Within a first few days after osmium tetroxide injection complete necrosis of the synovial intima and inflammatory response in the subintimal connective tissue occurred. The resurfacing of the denuded synovium was performed by proliferation of immature synovioblasts which appeared to be derived from mesenchymal cells in the transition zone. The synovial intima completely restored the original structure, while repair of the subintimal tissue was accomplished by a scarring.

Osmium-induced alteration in DNA structure

In the presence of pyridine and other ligands osmium tetroxide binds covalently to pyrimidine bases in DNA. Properties of osmium-modified native and denatured calf thymus DNA, and plasmid Co1E1 DNA were investigated by means of differential pulse polarography, absorption spectrophotometry, circular dichroism, agarose gel electrophoresis, and nuclease S1 digestion. A great difference in the reaction kinetics of native and denatured DNAs with osmium, pyridine was observed. On the ground of the slow stepwise reaction kinetics of native DNA in the initial stage of its modification by osmium it has been suggested that the primary reaction sites do not include bases contained in the intact double helix. Osmium binding to sporadic primary reaction sites (represented e.g. by bases in the vicinity of a single-strand break) in native calf thymus DNA resulted in local changes in DNA conformation limited to a close neighbourhood of the binding site. At higher osmium/nucleotide ratios disordering of the DNA structure over a region extending beyond the immediate binding site was observed. With denatured DNA the same type of structure disordering was detected already in the initial stage of the reaction at osmium/nucleotide ratios as low as 0.01. Osmium binding to the supercoiled Co1E1 DNA resulted in its relaxation without nicking and it increased its sensitivity to linearization by cleavage with nuclease S1. The behaviour of Co1E1 DNA has been explained by the formation of a denatured region in the molecule (accompanied by a coupled loss of duplex and superhelical turns). It has been suggested that osmium can be used to label and to visualize distorted regions in the DNA double helix.

Osmium tetroxide: a new probe for site-specific distortions in supercoiled DNAs

Supercoiled plasmids Col E1 and cDm 506 (a Col E1 derivative carrying the D. melanogaster histone gene repeat) were treated with OsO4 in presence of pyridine and the reaction products were analyzed using different approaches. Gel electrophoresis showed that OsO4 binding to supercoiled DNA induced its relaxation without nicking. The amount of osmium bound to DNA (as determined electrochemically) increased with the extent of DNA relaxation. As a result of osmium modification of supercoiled cDm 506, a single denaturation "bubble" was observed in the electron microscope. Mapping of the osmium binding site by S1 nuclease cleavage followed by restriction enzyme digestion has revealed one major site in the intergenic spacer between the H1 and H3 histone genes of D. melanogaster. This site differs from the site cleaved by S1 nuclease in supercoiled DNA in the absence of osmium.

[Amanita virosa peptides: viroidin and viroisin are more effective than phalloidin for the in vitro protection of actin against the effects of osmic acid]

Virotoxins are a group of monocyclic peptides recently identified in the deadly mushroom Amanita virosa by Faulstich and coll. We found that two of these peptides, which have a methyl sulfonyl group, namely viroidin and viroisin are very effective to protect F-actin against oxidative degradation by osmium tetroxide in vitro. Their desoxo analogs, which have a methyl sulfoxyde group instead of methyl sulfonyl are less active, therefore there exists a relationship between the chemistry of the sulfur group and the activity of the peptides.

The effect of osmium postfixation and uranyl and lead staining on the ultrastructure of young enamel in the rat incisor

