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Biomedical subjects

P J O'Brien

Publications and source records attributed to P J O'Brien.

At least 271 records · Page 15Linked to original sources
PubMed1979

Singlet oxygen formation by a peroxidase, H2O2 and halide system.

Evidence for singlet oxygen formation has been obtained for the lactoperoxidase, H2O2 and bromide system by monitoring 2,3-diphenylfuran and diphenylisobenzofuran oxidation, O2 evolution, and chemiluminescence. This could provide an explanation for the cytotoxic and microbicidal activity of peroxidases and polymorphonuclear leukocytes. Evidence for singlet oxygen formation included the following. (a) Chemiluminescence accompanying the enzymic reaction was doubled in a deuterated buffer and inhibited by singlet oxygen traps. (b) The singlet oxygen traps, diphenylfuran and diphenylisobenzofuran, were oxidized to their known singlet oxygen oxidation products in the presence of lactoperoxidase, hydrogen peroxide and bromide. (c) The rate of oxidation of diphenylfuran and diphenylisobenzofuran was inhibited when monitored in the presence of known singlet oxygen traps or quenchers. (d) Oxygen evolution from the enzymic reaction was inhibited by singlet oxygen traps but not by singlet oxygen quenchers. (e) The traps or quenchers which were effective inhibitors in the experiments above did not inhibit peroxidase activity, were not competitive peroxidase substrates and did not react with the hypobromite intermediate since they did not inhibit hydrogen peroxide consumption by the enzyme. Using these criteria, various biological molecules were tested for their reactivity with singlet oxygen. Furthermore, by studying their effect on oxygen release by the enzymic reaction, it could be ascertained whether they were acting as singlet oxygen traps or quenchers.

Bromides↗
PubMed1979

Differential effects of antioxidants, steroids and other compounds on benzo(a)pyrene 3-hydroxylase activity in various tissues of rat.

Antioxidants were found to inhibit the mixed-function oxidation of benzo(a)pyrene in several tissues of untreated and 3-methylcholanthrene-pretreated rats. The enzyme systems in the liver, kidney and stomach were much more susceptible to inhibition than those in the lung, adrenal, colon and small intestine. In all tissues except the stomach it was found that 3-methylcholanthrene pretreatment led to a decrease in inhibition of benzo(a)pyrene 3-hydroxylase activity. It is suggested that antioxidants exert their protective effect against cancer by inhibiting the formation of carcinogenic metabolites. Of the various steroids tested, only 17 beta-oestradiol and oestrone were significantly inhibitory in most tissues. Cholesterol was found to increase benzo(a)pyrene 3-hydroxylase activity in the gastrointestinal tract.

Adrenal Glands↗
PubMed1979

Structural and biochemical changes in vitamin A--deficient rat retinas.

The levels of rhodopsin and opsin were investigated in relation to the maintenance of retinal structure in retinas of vitamin A--deficient rats in low levels of cyclic illumination (1.5 to 2 foot-candles). Rhodopsin levels decreased in the deficient retinas to approximately 20% of control at 9 weeks, and this level was retained through 39 weeks on the deficient diet. Opsin levels decreased at a slower rate but reached about 20% of control levels at 32 weeks. Despite the decrease in rhodopsin levels, obvious deterioration of disc structure was not observed until 16 weeks of deficiency, when opsin levels had already decreased to 60% to 70% of control. The structural disruption of photoreceptor outer segments was localized initially in discs of the distal third. Rod cell degeneration preceded cone cell degeneration in vitamin A--deficient retinas. Most of the rods and cones persisted in the posterior retina at 23 weeks on the deficient diet; however, by 40 weeks, only 11% of the rod nuclei remained. In contrast, about 63% of the cone nuclei were present at 40 weeks of deficiency. The photoreceptor cells were affected by the deficiency to a greater extent in the inferior hemisphere than in the superior hemisphere of the eye.

Animals↗
PubMed1978

Phagocytosis in the retinal pigment epithelium of the RCS rat.

The retinal pigment epithelium of RCS rats, previously thought not to phagocytize photoreceptor outer segments, exhibited a peak of phagocytosis in vivo when animals were kept under conditions of cyclic lighting (12 hours of darkness and 12 hours of light). The peak occurred at 1 hour after the onset of light, with maximum and minimum levels of phagocytosis averaging about 5 percent of that found in the pigment epithelium of Osborn-Mendel rats used as a control. Eyecups that were obtained from Osborn-Mendel rats and maintained for up to 3 hours in organ culture demonstrated levels of phagocytosis that were sevenfold greater than those of unincubated controls. Likewise a tenfold increase occurred in incubated as opposed to unicubated RCS eyes, raising the possibility that phagocytosis could be experimentally stimulated in vivo.

