Modulation of rat liver aryl hydrocarbon (benzo[a]pyrene) hydroxylase activity by nutritional effects

Hepatic enzyme induction and mutagenicity of airborne particulate matter from Santiago, Chile in the nourished and malnourished rat

1. Respirable, airborne particles in the ambient air in downtown Santiago, Chile, have been characterized for the seasonal variation in total polycyclic aromatic hydrocarbon content, 13 of which have been identified including the mutagens (benzo(a)pyrene, dibenzo(a,h)anthracene, benzo(a)anthracene, benzo(b)fluoranthene and indeno(1,2,3, c,d)pyrene amongst others. 2. Organic extracts derived from these particles were administered to both the nourished and malnourished rat and resulted in modulation of the hepatic mixed function oxidase system including induction of NADPH-cytochrome P450 reductase, cytochrome P4501A as determined by Western blot analysis and the associated ethoxyresorufin O-deethylase and aryl hydrocarbon hydroxylase activities. 3. The cytochrome P4504A1-dependent 12-hydroxylation of lauric acid was induced in the malnourished state, but this activity was significantly inhibited by treatment of the animals with particle extracts in both nutritional states. 4. The particle extracts contained both direct and indirect-acting mutagens in the Ames test, and depending on the relative complement of both, resulted in either increased or decreased mutagenicity in the presence of S9 activation systems derived from both nourished and malnourished animals. 5. These results are discussed in the context of the interindividual risk assessment of airborne, particulate matter to man.

Induction of rat hepatic P4501A1 by organic extracts from airborne particulate matter in Santiago, Chile

1. Organic extracts from particulate matter collected in downtown Santiago, Chile, in 1990 were administered to female Wistar rats. 2. Extracts shifted the maximal absorption wavelength P450 spectra of the reduced-CO complex of hepatic microsomal P450 from 450 to 448 nm, and enhanced the total content of P450. In addition, substantial increases in aryl hydrocarbon hydroxylase, ethoxyresorufin O-deethylase and ethoxycoumarin O-deethylase activities were observed, whereas aminopyrine N-demethylase activity was not affected by treatment. 3. Western blotting using polyclonal antibodies against P4501A isozymes showed the appearance of a distinct and intense P4501A1 band in microsomes from rat pretreated with air particle extracts, and was not observed in microsomes from control rat. On the other hand, the intensity of the P4501A2 isoenzyme was apparently not affected. 4. These findings suggest that the organic extracts from airborne particulate matter modify the composition of rat liver P450 isozymes by inducing those isoforms responsible for the activation of precarcinogenic to carcinogenic agents.

Disruption effects of carbon tetrachloride on rat liver microsomes

Lipid fluidity in rat liver microsomes was assessed by the steady state fluorescence polarization of stearic acid labelled at positions 2, 7 or 12 with 9-anthroyl groups. Oxygen solubility and/or mobility was determined by measuring the quenching rate of different pyrene derivatives containing an aromatic group separated from the microsome surface by 1, 4 or 11 methylene groups. The fluorescence of these compounds, as well as that from methylpyrene and benzo(alpha)pyrene, was quenched by carbon tetrachloride. The effect of the latter compound and of n-heptanol on the fluorescence polarization of the anthracene derivatives and the quenching rate by oxygen was also determined. The results obtained indicate that the lipid order decreases and the solubility and/or mobility of oxygen increases towards the bilayer core. The addition of carbon tetrachloride decreases the order of the membrane and increases the rate of fluorescence quenching by oxygen. The largest effect of the additive on the fluorescence quenching rate by oxygen is observed at intermediate positions in the bilayer. n-Heptanol addition decreases the membrane order and increases the rate of fluorescence quenching by oxygen or carbon tetrachloride, the maximum effect being observed at the microsomal surface. The differences between the effects of the two additives are discussed in terms of their different localizations. Fluorescence quenching by oxygen is considerably more affected by carbon tetrachloride than fluorescence depolarization. In addition, the maximum effect takes place at different positions in the membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

Flumecinol, a novel inducer of testosterone 16 alpha-hydroxylation in male rats

1. Flumecinol, a new inducer of the cytochrome P-450 monooxygenases, was studied in rats as a possible effector of liver microsomal testosterone oxidases. The drug enhanced the total content of liver cytochrome P-450 in immature and adult rats. 2. When total testosterone oxidation activity was compared in liver microsomes of treated and untreated rats, no differences in activities were observed in 60-day-old-rats, but a slight decrease was found in 35-day-old treated rats. 3. Several regio- and stereo-specific hydroxylases were modified by flumecinol administration; in 35-day-old rats only 16 alpha-hydroxylation was induced, whereas in 60-day-old rats a slight increase in 2 alpha-hydroxylation was also observed.

