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Biomedical subjects

P A Riley

Publications and source records attributed to P A Riley.

At least 73 records · Page 4Linked to original sources
PubMed1991

Video absorbtiometry in diagnostic cytology: description of a new technique and a preliminary statistical evaluation.

Cytological specimens from 21 patients were examined using digital video microscopy. The staining characteristics of cells incubated with hydroquinone and the tetrazolium stain MTT were analysed using a novel procedure that is independent of morphological features and relies on characteristics of the frequency distribution of the localised (pixel) absorbances of the cellular images. The properties of the distribution of absorbances were assessed by a number of procedures. Comparisons were made of nine malignant and 12 non-malignant cytological samples, obtained mainly from pleural and peritoneal fluid. The data showed significant differences between the sets of samples which may be of value in cytological diagnosis. In the material examined all the malignant samples exhibited a positive skewness with the alpha-3 test (the mean alpha-3 value being greater than 0.5); this was true in only two of 12 non-malignant samples. This analytical technique, which is largely independent of morphology, magnification, and absolute absorbance values, seems to hold promise for the simplification and automation of cytological screening procedures.

Absorptiometry, Photon↗
PubMed1991

Possible relationship between abnormal melanosome structure and cytotoxic phenomena in malignant melanoma.

Melanogenesis has been regarded as a hazard for pigment cells which are endangered by reactive quinones and semiquinones generated by this process. Normally the potentially cytotoxic species are confined to melanosomes by a limiting membrane and thus separated from the rest of the cell. Our electron microscopic investigation has demonstrated the presence of abnormal and incomplete melanosomes in human melanomas from epidermal and mucosal sites, in melanoma metastases, and in B16 mouse melanoma. We conclude that significant leakage of reactive melanin precursors including free radical species may occur from aberrant melanosomes in pigmented tumors. This would be expected to be reflected by fully extended physiological scavenging mechanisms, and by local and distant manifestations of cytotoxicity: Among these manifestations is free radical damage to the liver, detected by a thiobarbituric acid-reactive substances assay, in B16 melanoma-bearing mice. The efflux of toxic species from abnormal melanosomes may explain both the observed frequent occurrence of necrosis in melanomas and the therapeutic efficacy of tyrosinase substrates and may also be one of the factors influencing the extent of melanogenuria.

Animals↗
PubMed1990

A pulse radiolysis investigation of the oxidation of methoxylated metabolites of indolic melanin precursors.

The rate constants associated with the series of successive transient absorptions initiated by one-electron oxidation of 6-hydroxy-5-methoxyindole (6H5MI) and its isomer 5-hydroxy-6-methoxyindole (5H6MI) have been studied by pulse radiolysis. These close analogues of 5,6-dihydroxyindole (DHI) are metabolites of the oxidative melanogenic pathway. The species initially produced from N3. oxidation of both methoxyindoles at pH 7.2-7.4 are assigned as the corresponding semiquinones. That from 6H5MI shows peak at 500, 370 and 330 nm, very close to those of the semiquinone of DHI, whereas the semiquinone of 5H6MI shows no absorption at 500 nm but bands at 420 and 340 nm. These spectral differences are attributed to marked changes in the degrees of electron delocalisation for the two types of radical, both rings of the indole being involved for the 6H5MI radical but only the benzenoid moiety for the 5H6MI radical. In both cases, the radicals decayed, probably by disproportionation, into products which absorbed in the 400-420 nm region. For 6H5MI, the subsequent decay in this region was best fitted by two consecutive first-order processes which were both strongly base-catalysed. The first of these processes is assigned to partial decay via deprotonation of the corresponding quinonoid cation to form an equilibrium mixture of this cation and the corresponding quinone methide. The second process is assigned to reaction of the quinone methide with water yielding hydroxylated product(s) which may subsequently react with remaining quinonoid cation or quinone methide to give dimeric product(s) with broad absorption centreing in the 550 nm region detected 0.5 s after the pulse. For 5H6MI, the decay at 430 nm fitted a single first-order process, which was weakly base-catalysed. This process is attributed to deprotonation of the corresponding quinonoid cation to the corresponding quinone imine absorbing below 350 nm, which was stable for at least tens of seconds. The current experiments suggest that our previous analogues observations (Lambert et al. (1989) Biochim. Biophys. Acta 993, 12-20) on the oxidation of the melanogenic precursors DHI and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) may be interpreted, as with 6H5MI, in terms of the corresponding indolequinones decaying into equilibrium mixtures of quinone, quinone imine and quinone methide. These decay via reaction of the methide with water generating hydroxylated species which proceed to give the coloured product(s) absorbing in the 550 nm region.

