Safrole: its metabolism, carcinogenicity and interactions with cytochrome P-450

Molecular dimensions of the substrate binding site of cytochrome P-448

The molecular geometries of specific substrates, inhibitors and inducers of cytochrome P-448 activity were determined using computer-graphic techniques for use in defining the molecular dimensions of the substrate binding site of this enzyme. Specific substrates of cytochrome P-448 are essentially planar molecules characterised by a small depth and a large area/depth ratio. In contrast, compounds that do not serve as substrates of cytochrome P-448 are bulky, non-planar molecules characterised by small area/depth ratios and greater flexibility in molecular conformation. Specific inhibitors of cytochrome P-448 whose effect is mediated through interaction with the haem still meet the dimensional criteria for substrates indicating that they must also interact with the substrate binding-site, which is probably located in proximity to the haem. Inducers of cytochrome P-448 activity exhibit similar molecular geometries to the substrates from which it may be inferred that the cytosolic receptor associated with the induction of cytochrome P-448 activity is structurally related to the active site of the cytochrome.

Interactions of safrole and isosafrole and their metabolites with cytochromes P-450

The structural features which determine interaction of safrole and related methylenedioxyphenyl compounds with cytochromes P-450 or P-448, and determine the induction of these two classes of the cytochrome, have been studied. All methylenedioxyphenyl compounds studied interact with both cytochromes P-450 and P-448 eliciting type I spectral changes and it has been found that the allyl 4-substituent is important in these interactions. Methylenedioxyphenyl compounds with an oxidised allyl 4-substituent exhibited higher affinity for cytochrome P-448 while those possessing an intact allyl or methylvinyl group generally showed higher affinity for cytochrome P-450. Compounds possessing intact allyl and methylenedioxyphenyl groups (safrole, isosafrole and myristicine) were the most potent inducers of cytochromes P-450 and P-448; compounds containing an intact allyl group only (estragole, allybenzene and eugenol methyl ether) or an oxidized allyl group and an intact methylenedioxyphenyl group (epoxysafrole) were inducers of P-448 only.

Ligand-complex formation between cytochromes P-450 and P-448 and methylenedioxyphenyl compounds

The formation of ligand complexes between hepatic microsomal cytochrome P-450 and safrole, isosafrole and other methylenedioxyphenyl compounds was studied in vivo and in vitro in rats pretreated with either phenobarbital or 3-methylcholanthrene. Both the phenobarbital-induced cytochrome P-450 and the 3-methylcholanthrene-induced cytochrome P-448 metabolically convert safrole, isosafrole, and those metabolites possessing an intact methylenedioxy group, to reactive metabolites which then interact with the cytochromes to form ligand complexes. Formation of these ligand complexes was accompanied by loss of mixed-function oxidase activities, and dissociation of the complexes with the type I substrate biphenyl restored activities. Safrole and, to a lesser extent, 1'-hydroxysafrole formed complexes in vivo when administered to phenobarbital-pretreated rats; none was obtained with epoxysafrole. However, when administered to 3-methylcholanthrene-pretreated animals all three compounds formed complexes, safrole being the least effective. Epoxysafrole and 1'-hydroxysafrole administered to phenobarbital-pretreated rats resulted in slight inhibition of the type I binding of safrole to liver microsomal P-450 in vitro; in contrast, with 3-methylcholanthrene-pretreated animals marked competitive inhibition was observed. This study shows that oxidation of the allyl chain of safrole analogues enhances their affinity for cytochrome P-448, but not for cytochrome P-450, and further demonstrates that these cytochromes possess distinctly different binding sites.

Cytochrome P-448 and the activation of toxic chemicals and carcinogens

The metabolic activation of carcinogens and some toxic chemicals appears to involve oxygenation in conformationally hindered positions in the chemical molecules. Oxygenation of xenobiotics in hindered positions is effected by cytochrome P-448 (LM4) but not by cytochrome P-450 (LM2). Substrate-interaction spectra show that cytochrome P-448 has an active site with a conformation different from that of cytochrome P-450. Induction of cytochrome P-448, as specifically measured by ethoxyresorufin O-deethylase activity, occurs in rat liver, kidney and lung after administration of the carcinogens, 3-methylcholanthrene, Aroclor 1254, 2-anthramine, safrole, 7,12-dimethylbenz[a]anthracene, MNNG and 2-acetamidofluorene. The doubtful carcinogens, saccharin, DDT and aldrin, resulted in no significant induction. The drugs paracetamol, antipyrine, imipramine and rifampicin resulted in diminished enzyme activity, indicating the absence of any induction of cytochrome P-448. In studies with the matched pairs of carcinogens and non-carcinogens, benzo[a]pyrene and benzo[e]pyrene, and 1,2,5,6-dibenzanthracene and anthracene, only the carcinogenic analogue resulted in induction of cytochrome P-448. With alpha- and beta-naphthylamine, both resulted in marked induction of cytochrome P-448 in liver, kidney and lung, indicating that both isomers might be carcinogenic.

