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Biomedical subjects

A Y Lu

Publications and source records attributed to A Y Lu.

At least 91 records · Page 5Linked to original sources
PubMed1982

Drug residue formation from ronidazole, a 5-nitroimidazole. III. Studies on the mechanism of protein alkylation in vitro.

Ronidazole (1-methyl-5-nitroimidazole-2-methanol carbamate) is reductively metabolized by liver microsomal and purified NADPH-cytochrome P-450 reductase preparations to reactive metabolites that covalently bind to tissue proteins. Kinetic experiments and studies employing immobilized cysteine or blocked cysteine thiols have shown that the principal targets of protein alkylation ara cysteine thiols. Furthermore, ronidazole specifically radiolabelled with 14C in the 4,5-ring, N-methyl or 2-methylene positions give rise to equivalent apparent covalent binding suggesting that the imidazole nucleus is retained in the bound residue. In contrast, the carbonyl-14C-labeled ronidazole gives approx. 6--15-fold less apparent covalent binding indicating that the carbamoyl group is lost during the reaction leading to the covalently bound metabolite. The conversion of ronidazole to reactive metabolite(s) is quantitative and reflects the amazing efficiency by which this compound is activated by microsomal enzymes. However, only about 5% of this metabolite can be accounted for as protein-bound products under the conditions employed in these studies. Consequently, approx. 95% of the reactive ronidazole metabolite(s) can react with other constituents in the reaction media such as other thiols or water. Based on these results, a mechanism is proposed for the metabolic activation of ronidazole.

Alkylation↗
PubMed1982

Problems associated with the use of cycloheximide to distinguish between animal drug residues bound to protein and those incorporated into protein.

The user of cycloheximide to distinguish between covalently-bound drug residues in animals and residues due to the incorporation of drug fragments into endogenous molecules was explored. The results indicated that cycloheximide prevented the absorption of both glycine and ronidazole from the gastro-intestinal tract, an effect that complicates its use in the characterization of drug residues in animals.

Animals↗
PubMed1982

Tumor-initiating ability of the twelve monomethylbenz[a]anthracenes.

The 12 possible monomethylbenz[a]anthracenes (MBAs) were examined for their tumor-initiating activity in the classical two-stage initiation-promotion experiment. Based on the average number of tumors/mouse, 7-MBA was the most tumorigenic compound of the series causing 4.9 papillomas/mouse at an initiating dose of 400 nmol/mouse. At this same dose level 8-MBA and 12-MBA were equally potent causing 1.0 papillomas/mouse. 6- an 9-MBAs caused -0.6 papillomas/mouse at this dose level. These data are in general agreement with previous experiments using other model systems. Of interest is the low tumorigenicity of the 1-, 2-, 3-and 4-MBAs which has been cited in support of the bay region theory of polycyclic aromatic hydrocarbon carcinogenesis. The tumor-initiating ability of anthracene and 9,10-dimethylanthracene was also examined. Neither of these compounds possessed tumor-initiating activity.

Animals↗
PubMed1981

Effect of phenobarbital on the level of translatable rat liver epoxide hydrolase mRNA.

Liver poly(A)+RNA isolated from untreated and phenobarbital-treated rats has been translated in the rabbit reticulocyte cell-fre system in order to determine the level of translationally active epoxide hydrolase (EC 3.3.2.3) mRNA. The in vitro translation systems were immunoprecipitated with rabbit IgG prepared against purified epoxide hydrolase, and the amount of epoxide hydrolase synthesized by the lysate programmed with control and phenobarbital poly(A)+RNA was quantitated. The level of translatable epoxide hydrolase mRNA is increased 3-fold after chronic phenobarbital administration. This level of induction correlates well with the 5-fold induction in catalytic activity of epoxide hydrolase (using styrene 7,8-oxide as substrate) in microsomes isolated from phenobarbital-treated rats. Therefore, we suggest that chronic phenobarbital administration increases the amount of functional epoxide hydrolase in rat liver microsomes by way of an increase in the translatable mRNA level encoding for the enzyme. We do not know whether the increase in mRNA is the result of increased transcription or messenger stability.

Animals↗
PubMed1981

Tumor-initiating activity of the dihydrodiols of 8-methylbenz[a]anthracene and 8-hydroxymethylbenz[a]anthracene.

The tumor-initiating activity of the 3,4-dihydrodiols (diols) as well as other metabolites of 8-methylbenz[a]anthracene (8-MBA) and 8-hydroxymethylbenz[a]anthracene (8-OHMBA) were examined in the classical 2-stage initiation-promotion model on mouse skin. The 3,4-diol of 8-MBA caused 2.2 times as many papillomas/mouse as did 8-MBA. The 3,4-diol of 8-OHMBA was not more tumorigenic than either 8-MBA or 8-OHMBA. None of the other diols tested, including the 5,6- and 8,9-diol of 8-MBA and the 5,6 and 10,11-diol of 8-OHMBA were remarkably tumorigenic. These data indicate that the 3,4-diol of 8-MBA is a good candidate as a proximate carcinogen of 8-MBA and further suggest that the bay region 3,4-diol-1,2-epoxide is a likely ultimate carcinogen of this compound on mouse skin.

Animals↗
PubMed1981

Tumorigenicity of 7,12-dimethylbenz[a]anthracene, its hydroxymethylated derivatives and selected dihydrodiols in the newborn mouse.

