Alkaloid N-oxides

Pyrrolizidine Alkaloids—Genotoxicity, Metabolism Enzymes, Metabolic Activation, and Mechanisms

Pyrrolizidine alkaloid-containing plants are widely distributed in the world and are probably the most common poisonous plants affecting livestock, wildlife, and humans. Because of their abundance and potent toxicities, the mechanisms by which pyrrolizidine alkaloids induce genotoxicities, particularly carcinogenicity, were extensively studied for several decades but not exclusively elucidated until recently. To date, the pyrrolizidine alkaloid-induced genotoxicities were revealed to be elicited by the hepatic metabolism of these naturally occurring toxins. In this review, we present updated information on the metabolism, metabolizing enzymes, and the mechanisms by which pyrrolizidine alkaloids exert genotoxicity and tumorigenicity.

Riddelliine N-oxide is a phytochemical and mammalian metabolite with genotoxic activity that is comparable to the parent pyrrolizidine alkaloid riddelliine

Pyrrolizidine alkaloids (PAs) and their N-oxide derivatives are naturally-formed genotoxic phytochemicals that are widely distributed throughout the world. Although, the quantities of PAs and PA N-oxides in plants are nearly equal, the biological and genotoxic activities of PA N-oxides have not been studied extensively. PA N-oxides are major metabolites of PAs and are generally regarded as detoxification products. However, in this study, we report that rat liver microsomes converted riddelliine N-oxide to the genotoxic 6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP) metabolite. Metabolism of riddelliine N-oxide by rat liver microsomes under hypoxic conditions (argon) generated predominantly the parent PA, riddelliine. The reduction of riddelliine N-oxide to riddelliine was diminished, when the metabolism of riddelliine N-oxide with rat liver microsomes was conducted aerobically. Rat liver microsomal incubations of riddelliine N-oxide in the presence of calf thymus DNA produced a set of DHP-derived DNA adducts as detected and quantified by 32P-postlabeling/HPLC. The same DHP-derived DNA adducts were also found in liver DNA of F344 rats fed riddelliine N-oxide or riddelliine. When rats received doses of 1.0 mg/kg riddelliine N-oxide for three consecutive days, the level of DNA adducts was 39.9 +/- 0.6 adducts/10(7) nucleotides, which was 2.6-fold less than that measured in rats treated with riddelliine at the same dose. We have previously shown that these DHP-derived DNA adducts are produced by chronic feeding of riddelliine and that the adduct levels correlated with liver tumor formation. Results presented in this paper indicate that riddelliine N-oxide, through its conversion to riddelliine, is also a potential genotoxic hepatocarcinogen.

Enzymatic and non-enzymatic reduction of brucine N-oxide by aldehyde oxidase and catalase

1. Brucine N-oxide was reduced by aldehyde oxidase in rabbit liver cytosol in the presence of an electron donor, 2-hydroxypyrimidine, under anaerobic conditions. The flavoprotein purified from rabbit liver exhibited significant reductase activity in the presence of electron donors. 2. Brucine N-oxide was also reduced by rabbit liver cytosol and blood in the presence of both a reduced pyridine nucleotide and FAD under anaerobic conditions. The N-oxide reductase activities were inhibited by carbon monoxide and air. However, these activities were not abolished whe n liver cytosol and blood were boiled. Rabbit erythrocytes exhibited the reductase activity, but not plasma. 3. When liver cytosol or blood was separated by DEAE-cellulose column chromatography, the fractions with the reducing activity in the presence of both NADH and FAD also showed catalase activity. 4. Catalase catalysed the brucine N-oxide reduction in the presence of both NAD(P)H and FAD. Hematin also exhibited the reductase activity in the presence of both NAD(P)H and FAD. Photochemically reduced FAD was effective in the reduction instead of NAD(P)H and FAD. 5. Bricine N-oxide reduction proceeds via two routes in liver cytosol and blood. One is enzymatic reduction by aldehyde oxidase; the other is non-enzymatic reduction catalysed by the haem group of catalase in the presence of reduced flavin.

A matter of some sensitivity

The development of sensitive chromatographic and spectroscopic techniques for the isolation and structure determination of natural products has greatly facilitated phytochemical investigations. Chemical investigations of herbarium material have resulted in the isolation of indole, quinoline and isoquinoline alkaloids from a wide number of plants. Examples of novel natural products from higher plants are given and include alkaloids, terpenoids, phenolics and quinones. Some plants investigated have not yielded the types of constituents which would have been predicted from them. Plant tissue cultures provide alternative sources of biologically active compounds and examples investigated include Cinchona, Ailanthus, Brucea and Artemisia for antiprotozoal compounds and Datura for tropane alkaloids. Biological tests are useful for bioassay-guided fractionation of plant extracts and examples of the isolation of a series of natural products with antiprotozoal and cytotoxic activities are given. Chemical and biological investigations into the traditional medicine Dragon's blood (Croton lechleri) from S. America and a Chinese prescription for the treatment of atopic eczema are described. The use of radio-ligand binding assays for the detection of a wide range of biological activities is discussed. Sensitivity of chemical and biological techniques has greatly improved prospects for finding new drug entities from plants and for investigating traditional medicines. Basic phytochemical investigations should continue to be encouraged especially in view of the rapid loss of plant species.