Active metabolites in toxicology: the role of cytochrome P-448 and flavoprotein oxidases

The activation of toxic chemicals and carcinogens into reactive intermediates involves oxygenation in hindered positions of the molecule, by cytochrome P-448 (LM4), flavoprotein oxidoreductases, or transoxygenation during prostaglandin biosynthesis. Cytochrome P-450 (LM2) does not oxygenate in hindered positions and therefore generally detoxicates carcinogens and toxic chemicals. Cytochrome P-448 has a different active site from cytochrome P-450, which enables it to oxygenate substrates in conformationally-hindered positions.

Dietary carcinogens and anticarcinogens. Oxygen radicals and degenerative diseases

The human diet contains a great variety of natural mutagens and carcinogens, as well as many natural antimutagens and anticarcinogens. Many of these mutagens and carcinogens may act through the generation of oxygen radicals. Oxygen radicals may also play a major role as endogenous initiators of degenerative processes, such as DNA damage and mutation (and promotion), that may be related to cancer, heart disease, and aging. Dietary intake of natural antioxidants could be an important aspect of the body's defense mechanism against these agents. Many antioxidants are being identified as anticarcinogens. Characterizing and optimizing such defense systems may be an important part of a strategy of minimizing cancer and other age-related diseases.

Induction of cytochrome P-450 by methylenedioxyphenyl compounds: importance of the methylene carbon

Induction of hepatic microsomal cytochrome P-450 in Dub:ICR male mice treated with phenobarbital, 3-methylcholanthrene, safrole, isosafrole, 5-tert.-butyl-1,3-benzodioxole (BBD), 2-methyl-5-tert.-butyl-1,3-benzodioxole (MBBD), and 2,2-dimethyl-5-tert.-butyl-1,3-benzodiozole (DBBD) was evaluated by measuring the cytochrome P-450 content, Type II:Type 1 binding ratio, ethylisocyanide pH equilibrium point, biphenyl 2- and 4-hydroxylase, ethylmorphine N-demethylase, ethoxyresorufin O-deethylase, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Safrole and isosafrole treatment of mice produced a phenobarbital-type induction. BBD, but not MBBD and DBBD, induced cytochrome P-450 and formed a Type III metabolite-cytochrome P-450 complex, in vitro and in vivo. SDS-PAGE revealed that DBBD does induce proteins other than cytochrome P-450. These data suggest that the methylene carbon plays an important role in cytochrome P-450 induction.

Metabolism of safrole in the rat

The urinary metabolites of safrole (4-allyl-1,2-methylenedioxybenzene) in the rat were identified using gas chromatographic - mass spectrometric methods. The amounts of the individual metabolites excreted were determined gas chromatographically. Metabolite excretion was 93% in 72 hrs and most of this material (86%) consisted of metabolites formed via demethylenation of the methylenedioxy moiety. The other metabolic routes observed were allylic hydroxylation and the epoxide-diol pathway.

Selective inhibition of the safrole-induced mixed-function oxidase activities by 9-hydroxyellipticine

Intraperitoneal administration of 9-hydroxyellipticine, a specific cytochrome P-448 inhibitor, inhibited 3-methylcholanthrene-induced cytochrome P-448 activity (ethoxyresorufin O-deethylase, biphenyl 2-hydroxylase) and formation of the safrole carbene ligand complex with this cytochrome, but did not inhibit phenobarbital-induced cytochrome P-450 activity (ethyl-morphine N-demethylase) or formation of the safrole carbene ligand complex with this cytochrome. Biphenyl displaced the 9-hydroxyellipticine ligand from cytochrome P-448 leading to increased free cytochrome, but with no corresponding increase in mixed-function oxidase activity when biphenyl was used as substrate. It is concluded that following dissociation of the ligand complex, 9-hydroxyellipticine, which also exhibits type I binding, competes with biphenyl for the substrate binding site. Administration of 9-hydroxyellipticine to safrole-pretreated rats inhibited the cytochrome P-448-catalysed activity, but had no effect on the cytochrome P-450-catalysed activity. These results indicate that safrole induces a mixture of cytochromes P-450 and P-448 rather than a single novel haemo-protein. The type I substrate biphenyl displaced both the safrole carbene and the 9-hydroxyellipticine ligands from cytochrome P-448 resulting in increased free cytochrome. Displacement of the safrole carbene ligand was accompanied by increased mixed-function oxidase activity but, in contrast, displacement of the 9-hydroxyellipticine ligand resulted in no increase in mixed-function oxidase activity.

Metabolism of isosafrole and dihydrosafrole in the rat

The urinary metabolites of isosafrole (1,2-methylenedioxy-4-propenylbenzene) and dihydrosafrole (1,2-methylenedioxy-4-propylbenzene) in the rat were identified using gas chromatographic mass spectrometric methods. Additionally, the amounts of the individual metabolites excreted were determined gas chromatographically. Metabolite excretion was 89% (isosafrole) and 97% (dihydrosafrole) of the dose in 72 h. Although isosafrole was metabolized by allylic hydroxylation and via the epoxide-diol pathway, demethylenation leading mainly to 1,2-dihydroxy-4-propenylbenzene was by far the most prominent reaction. This was similarly true with dihydrosafrole which was metabolized mainly to 1,2-dihydroxy-4-(1-propyl) benzene. The total amount of demethylenated metabolites formed were 92% (isosafrole) and 95% (dihydrosafrole) of the identified material.