Clare L. Ritter

Mixed function oxidase in the mammary gland and liver microsomes of lactating rats: Effects of 3-methylcholanthrene and β-naphthofla vone

Mammary gland and liver microsomes of lactating rats were examined for the components of mixed function oxidase and related enzyme activities. Cytochrome b5, NADH- and NADPH- dependent cytochrome c reductase activities were 15-, 6- and 10-fold lower, respectively, in the mammary gland than in the liver microsomes. The determination of cytochrome P-450 (P-448) in the mammary gland microsomes required elimination of the spectral interferences by hemoglobin and cytochrome aa3. The presence of the latter in this fraction was also shown by cytochrome c oxidase activity. Cytochrome aa3 was reduced by anaerobic incubation of mammary gland microsomes, in the presence of antimycin A, with sodium succinate, phenazine ethosulfate, and sodium ascorbate for 30 min at room temperature. Spectral resolution of the dithionite-reduced cytochrome P-450 (P-488) carbon monoxide complex occurred 30 min after gassing. The basal level of cytochrome P-450 was about 500-fold greater in the liver than in the mammary gland microsomes. Pretreatment of lactating rats with the inducers of hepatic cytochrome P-448, 3-methylcholanthrene and beta-naphthoflavone, increased the cytochrome content 3- to 10-fold, in the mammary gland and liver microsomes, respectively. The induction of cytochrome P-448 in microsomes of both tissues was also shown by type I binding spectra obtained with N-2-fluorenylacetamide. Using hydroxylation of benzo[a]pyrene and N-2-fluorenylacetamide as a measure of mixed function oxidase activity, we found that the basal activities, which were 4- to 8-fold greater in the liver microsomes, were increased in both tissues after treatment of rats with the inducers. The induced activities were inhibited by 0.1 micrometers alpha-napthoflavone in vitro, indicating a dependence on cytochrome P-448. The data suggest that the mammary gland, an extrahepatic target for carcinogens, is capable of their metabolism.

Chloramphenicol Binding Site with Analogues of Chloramphenicol and Puromycin

The effect of a series of puromycin analogues and aminoacyl chloramphenicol derivatives on poly(U,C)-directed polyphenylalanine synthesis in an Escherichia coli cell-free system was examined. A comparison between the structures and activities of the puromycin and chloramphenicol analogues was made to examine the proposal that ribosomal binding sites for both antibiotics overlap. Our results suggest that the dichloroacetamido group in the chloramphenicol molecule does not correspond to the role of the aminoacyl moieties of either puromycin or aminoacyl transfer ribonucleic acid. These results comparing the structures and activities of puromycin and chloramphenicol analogues also seem inconsistent with a common binding site for the p-substituted phenyl moieties of the two antibiotics. Previous data have indicated that both sites are mutually affected by the prior binding of either antibiotic. Although it is possible that chloramphenicol and puromycin may have overlapping bindings sites, no common structural features between the two antibiotics are supported by our data.

A novel oxidation of the carcinogen N-hydroxy-N-2-fluorenylacetamide catalyzed by peroxidase/H2O2/Br−

N-Hydroxy-N-2-fluorenylacetamide, a proximate carcinogenic metabolite of N-2-fluorenylacetamide, is oxidized largely to 2-nitrosofluorene by lactoperoxidase or extract of peroxidative activity of rat uterus in an H2O2- and Br- -dependent reaction. Evidence is presented that the oxidizing species includes OBr- (HOBr). This novel oxidation may be involved in carcinogenesis by N-arylhydroxamic acids.

Modifications of carcinogen metabolism in hepatic microsomes of suckling young by 3-methylcholanthrene or β-naphthoflavone administered to lactating rats