Nutritionally triggered alterations in the regiospecificity of arachidonic acid oxygenation by rat liver microsomal cytochrome P450

Cytochrome P450-dependent oxidation of arachidonic acid was studied in liver microsomes from normal fed, protein-energy malnourished, and refed rats. The overall rate of arachidonic acid oxidation was very similar in microsomes from the three groups, but microsomes from malnourished rats showed a higher turnover rate than microsomes from normal fed and refed rats. The regiospecificity of cytochrome P450 oxidation of arachidonic acid was drastically altered by the animal nutritional status. Thus, protein-energy malnutrition results in a clear stimulation of total omega and omega-1 hydroxylation, concomitant with a marked decrease in olefin epoxidation and allyllic oxidations. These changes, as well as the documented biological activity of some of the cytochrome P450 arachidonate metabolites, suggest that protein-energy deficiency might help to select P450 isozymes which are probably involved in key monooxygenation reactions of physiological substrates.

Effects of human diets on biotransformation enzyme activity and metabolic activation of carcinogens in rat liver

We studied the effects of a complete human diet, based on mean consumption figures in The Netherlands, the heating of food, and the presence of vegetables and fruit in the diet on the drug metabolizing capacity of the rat liver and on metabolic activation of known carcinogens. Groups of five male and five female Wistar rats were given ad lib. one of six different diets for 3 months. Each diet contained 40 energy (E)% fat, 13 E% protein, 47 E% carbohydrate and 5% fibre (w/w). The diets were as follows: a control diet of semi-synthetic materials (A); a human diet of meat, bread and eggs without processing (B); diet B heated under usual household conditions (C); a diet representing a complete human meal including (summer) vegetables and fruit (D); diets consisting of winter vegetables (E) or summer vegetables (F) with fruit. Semi-synthetic components were added to diets B-F to achieve the desired composition. There were differences between male and female rats on the effects of the different diets on hepatic enzyme activity. In female rats, but not in males, ethoxyresorufin-O-deethylase activity was increased significantly (P less than 0.05) in groups C, D and E in comparison with the controls (group A). In male rats ethoxycoumarin-O-deethylase activity was enhanced in groups D, E and F, and glutathione-S-transferase was markedly induced in group F (P less than 0.01). In males, hepatic cytochrome P-450 was significantly (P less than 0.05) increased in groups B, C and E. There was no effect on aminopyrine-N-demethylase activity and almost no effect on UDP-glucuronyltransferase activity in either sex. Microsomes from rats fed heated food (C) markedly increased the mutagenicity of benzo[a]pyrene (B[a]P) in the Ames assay using Salmonella typhimurium strain TA98, in comparison with levels using microsomes from rats fed the raw food (B). Vegetables and fruit decreased B[a]P mutagenicity. All human diets except D decreased the mutagenicity of N-nitrosodimethylamine in tester strain TA100. The results indicate that the influence of components of human diets on rat-liver drug metabolism may have quite different effects on the biotransformation of carcinogens activated by different metabolic pathways.

Nutrition-related alterations in liver microsomal testosterone hydroxylases

The oxidation products of testosterone formed by liver microsomes from normal-fed and protein-energy malnourished male rats have been analysed by HPLC. Microsomes from normal-fed rats oxidized testosterone at a rate of 4.52 nmol/min/mg protein. The major products formed were: 6 beta-, 7 alpha- and 16-alpha-hydroxytestosterone; these three metabolites represented 65% of the total testosterone metabolism. Microsomes from protein-energy malnourished rats oxidized testosterone at a reduced rate of 2.03 nmol/min/mg protein. The major product formed was 7 alpha-hydroxytestosterone, which accounted for 43% of total testosterone oxidation. Microsomes from protein-energy malnourished rats showed a CO-reduced cytochrome P-450 spectra with a maxima at 452 nm, and a 38% decrease in the total content of cytochrome P-450. Some testosterone hydroxylases were drastically affected by protein-energy malnutrition but others, such as 7 alpha-hydroxylase, remained unchanged. The present results suggest that nutritional status can modify the relative amounts of individual cytochrome P-450 isozymes, thus explaining the observed changes in several testosterone hydroxylases. Protein-energy malnutrition seems to be an excellent tool with which to obtain a microsomal fraction containing predominantly P-450 isozymes, which are probably involved in key mono-oxygenations of physiological substrates.