Free Radicals↗
PubMed1990

Reaction kinetics of 4-methoxy ortho benzoquinone in relation to its mechanism of cytotoxicity: a pulse radiolysis study.

Rate constants quantifying the reactivity of 4-methoxy ortho benzoquinone, formed in the metabolic activation of 4-hydroxyanisole, a possible melanocytotoxic drug under current assessment as a treatment for malignant melanoma, have been determined by pulse radiolysis. The quinone is reactive towards the thiols cysteine (k = 3.5x10(5)M-1sec-1), glutathione (k = 3.1x10(5)M-1sec-1) and dithiothreitol (k = 3.5x10(5)M-1sec-1), but relatively unreactive towards other nucleophiles such as arginine (k less than or equal to 1M-1sec-1) and glutamine (k less than or equal to 1M-1sec-1). Redox exchange with ascorbate also occurs (k = 1.0x10(4)M-1sec-1). In view of the low reactivity of 4-methoxy ortho benzosemiquinone towards oxygen (k less than or equal to 10(5)M-1sec-1) and the model lipid trans-2-butenoic acid (k less than or equal to 2x10(5)M-1sec-1), it is unlikely that initiation of lipid peroxidation by the semiquinone is a major source of cytotoxicity. A more likely toxicity pathway appears to be covalent addition reactions of 4-methoxy ortho benzoquinone with cellular nucleophiles, especially thiols, and/or redox exchange reactions of the quinone leading to antioxidant depletion.

Antineoplastic Agents↗
PubMed1990

In vitro transformation induces abnormal protein thiol levels in rat liver cells.

Cells from an established line of normal rat hepatocytes (IAR 20) were irradiated with 3 Gy gamma-radiation from a cobalt source to generate transformed clones. Cells from four transformed colonies were compared with the parent cell line by flow cytometry following staining with ortho-phthalaldehyde and propidium iodide to quantitate protein thiols and DNA respectively. Transformed cells exhibited an increased variability in cellular protein thiols which was most evident in G1 and S phase. The altered pattern of macromolecular thiol distribution implies early changes in redox state and/or modification of the amounts or types of sulphur-containing proteins in transformed cells.

Animals↗
PubMed1990

Is the initial event in carcinogenesis an enhancement of the mutation rate?

The age-specific incidence rates of adult cancer indicate that the carcinogenic process is a power function of elapsed time. If the malignant clone arises by a series of mutations, and each mutation is regarded as an independent event with a small instantaneous probability of occurrence, then the slope of the age-specific incidence gives an indication of the number of genes involved. In most cases, the epidemiological data exhibit an age-specific incidence the slope of which is between a fourth and a seventh power of age. Assuming that the mutations involved are deletions and must occur in otherwise viable and proliferative cells, the mutation rate required to generate enough mutant cells to fit the cancer incidence data must be remarkably high in the pre-malignant cell population. The initiation process may thus be an event that results in a raised mutation rate in the affected cell and its progeny. It is proposed that the process of induction involves one or more mutations that induce a metabolic lesion that has the effect of increasing the subsequent mutation rate in the affected clone. A possible mechanism involving free radical generation is suggested and some of the biological implications of the proposal examined.

Adult↗
PubMed1989

Synthesis and evaluation of some novel phosphate and phosphinate derivatives of araA. Studies on the mechanism of action of phosphate triesters.

A number of novel phosphinate and phosphate triester derivatives of the anti-viral nucleoside analogue araA have been prepared. Spectroscopic and analytical data have been collected on both the reagents and the nucleotides. An in vitro assay indicated inhibition of DNA synthesis by mammalian cells, by each of the nucleotide derivatives, in the range 3-30 microM. Inhibition was reduced, but not abolished, for the phosphinates relative to the phosphates. These results are consistent with a mode of action involving release of the free nucleoside araA and the nucleotide araAMP.