The newborn mouse lung adenoma model has been shown to be a sensitive test for studying the tumorigenicity of bay region diol epoxides and their precursor dihydrodiols. When a total dose of 28 nmol of 7,12-dimethylbenz[a]anthracene (DMBA) or its derivatives was injected i.p. into the preweaning mice, it was found that the 3,4-dihydrodiols of both DMBA and 7-hydroxymethyl-12-methylbenz[a]anthracene caused 13.3 and 4.1 times more lung adenomas than DMBA, respectively. The mice treated with the 5,6- and 8,9-dihydrodiols of DMBA, 7-hydroxymethyl-12-methylbenz[a]anthracene and its 5,6- and 8,9- and 10,11-dihydrodiols, 7-methyl-12-hydroxymethylbenz[a]anthracene and 7,12-dihydroxymethylbenz[a]anthracene developed a level of lung adenomas/mouse less than 2-fold higher than that found in the DMSO-treated control group. Liver tumors also developed in some of the mice. The percentage of mice with liver tumors also indicated that the 3,4-dihydrodiols of both DMBA and 7-hydroxymethyl-12-methylbenz[a]anthracene were more tumorigenic than DMBA itself. These data indicate that the 3,4-dihydrodiols of both DMBA and its 7-hydroxymethyl derivative may be proximate carcinogenic metabolites of DMBA in the newborn mouse.

9,10-Dimethyl-1,2-benzanthracene↗
PubMed1980

Kinetic isotope effects in cytochrome P-450-catalyzed oxidation reactions. Intermolecular and intramolecular deuterium isotope effects during the N-demethylation of N,N-dimethylphentermine.

Two N,N-dimethylphentermine (N,N-dimethyl-2-amino-2-methyl-3-phenylpropane) substrates differing in deuterium substitution have been used to determine the intermolecular and intramolecular isotope effects associated with the cytochrome P-450-dependent N-demethylation of this substrate. No intermolecular isotope effect was observed in Vmax or Vmax/Km when the reaction rates for this substrate were compared to those for the substrate in which both N-methyl groups contained deuterium. In contrast, identical isotope effects of 1.6 to 2.0 were observed in both Vmax and Vmax/Km when this reaction was studied with a substrate in which only one of the two N-methyl groups was substituted with deuterium. Furthermore, both the intermolecular and intramolecular isotope effects were independent of the cytochrome P-450/NADPH-cytochrome P-450 reductase mole ratio. From these data, it is concluded that: 1) the carbon-hydrogen bond cleavage step does not contribute significantly to Vmax; 2) the contribution of the carbon-hydrogen bond cleavage step to Vmax is not detectably increased through changes in the cytochrome P-450/NADPH-cytochrome P-450 reductase mole ratio; 3) the N-methyl groups are free to exchange at the enzyme active site. The basis for these conclusions is the proposal of a new kinetic model for interpretation of intramolecular isotope effects which shows that intramolecular isotope effects are not necessarily equal to intrinsic isotope effects and, in fact, may be much smaller.

Animals↗
PubMed1980

Activating enzymes: multiple monooxygenase forms with different substrate preferences as related to toxic effects.

The presence of multiple molecular forms of cytochrome P-450 in experimental animals and perhaps is of considerable importance in dealing with the metabolism of drugs, mutagens, carcinogens and other toxic compounds. Differences in substrate specificity, positional specificity and stereoselectivity of various forms of cytochrome P-450 play an important role in regulating the balance between activation and inactivation pathways of a given chemical. The relative proportions of various forms of cytochrome P-450 in a given tissue or individual may be an important factor in determining the cytotoxic and carcinogenic action of many toxic compounds.

2-Acetylaminofluorene↗
PubMed1980

Aldrin epoxidation catalyzed by purified rat-liver cytochromes P-450 and P-448. High selectivity for cytochrome P-450.

Aldrin epoxidation was studied in monooxygenase systems reconstituted from purified rat liver microsomal cytochrome P-450 or P-448, NADPH-cytochrome c reductase, dilauroylphosphatidylcholine and sodium cholate. Cytochrome P-450, purified from hepatic microsomes of phenobarbital-treated rats, exhibited a high rate of dieldrin formation. The low enzyme activity observed in the absence of the lipid and sodium cholate was increased threefold by addition of dilauroylphosphatidylcholine and was further stimulated twofold by addition of sodium cholate. The apparent Km for aldrin in the complete system was 7 +/- 2 microM. SKF 525-A, at a concentration of 250 microM, inhibited aldrin epoxidation by 65%, whereas 7,8-benzoflavone had no inhibitory effect at concentrations up to 250 microM. Addition of ethanol markedly increased epoxidase activity. The increase was threefold in the presence of 5% ethanol. When cytochrome P-448 purified from hepatic microsomes of 3-methylcholanthrene-treated rats was used, a very low rate of epoxidation was observed which was less than 3% of the activity mediated by cytochrome P-450 under similar assay conditions. Enzyme activity was independent of the lipid factor dilauroylphosphatidylcholine. The apparent Km for aldrin was 27 +/- 7 microM. The modifiers of monooxygenase reactions, 7,8-benzoflavone, SKF 525-A and ethanol, inhibited the activity mediated by cytochrome P-448. The I50 was 0.05, 0.2 and 800 mM, respectively. These results indicate that aldrin is a highly selective substrate for cytochrome P-450 species present in microsomes of phenobarbital-treated animals and is a poor substrate for cytochrome P-448. The two forms of aldrin epoxidase can be characterised by their turnover number, their apparent Km and their sensitivity to modifiers, like 7,8-benzoflavone and ethanol.

Aldrin↗