Enamel crystallites are electron opaque without osmium or heavy metal staining and give a crystalline electron diffraction pattern. Since the opacity and diffraction pattern are abolished from ultrathin sections of young enamel by floating on distilled water (Bishop and Warshawsky, 1982), the possibility that aqueous staining may also remove crystallites was tested. In addition, the effect of osmium postfixation on crystallite structure was examined. Rat incisors fixed by perfusion with a mixture of aldehydes were either nonosmicated or osmicated prior to dehydration. Incisor segments in the region of inner enamel secretion were embedded in the same Epon block to ensure reliable comparison. Osmicated enamel was more intensely stained with toluidine blue and more electron opaque than nonosmicated enamel. No other structural differences were seen. However, crystallites in osmicated enamel were more resistant to grid demineralization and electron beam damage. Routine staining was done by floating sections on solutions of uranyl acetate and lead citrate; sections were also floated on similar solutions from which the heavy metals were omitted. These solutions removed the electron opaque crystallites from the youngest enamel. Stained sections showed electron opaque crystallite-like structures similar to unstained enamel. When sections that were extracted by the solutions from which the metals were omitted were restained, they appeared identical to routinely stained enamel. It was concluded that staining of young enamel removes the crystallites and reveals only the organic matrix.

Variation in epidermal structure as function of different fixation methods. A stereological and morphological study

Ultrastructural and volume variations in intact human epidermis due to differences in composition of the fixative have been investigated by electron microscopy and stereology. It was found that with glutaraldehyde as fixative variations in buffer osmolality in the range of 120-390 mosmol caused readily detectable changes in volume of the intercellular space and keratinocyte morphology. Also the epidermal Langerhans' cell was effected by variations in buffer osmolality. Three commonly used fixatives (glutaraldehyde, glutaraldehyde + paraformaldehyde, and osmium tetroxide) were found to result in different morphological and volume relations.

The localisation of intracellular immunoglobulin and alpha-1-antitrypsin by immunoelectron staining of post-osmicated, resin-embedded tissue

An indirect immunoperoxidase method is described to demonstrate intracellular immunoglobulins and alpha-1-antitrypsin in semithin and ultrathin sections from human tissue. The tissue was primarily fixed in glutaraldehyde, post-osmicated and resin-embedded.

The effects of glutaraldehyde and osmium tetroxide on the erythrocyte membrane. A spin label study

A nitroxide spin label probe technique was applied to study the interaction between glutaraldehyde or osmium tetroxide (OsO4) amd the membranes of horse erythrocytes, ghosts and liposomes prepared from erythrocyte lipids. Two major conclusions have been established: (1) Reaction of the fixation reagents with the membrane is selective. OsO4 reacts predominantly with lipids and glutaraldehyde with membrane proteins. (2) The lipid-protein interactions change after pretreatment by OsO4 or glutaraldehyde.

Effects of glutaraldehyde or osmium tetroxide fixation on the osmotic properties of lung cells

The osmotic properties of lung cells have been tested before and after perfusion fixation of isolated, perfused lungs with either glutaraldehyde or osmium tetroxide. The testing procedure was to add hypertonic sucrose to the perfusate for several minutes and monitor the lung weight response (an 'osmotic transient'). Each lung was perfused with one or the other fixative solutions for 10 min, then the perfusate was changed back to Ringer-lactate before the post-fixation test was conducted. The results indicate that osmium tetroxide makes the cell membranes as permeable to sucrose as to water, and that sucrose thus causes no osmotic volume change. Glutaraldehyde, on the other hand, apparently preserves the impermeability of the cell membranes to sucrose, but the osmotic volume response is attenuated, indicating that significant changes in the cells have occurred.

[Phalloidin counteracts the destruction of F-actin by osmic acid. II. Protection by phalloidin of F-actin crosslinked with aldehydes (author's transl)]

We have studied by viscometry and spectrophotometry the effects of glutaraldehyde, acetaldehyde, acrolein and formaldehyde on F-actin in vitro. Pretreatment with acrolein and, to a lesser extent, with glutaraldehyde, results in increased destruction of F-actin during the subsequent degradation with osmic acid. Formaldehyde alone disintegrates actin filaments. Acetaldehyde gives the best results and does not seem to damage F-actin. Phalloidin protects F-actin against destruction by osmic acid and this protection is also observed in the F-actin which has been pretreated with the four aldehydes mentioned.