Animals↗
PubMed1989

Hydrogen peroxide cytotoxicity. Low-temperature enhancement by ascorbate or reduced lipoate.

The principal mechanism of H2O2 toxicity is thought to involve the generation of hydroxyl (HO.) radicals through its interactions with Fe2+ ions by the Fenton reaction. Of particular interest has been the demonstration by Ward, Blakely & Joner [(1985) Radiat. Res. 103, 383-392] that the cytotoxicity of H2O2 is diminished at low temperature. We have now examined this phenomenon further with a mammalian epithelial cell line (CNCMI-221). Resistance of these cells to 100 microM-H2O2 added extracellularly exhibits a transition in the temperature range between 27 degrees C and 22 degrees C. We have found that the low-temperature resistance to cytotoxic concentrations of H2O2 is abolished by preincubation of cells with reductants such as ascorbate or reduced lipoic acid. This implies that the low-temperature resistance to H2O2 cytotoxicity may be due to inhibition of cellular reductive processes. The restoration of the cytotoxic action of H2O2 at 4 degrees C by ascorbate is prevented by pre-exposure of cells to desferrioxamine. This is evidence that transition-metal ions (such as iron ions) are involved in the cytotoxicity and is consistent with a mechanism of cell damage that depends on the Fenton reaction and a metal ion in the reduced state. Restoration of H2O2 cytotoxicity at low temperature by ascorbate is consistent with the artificial production of an intracellular reducing environment that at normal temperatures is sustained by cellular metabolism.

Animals↗
PubMed1989

A pulse radiolysis investigation of the oxidation of indolic melanin precursors: evidence for indolequinones and subsequent intermediates.

The rate constants associated with the series of successive transient absorptions initiated by one-electron oxidation of 5,6-dihydroxyindole (DHI), 5,6-dihydroxyindole-2-carboxylic acid (DHICA), precursors of melanin, and N-methyl-5,6-dihydroxyindole (NMDHI), a model compound, have been studied by pulse radiolysis. The initial transient species resulting from N3. oxidation reaction at pH 7.3-7.4 are assigned as the corresponding semiquinones. In each case, these radicals decayed, probably by disproportionation, into products most readily monitored in the 400-430 nm region. For DHI, the decay in this region could be fitted by two parent concentration independent first-order processes. These may correspond to transformations between 5,6-indolequinone, and its quinone-imine and quinone-methide tautomers. With NMDHI, on the other hand, a single longer-lived product with a peak around 430 nm predominated after decay of the corresponding radical, due almost certainly to N-methyl-5,6-indolequinone. The data appear to exclude significant melanin polymerisation by condensation of semiquinones, reaction of semiquinones with dihydroxyindoles, self-addition of indolequinones or tautomers, or reaction of indolequinones or tautomers with the parent dihydroxyindoles. It is suggested that polymerisation of melanin may rather occur by stepwise addition of indolequinone methide/imine to reduced oligomeric species.

Indoles↗
PubMed1989

Synthesis and biological evaluation of some phosphate triester derivatives of the anti-cancer drug araC.

A number of novel phosphate triester derivatives of the anti-cancer nucleoside analogue araC have been prepared by a rapid 2-step procedure, not necessitating prior sugar protection. Spectroscopic and lipophilicity data have been collected on these compounds. An in vitro assay indicated inhibition of thymidine incorporation by mammalian epithelial cells, by each of these compounds, in the range 3-300 microM. Moreover, the degree of inhibition showed a close correlation to chemical structure; in particular, there was a clear relationship between inhibition of thymidine incorporation and log(P). These results are consistent with cellular penetration by the intact phosphate triesters and intracellular action by an unspecified mechanism. Triethyl phosphate is inactive under the conditions of the test.

Animals↗
PubMed1989

Synthesis and biological evaluation of some phosphate triester derivatives of the anti-viral drug AraA.