Purification and characterization of liver cytochrome P-446 isolated from protein energy malnourished rats

A liver cytochrome P-450 isozyme has been purified to homogeneity from protein-energy malnourished rats induced with beta-naphthoflavone (beta-NF). The purification steps included chromatography on DEAE-Sephadex-A-25, DEAE-cellulose (DE-53), hydroxylapatite (HA) and carboxymethyl-sephadex (CM) columns. The reduced carbon monoxide difference and absolute spectra showed a Soret peak at 446.5 nm. The wavelength maxima for the oxidized and reduced spectra were at 416 and 408 nm, respectively. Cytochrome P-446 appears to have a predominantly low spin ferric iron, migrates as a single band of molecular weight 56,000 in sodium dodecyl sulfate polyacrylamide gels and has a specific content of 14 nmol/mg of protein. P-446 oxidized various substrates at different rates in a reconstituted system with NADPH-cytochrome P-450 reductase and dilauroyl-phosphatidylcholine. In this system turnover rates for benzo[alpha]pyrene, testosterone and benzphetamine oxidation were: 81.10; 1.85 and 1.42 nmoles product/min/nmol P-446 respectively. While NH2 terminal amino acid sequence analysis of 18 of the first 20 residues suggests that the cytochrome P-446 isolated from malnourished rats is identical with form c, the catalytic activities suggest that this isozyme may be a more effective or efficient catalyst for some substrates.

Effect of unbalanced diets on incorporation of delta-aminolevulinic acid into cytochrome P-450

The in vivo syntheses of two liver microsomal cytochromes P-450 PB3a, P-450 UT50 [(1987) Eur. J. Biochem., submitted] (Mr 50,000, 52,000) have been estimated by measuring the specific activity 2 h after i.p. administration of delta-[3H]aminolevulinic acid to male Sprague Dawley rats. The animals were fed either a standard rat chow (5% lard, 22% casein) or unbalanced diets (high lipid, 30% lard or low protein, 6% casein) with or without 50 ppm Phenoclor DP6. The high-lipid diet supported a more rapid body weight gain but had little impact on cytochrome P-450 content, expressed either per whole liver or per mg microsomal protein, and on the incorporation of the precursor into cytochrome P-450. The latter was determined by measuring the radioactivity incorporated into the cytochrome P-450 fraction, partially purified by affinity chromatography, as well as into two cytochrome P-450 isozymes (Mr 50,000 or 52,000) purified by DEAE-52 cellulose ion-exchange chromatography. The low-protein diet, on the other hand, severely depressed body weight gain and cytochrome P-450 content as well as incorporation of radioactivity, the lower-Mr cytochrome (Mr 50,000) being particularly affected. However, when a potent inducer, Phenoclor DP6, was added to the low-protein diet, cytochrome synthesis was restored indicating that the effect was reversible.

Effect of benzo[a]pyrene on DNA synthesis and DNA polymerase activity of rat liver nuclei

When benzo[a]pyrene (B[a]P) was administered intraperitoneally to rats 48 hr before they were killed, the DNA-synthesizing capability of isolated rat liver nuclei was decreased as compared with control animals. B[a]P also inhibited in vitro DNA synthesis in nuclei purified from control animals; this effect was enhanced by NADPH. DNA polymerases solubilized from purified nuclei of B[a]P-treated animals were less active than those of control animals. DNA polymerase alpha was more inhibited than DNA polymerase beta. Purified rat liver nuclei devoid of cytoplasmic contamination possess an NADPH-dependent B[a]P hydroxylase activity. The observed inhibition of DNA synthesis in nuclei isolated from B[a]P-treated rats was increased by NADPH. Moreover, there was an increased inhibition of DNA polymerase activity by nuclear membranes obtained from B[a]P-treated animals when the incubations were performed in the presence of NADPH. Also, the derivative B[a]P-trans-9,10-dihydrodiol was a potent inhibitor of DNA polymerase alpha under conditions where DNA polymerase beta was less affected. These results suggest that nuclear B[a]P hydroxylase might be involved in the inhibition of DNA synthesis probably at the level of DNA polymerase alpha. As in the in vivo studies, the nuclear polymerase most affected by the hydrocarbon in vitro was DNA polymerase alpha.

Nutritional effects on mitochondrial bioenergetics. Alterations in oxidative phosphorylation by rat liver mitochondria

Rats malnourished since birth and fed on a protein-free diet for 2 weeks showed a 23-27% decrease in the State-3 oxidation of glutamate, succinate and ascorbate + NNN' N'-tetramethyl-p-phenylenediamine by liver mitochondria compared with control fed animals. ATP synthesis and the respiratory control index were diminished at the three coupling sites, but significant alterations were not observed in ADP/O ratios. Vmax. for NADH oxidation in electron-transport particles was 40% lower. Mitochondrial cytochromes b and c1 remained unchanged, but cytochrome c was increased by 26%. Cytochromes a + a3 were diminished by 22%. Vmax. for mitochondrial ATPase was 23% lower. These results suggest that the lower content of cytochrome a + a3 at the rate-controlling step of oxidative phosphorylation in malnourished rats might be mainly responsible for the decrease in substrate oxidations as well as ATP synthesis at the three coupling sites. The decreased synthesis and hydrolysis of ATP suggests that other energy-dependent mitochondrial processes could be decreased during malnutrition.