Animals↗
PubMed1978

The mechanism of action of cytochrome P-450. Occurrence of the 'NIH shift' during hydroperoxide-dependent aromatic hydroxylations.

The mechanism of liver microsomal aromatic hydroxylation has been investigated by using cumene hydroperoxide as the hydroxylating agent and comparing this reaction with the NADPH-dependent reaction. The conversion of [4-(3)H]acetanilide to 4-hydroxyacetanilide by rat liver microsomes (or purified cytochrome P-450) in the presence of either cumene hydroperoxide or NADPH is attended by comparable 'NIH shifts'. This indicates that hydroxylation in the two systems proceeds via a common intermediate, presumably an arene oxide. The intermediacy of an arene oxide, phenanthrene-9,10-oxide, is established by incubating [3-(3)H]-phenanthrene with rat-liver microsomes and cumene hydroperoxide in the presence of either non-radioactive phenanthrene-9,10-oxide as a 'trap' or in the presence of cyclohexene oxide, an inhibitor of the enzyme epoxide hydrase. Incubation of phenanthrene with cumene hydroperoxide in an 18O-enriched medium has confirmed that the oxygen atom in phenanthrene-9,10-oxide is derived from the hydroperoxide and not from the medium.

Acetanilides↗
PubMed1977

The role of cytochrome P-450 in the hydroperoxide-catalyzed oxidation of alcohols by rat-liver microsomes.

The organic hydroperoxide cumene hydroperoxide is capable of oxidizing ethanol to acetaldehyde in the presence of either catalase, purified cytochrome P-450 or rat liver microsomes. Other hemoproteins like horseradish peroxidase, cytochrome c or hemoglobin were ineffective. In addition to ethanol, higher alcohols like 1-propanol, 1-butanol and 1-pentanol are also oxidized to their corresponding aldehydes to a lesser extent. Other organic hydroxyperoxides will replace cumene hydroperoxide in oxidizing ethanol but less effectively. The cumene-hydroperoxide-dependent ethanol oxidation in microsomes was inhibited partially by cytochrome P-450 inhibitors but was unaffected by catalase inhibitors. Phenobarbital pretreatment of rats increased the specific activity of the cumene-hydroperoxide-dependent ethanol oxidation per mg of microsomes about seven-fold. The evidence suggests that cytochrome P-450 rather than catalase is the enzyme responsible for hydroperoxide-dependent ethanol oxidation. However, when H2O2 is used in place of cumene hydroperoxide, the microsomal ethanol oxidation closely resembles the catalase system.

Alcohols↗
PubMed1977

The effects of antioxidants on the metabolism and mutagenicity of benzo[a]pyrene in vitro.

Antioxidants inhibit the rat liver microsomal mixed-function-oxidase-catalysed hydroxylation of benzo[a]pyrene. These antioxidants also decrease the formation of mutagenic products from benzo[a]pyrene as judged by the Ames bacterial-mutagenicity assay [B.N. Ames, J. McCann & E. Yamazaki (1975) Mutat. Res. 31, 347-364]. It is suggested that antioxidants exert their protective effect against cancer by inhibiting the formation of carcinogenic metabolites.

Animals↗
PubMed1977

Differential effects of puromycin on the incorporation of precursors of rhodopsin in bovine retina.

Bovine retinas incubated in vitro sustain the synthesis of opsin and rhodopsin as monitored by the incorporation of labeled leucine, mannose, and glucosamine. Puromycin, an inhibitor of protein synthesis, effectively blocks the incorporation of leucine and mannose into opsin and rhodopsin of rod outer segments. However, the incorporation of glucosamine into opsin and rhodopsin is not immediately blocked. Instead, it continues for a time suggesting not only core oligosaccharide synthesis but also the secondary glycosylation of a pool of preformed opsin which is thought to be transiently accumulated in the photoreceptor Golgi complex. Galactose, not normally found in rhodopsin, is also incorporated into both opsin and rhodopsin. This incorporation appears to be completely insensitive to puromycin, suggesting that it may occur in the rod outer segments involving only preexisting glycoproteins.

Animals↗