A number of novel phosphate triester derivatives of the anti-viral nucleoside analogue araA have been prepared by a rapid 2-step procedure, not necessitating prior sugar protection. Spectroscopic and lipophilicity data have been collected on these compounds, and they have been assayed with a range of hydrolytic enzymes. The compounds have been found to be highly resistant to hydrolysis at physiological pH, enzymatic or otherwise. An in vitro assay indicated inhibition of DNA synthesis by mammalian cells, by each of these compounds, in the range 3-300 microM. Moreover, the degree of inhibition showed a close correlation to chemical structure; in particular, there was a direct relationship between inhibition of thymidine incorporation and lipophilicity. These results suggest cellular penetration by the phosphate triesters and intracellular hydrolysis, by an unspecified mechanism, to the free nucleotide or nucleoside.

Adenosine Deaminase↗
PubMed1989

Studies on the kinetics of oxidation of 4-hydroxyanisole by tyrosinase.

The potential use of 4-hydroxyanisole as a chemotherapeutic agent in the treatment of malignant melanoma led us to investigate the kinetics of oxidation of this tyrosine analogue by tyrosinase. We found that addition of amino acids accelerated the reaction, resulting in a reduction in length of the characteristic lag period of monohydric phenol oxidation. The lag period was abolished completely by an aliquot of exhausted 4-hydroxyanisole/tyrosinase reaction mixture and by very low concentrations of thiol-containing compounds. We conclude that the reaction-accelerating property of non-thiol amino acids is due to the reductive addition of the ortho-quinone reaction product to nucleophilic groups of the amino acids. The dihydric phenol product which results is capable of met-tyrosinase recruitment by electron donation to the cupric active site generating the cuprous form of the enzyme which binds oxygen and is able to oxidise monohydric phenols. Abolition of the lag period by an aliquot of exhausted reaction mixture is probably due to recruitment of the met-enzyme by catecholic oligomers of the quinone product. Thiol containing compounds are able to abolish the lag period due to the ability of these compounds to reduce met-tyrosinase directly.

Amino Acids↗
PubMed1989

Flow cytometric analysis of protein thiol groups in relation to the cell cycle and the intracellular content of glutathione in rat hepatocytes.

The relationship between protein thiols (PSH) and cell proliferation was examined in ethanol-fixed rat hepatocytes. A new protocol was developed for simultaneous measurement of protein thiol vs. DNA content by flow cytometry. The fluorescent dye o-phthalaldehyde (OPT) was used for flow cytometric measurements of protein thiol groups. The influence of nonprotein thiols was examined by monitoring the cell cycle of cells in which the glutathione content (GSH) was modified by treatment with buthionine sulphoximine (BSO). Three rat liver cell lines (IAR 20, IAR 6.1, IAR 6.1RT7) were used: these cell lines possess different growth characteristics and degrees of tumorigenicity, which made it possible to analyse changes in PSH during normal and deranged cell proliferation. The effects on the cell cycle of the changes in PSH due to the depletion of GSH were measured by 5-bromo-2'-deoxyuridine (BrdUrd) incorporation and flow cytometry. The data obtained can be summarised as follows: a) OPT fluorescence increases with increasing DNA content in all rat liver cell lines examined; b) the greatest variation in PSH content occurs in G1. There is a smaller variation in G2 + M, and PSH levels are relatively invariant throughout S-phase; c) a higher content of PSH is found in the tumorigenic cell lines; d) the amount and distribution of PSH is not affected by BSO treatment; e) kinetic studies indicate that BSO treatment has no effect on the ability of the IAR rat liver cell lines to progress through the cycle.

Animals↗
PubMed1989

Cytotoxicity of zinc in vitro.

The effect of zinc ions on B16 mouse melanoma lines, HeLa cells and I-221 epithelial cells was investigated in vitro in order to ascertain whether sensitivity to Zn2+ is a general feature of cells in vitro and in an attempt to elucidate the mechanism(s) of zinc cytotoxicity. The proliferation of B16, HeLa and I-221 cell lines was inhibited by 1.25 x 10(-4), 1.50 x 10(-4) and 1.50 x 10(-4) mol/l Zn2+, respectively. The free radical scavengers, methimazole and ethanol, did not suppress the toxicity of Zn2+, neither did superoxide dismutase or catalase. The addition of the chelating agent EDTA reduced the zinc cytotoxicity. It was possible to suppress the cytotoxicity of zinc by increasing the concentration of either Fe2+ or Ca2+ but not Mg2+, which suggests that a prerequisite for the toxic action of zinc is entry into cells using channels that are shared with iron or calcium. This view was supported by experiments in which transferrin intensified the cytotoxic action of zinc in serum-free medium. Another agent facilitating zinc transport, prostaglandin E2, inhibited the proliferation of the B16 melanoma cell line. There were no conspicuous differences in zinc toxicity to pigmented and unpigmented cells. The toxic effect of zinc in the cell systems studied exceeded that of iron, copper, manganese and cobalt in the same concentration range. In vitro, Zn2+ should be regarded as a dangerous cation.