The effects of treating lactating rats with 3-methylcholanthrene (3-MC) or beta-naphthoflavone (beta-NF) (three i.p. injections of 20 or 40 mg compound/kg of body weight) on hepatic microsomal enzymes of their suckling young were examined. This treatment had no apparent effect on the contents of cytochromes P-450 and b5 or on the activities of NADH- and NADPH-cytochrome c reductases in hepatic microsomes of the pups. However, these microsomes had 8- and 6-fold increased capacities for hydroxylations of benzo[a]pyrene (B[a]P) and N-2-fluorenylacetamide (2-FAA) respectively. These increases were about 5-fold greater in the hepatic microsomes of the dams, in which they were inhibited by 0.1 mM alpha-naphthoflavone (alpha-NF) in vitro 72-81 and 89-95% and by 0.1 mM beta-NF in vitro 12-41 and 60-76% respectively. In the pups, the induced activities were also inhibited, whereas the basal hydroxylations of B[a]P and 2-FAA were stimulated by alpha-NF 2.7- and 5.0-fold and by beta-NF 1.4- and 2.4-fold respectively. The inhibition of the induced hydroxylations by alpha-NF and beta-NF may be explained by their higher affinities (Ks, 0.14 and 0.28 microM, respectively) than those of B[a]P and 2-FAA (Ks, 4.4 to 8.8 and 2.4 to 3.1 microM, respectively) for cytochrome P-450. Whereas beta-NF gave a type I binding spectrum, alpha-NF gave a spectrum composed of type I and reverse-type I elements. Analysis of metabolites of 2-FAA showed differences in their type and amounts formed by hepatic microsomes of beta-NF-treated lactating rats and their pups. Thus, in the dams the formation of 1-, 3-, 5-, 7-, 9- and N-hydroxy-2-FAA was increased by 9-, 30-, 40-, 5-, 20- and 5-fold respectively. In the pups, the formation of 1-, 3-, 5-, 7- and N-hydroxy-2-FAA was increased by 2-, 30-, 18-, 4- and 27-fold respectively. All these increased hydroxylations were inhibited by 0.1 mM alpha-NF in vitro. In the hepatic microsomes of pups from the corn oil-treated dams, alpha-NF stimulated all ring-hydroxylations, but not N-hydroxylation of 2-FAA. The results support earlier findings that microsomal enzymes differ in immature and mature rat liver and suggest that N-hydroxylation of 2-FAA, the activation required for carcinogenesis, and specific ring-hydroxylations are catalyzed by different cytochrome P-450 isozymes.(ABSTRACT TRUNCATED AT 400 WORDS)

Determination of peroxidative halogenation in mixtures of chloride and bromide

A method for the differentiation of chlorinated and brominated products from peroxidative oxidation of mixtures of the halides is presented. Chlorination or bromination of monochlorodimedone (MCD) by fungal chloroperoxidase (CPO) was measured by loss of MCD absorbance. Although the Vmax was similar for both halides [approximately 0.08 mM (2 min)-1], the apparent Km for chlorination was 10 times greater than that for bromination (5.88 vs 0.67 mM). Chlorination was also quantitated as I3- produced from N-chlorotaurine and I-. The Vmax [0.076 mM (2 min)-1] and apparent Km (6.31 mM) determined by this method agreed with those determined with MCD. Selective reduction by H2O2 of the I-oxidizing potential of N-bromotaurine allowed determination of the brominated product from the difference between the amounts of halogenated MCD and N-chlorotaurine. The brominated product predominated at saturating and at physiologic halide levels. Hence, it is suggested that Br- plays a significant role in halogenation even though in vivo levels of Cl- are equal to or greater than 1000 times those Br-.

Debenzoylating and deacetylating activities of rat liver and mammary gland microsomes: Effect of ovariectomy