Effects of glutaraldehyde and osmium tetroxide on newborn rat epidermal keratin

The effects of glutaraldehyde and osmium tetroxide (OsO4) fixation on ultrastructural and biochemical changes of epidermal keratin were investigated. Morphometrically, the apparent size of keratohyalin granules was smallest when granular cells were fixed with OsO4, whereas those fixed with glutaraldehyde were larger, and those double-fixed with glutaraldehyde and OsO4 were largest. The ultrastructure of tonofilaments appeared different in each type of fixation. Biochemically, cross-linking glutaraldehyde with purified keratin from cornified cells formed a high molecular weight product with a blue shift of the absorption maximum and an increase in absorbance. Detection of the absorption spectra of OsO4- keratin reactions revealed that a new absorption maximum around 410 nm developed in glutaraldehyde-treated keratin, while one around 315 nm appeared in nontreated keratin. In addition, glutaraldehyde pretreatment increased by 40% the initial reaction rate and by 23% the total amount of OsO4 reduction. Sodium dodecyl sulfate gel electrophoresis demonstrated that degradation of keratin molecules occurred in both glutaraldehyde-treated and untreated keratin after 2 hr of exposure to OsO4. These findings indicate that keratin molecules are chemically modified by fixatives routinely used for electron microscopy. We consider that this chemical information plays an essential role in the appropriate interpretation of the ultrastructure of epidermal cells.

[Phalloidin counteracts the destructive effects of osmic acid and potassium permanganate on F-actin (author's transl)]

Osmium tetroxide and potassium permanganate destroy F-actin filaments in vitro, and split actin molecules into smaller peptides. It is shown by viscometry, spectrophotometry, electrophoresis and electron microscopy that the destruction of F-actin by these agents is inhibited by phalloidin. Other proteins are unprotected.

Use of osmium-ferrocyanide treatment for improved lysosomal acid trimetaphosphatase reaction and subcellular detail in thyroid follicular cells

The histochemical reaction for acid trimetaphosphatase in addition to secondary tissue treatment with an osmium-ferrocyanide mixture was used to study lysosomes and phagolysosomes in the mouse thyroid gland. The osmium-ferrocyanide postfixation enhanced reaction product localization, reduced diffuse reaction, and improved membrane contrast. In addition, the ultrathin tissue sections did not require heavy metal staining, thus eliminating potential stain artifacts due to precipitation. In view of the improved tissue preservation and enzyme localization, it is suggested that osmium-ferrocyanide postfixation be used after the acid trimetaphosphatase method.

A quantitative immunoperoxidase procedure employing energy dispersive x-ray analysis

A method is described for substituting gold for osmium as a marker in the unlabeled antibody technique. The gold marker can be detected in the light or electron microscope. The gold-labeled reaction product can be detected in lower concentrations than osmium and can be used as the basis for quantitating antigen concentrations in cells and tissues with the scanning electron microscope and energy dispersive x-ray analysis.

Ultrastructure of Bacteroides species: Bacteroides asaccharolyticus, Bacteroides fragilis, Bacteroides melaninogenicus subspecies melaninogenicus, and B. melaninogenicus subspecies intermedius

Representative strains of two subspecies of Bacteroides melaninogenicus (subspecies melaninogenicus and subspecies intermedius) and Bacteroides asaccharolyticus as well as B. asaccharolyticus strain 536B isolated from a human perirectal abscess and Bacteroides fragilis ATCC 25285 were examined by glutaraldehyde-osmium fixation, ruthenium red fixation and staining, and thorium hydroxyde staining as well as by the physical preparative techniques of critical point drying--transmission electron microscopy (CPD--TEM) and scanning electron microscopy (SEM). All strains, with the exception of B. fragilis 25285, possessed an electron-dense material external to their outer membranes. Ruthenium red staining further revealed a layer, external to the surface of the outer membrane, that was distinct for each species examined. Thorium hydroxide, as well as CPD--TEM and SEM, showed the cells to be interconnected by thin fibers that not only connected adjacent cells but also traversed several microns to connect cell aggregates.