Animals↗
PubMed1989

Clinical pharmacology of nedocromil sodium.

In attempts to define the clinical pharmacological activity of inhaled nedocromil sodium, various challenge systems, ranging from specific antigen challenge to provocation with chemical irritants such as sulphur dioxide, have been used. A single dose (4 mg) of nedocromil sodium taken before antigen challenge prevented both early and late asthmatic responses, whereas the same dose taken shortly after the early response delayed onset of the late reaction but did not affect its magnitude. Exercise-induced asthma was inhibited by pretreatment with nedocromil sodium, as were the bronchoconstrictor responses to hyperventilation of cold dry air and inhalation of ultrasonically nebulised distilled water ('fog'). Mast cells lying in the surface mucosa of the lung are thought to be less stable in asthmatic subjects and may be implicated in the mechanism of response to these 3 types of physical insult. However, in addition to the marked protective action on isolated mucosal mast cells which has been reported from preclinical studies, nedocromil sodium was also effective against bronchoconstriction induced by sulphur dioxide in hyper-responsive asthmatic and atopic subjects. The response to sulphur dioxide, in which axon reflexes are thought to be involved, is less likely to have an immunological mechanism and it is clear that in this type of situation the effect of nedocromil sodium can be more readily differentiated from that of sodium cromoglycate. The increased potency and wider scope of activity described for nedocromil sodium suggests a therapeutic advantage for this new compound in chronic inflammatory allergic lung disorders.

Anti-Inflammatory Agents, Non-Steroidal↗
PubMed1988

Role of hydrogen peroxide in the cytotoxicity of the xanthine/xanthine oxidase system.

1. The survival of mammalian epithelial cells exposed in vitro to the xanthine/xanthine oxidase system in phosphate-buffered saline (PBS) or serum-containing medium (SCMEM) was investigated. 2. The cytotoxic effect observed depended on the composition of the medium in which the enzymic reaction was carried out; a surviving fraction of 5 x 10(-5) was found for cells exposed in PBS and 5.2 x 10(-1) for those in SCMEM. 3. The cytotoxic product(s) formed by the xanthine/xanthine oxidase system was relatively stable in PBS; survival of cells incubated after completion of the enzymic reaction was always less than that found for cells exposed during the reaction in the same system. 4. Superoxide dismutase or mannitol present during the enzymic reaction did not inhibit the cytotoxic effect. 5. NaN3 (a single-oxygen quencher and a catalase inhibitor) added to the system in SCMEM caused a reduction in survival to the level observed for cells exposed to the enzymic reaction in PBS. 6. Catalase completely protected cells, but no protection was observed when both catalase and NaN3 were present in the reaction mixture. 7. A similar cytotoxic effect was produced when cells were treated with H2O2 alone. 8. The rate of H2O2 decomposition in medium was accelerated by the presence of serum, but this was completely inhibited by NaN3. 9. It is concluded that H2O2 is the major cytotoxic product formed by the xanthine/xanthine oxidase system.

Animals↗
PubMed1988

Initial mushroom tyrosinase-catalysed oxidation product of 4-hydroxyanisole is 4-methoxy-ortho-benzoquinone.

Evidence is presented that the first and major product of the oxidation of 4-hydroxyanisole (4HA) by tyrosinase is 4-methoxy ortho benzoquinone (4-MOB). 4-MOB was synthesized by oxidation of 4HA by potassium nitrodisulphonate and comparisons made between the synthetic quinone and an extract of a reaction mixture in which 4HA had been completely oxidized by mushroom tyrosinase. The chemical species were found to be identical in UV/visible absorption spectrum, 1H-NMR spectrum, and by thin-layer chromatography.

Anisoles↗