This study compared the rates of N-deacylations of N-hydroxy-N-2-fluorenylbenzamide (N-OH-2-FBA) with those of its analogue, N-hydroxy-N-2-fluorenylacetamide (N-OH-2-FAA), by the mammary gland (tumor target for both compounds) and the liver of female Sprague-Dawley rats and examined the effect of ovariectomy on these activities. N-Debenzoylation of N-OH-2-FBA was catalyzed by the mammary and liver microsomes of 50-day-old female rats at similar rates (approximately 24 nmol/min/mg). The activity of both tissues increased (up to 1.8 times) after ovariectomy at 42, 32 and, especially, 22 days of age. The rapid hydrolysis appeared to be unique for the benzoyl group since N-OH-2-FAA was deacylated only approximately 0.05 and 0.004 times as fast by the liver and mammary microsomes, respectively, and these low rates were unaffected by ovariectomy. Since such substrate specificity would be of significance in the metabolism of xenobiotics and drug design, esterase activity and its sensitivity to ovariectomy at 22 days of age were examined with several acetylated and benzoylated substrates in the liver and mammary microsomes and compared with those of male liver. Tissues of rats of both sexes had a greater capacity to hydrolyze carboxyl esters than amides. Expect for N-2-fluorenylacetamide (2-FAA) and o-nitrophenylacetate (o-NPA), all substrates were hydrolyzed by liver microsomes of the male up to 3.9 times faster than by those of the female. Microsomes of female liver hydrolyzed acetylated substrates 1.2 to 25 times faster than benzoylated analogues except for N-OH-2-FBA and benzamide. By contrast, mammary gland microsomes hydrolyzed benzoylated compounds 1.4 to 333 times faster except for 2-naphthyl benzoate. Respective rates of hydrolysis of o-NPA by microsomes of liver and mammary gland were 1.7 and 0.6 times those of p-NPA. After ovariectomy, deacylating activities increased (up to 1.6 times) except for those of 2-FAA and acetanilide. All deacylations were > 98% inhibited by 0.1 mM paraoxon, indicating catalysis by serine hydrolases. The results suggest involvement of multiple carboxylesterases and indicate that certain benzoylated xenobiotics may have a greater effect on the mammary gland than acetylated xenobiotics because of their greater vulnerability to hydrolysis by esterases of mammary gland.

Activation of Carcinogenic N-Arylhydroxamic Acids by Peroxidase/H2O2/Halide Systems: Route to C-Nitroso Aromatics

Evidence that locally carcinogenic N-arylhydroxamic acids may be activated by oxidants generated by leukocytes has been obtained from model systems including tissue peroxidases, myeloperoxidase (MPO)/H2O2/ halide (X-) or hypohalous acids (HOX). Peroxidative oxidations of N-hydroxy-N-2-fluorenylacetamide (N-OH-2-FAA) via one electron (le-) to equimolar 2-nitrosofluorene (2-NOF) and N-acetoxy-2-FAA, and via oxidative cleavage to 2-NOF were determined. The latter oxidation predominated with peroxidase-rich extracts of rat uterus and mammary gland and with eosinophils from intraperitoneal fluid in the presence of H2O2, Br- and cationic detergent. Contribution to 2-NOF by le- oxidation was up to 50% at 0.1 mM Br- and pH 7.4, but negligible at 10 mM Br- and pH 5.5. Oxidation of N-OH-2-FAA by MPO of human neutrophils in the presence of physiologic concentrations of Cl- (0.1 M) or Br- (0.1 mM) or their mixture was examined in the pH range of 4 to 6.5. At the respective pH optima (4 for Br- and 5 for Cl- or Cl- + Br-), oxidation of N-OH-2-FAA to 2-NOF by MPO/H2O2 was much more rapid with Br- and Br- + Cl- than with C1-. Since HOBr oxidized N-OH-2-FAA to 2-NOF much more rapidly than did HOC1, MPO/H2O2-catalyzed oxidation in the presence of C1-+Br- was possibly due in part to HOBr. le- oxidation of N-OH-2-FAA occurred to a lesser extent in chemical (HOX) than enzymatic systems (MPO/H2O2/X-), in which it appeared to be stimulated by Br- at pH > 5.5. In the presence of taurine, a scavenger of hypohalous acids in vivo, oxidation of N-OH-2-FAA to 2-NOF bv MPO/H2O2 was unaffected with Br-, inhibited with Cl-, and partially inhibited with Cl- + Br-. These results were linked to N-halotaurine formation since it was found that N-bromotaurine, but not N-chloro-taurine, oxidized N-OH-2-FAA chiefly to 2-NOF. The possibility that 2-NOF through its interactions with unsaturated lipids may initiate lipid peroxidation was investigated by measurements of malondialdehyde (MDA), a lipid degradation product, following aerobic incubations of 2-NOF with arachidonic, linolenic or linoleic acid at a molar ratio of 1:1. The amounts of MDA were optimal after 4 hr and were increased by 50% when 2-NOF was preincubated with the fatty acids for 24 hr under anaerobic conditions. Our studies indicate potential significance of: 1) Br--derived oxidants in activation of carcinogenic N-arylhydroxamic acids to C-nitroso aromatics, and 2) interactions of C-nitroso aromatics with unsaturated lipids leading to lipid peroxidation and thus, genotoxicity.