Factors involved in the N-oxidation of isomeric aromatic diazines by microsomal preparations

Factors affecting the metabolism of isomeric aromatic diazines in vitro were studied and the conditions which allow maximal metabolism established. The N-oxidation of isomeric diazines was linear with respect to microsomal concentration up to 0.5 g original liver weight per ml using a rabbit microsomal suspension. N-Oxidation was also linear up to between 20-30 minutes depending on substrate studied. The rate of N-oxide production increased with increase in substrate concentration up to about 10 mumol/incubate, after which the rate declined. By using the data obtained the appropriate kinetic factors, Km and Vmax, for the N-oxidation of pyrazine, pyrimidine and pyridazine by rabbit hepatic microsomal preparations were calculated. Spectral binding constants, Ks, of substrates to cytochrome P450 were also calculated and appeared to be related to the Km values.

Alkaloid N-oxides as transport and vacuolar storage compounds of pyrrolizidine alkaloids in Senecio vulgaris L

Cell-suspension cultures of pyrrolizidinealkaloid-producing species selectively take up and accumulate senecionine (sen) and its N-oxide (sen-Nox). Cultures established from non-alkaloid-producing species are unable to accumulate the alkaloids. The uptake and accumulation of (14)C-labelled alkaloids was studied using a Senecio vulgaris cell-suspension culture as well as protoplasts and vacuoles derived from it. The alkaloid uptake exhibits all characteristics of a carrier-mediated transport. The uptake of sen-Nox follows a multiphasic saturation kinetics. The Km-values for sen Nox of 53 μM and 310 μM are evaluated. Senecionine competitively inhibits sen-Nox uptake, indicating that the tertiary alkaloid and its N-oxide share the same membrane carrier. The N-oxide of sen shows a pH optimum below 5.5, whereas sen is taken up over a range from pH 4 to 8. Activation energies of 90 and 53 kJ·mol(-1) are calculated for sen-Nox and sen transport, respectively. At concentrations of 10 to 100 μM, sen-Nox is rapidly taken up by cells and protoplasts; within 2 h >90% of total N-oxide is within the cells. By contrast the uptake of sen is less efficient. Vacuoles isolated from protoplasts preloaded with sen-Nox totally retained the alkaloid N-oxide, whereas sen is rapidly lost during the procedure of vacuole preparation. N-oxidation converts the weak lipophilic tertiary base into a charged polar molecule which is excellently adapted to serve as the cellular transport and storage form of pyrrolizidine alkaloids.

Selective uptake of pyrrolizidine N-oxides by cell suspension cultures from pyrrolizidine alkaloid producing plants

The N-oxides of pyrrolizidine alkaloids such as senecionine or monocrotaline are rapidly taken up and accumulated by cell suspension cultures obtained from plants known to produce pyrrolizidines, i.e. Senecio vernalis, vulgaris, viscosus (Asteraceae) and Symphytum officinale (Boraginaceae). The transport of the N-oxides into the cells is a specific and selective process. Other alkaloid N-oxides such as sparteine N-oxide are not taken up. Cell cultures from plant species which do not synthesize pyrrolizidine alkaloids are unable to accumulate pyrrolizidine N-oxides. The suitability of the pyrrolizidine N-oxides in alkaloid storage and accumulation is emphasized.

Biotransformations of glafenine in the rat and in man

The biotransformations of a therapeutic dose of the non-narcotic analgesic, glafenine, have been studied in the rat and in man. In the rat, the ester bond is extensively hydrolysed to give glafenic acid which is the major metabolite excreted in bile and in urine. Two minor pathways have been identified one leading by hydroxylation of the benzene ring of glafenine or glafenic acid in para of the amino-substituent to the corresponding phenols, the other, by oxidation of the quinoline nitrogen of glafenic acid, to its N-oxide. In vivo this N-oxide is partly reduced into the parent compound. Hydroxyglafenic acid is the product of both direct oxidation of glafenic acid and hydrolysis of hydroxyglafenine. The glyceric esters are conjugated as glucuro-ethers and/or sulfo-esters and the carboxylic metabolites as acyl glucuronides. The conjugation rate, high for glafenine, its phenol homologue and glafenic acid, is low for hydroxyglafenic acid and the N-oxide. The analogous urinary excretion patterns in man and in the rat suggest a similarity in the biotransformation of glafenine in these two species.

Constituents of West African medicinal plants XIX: Funiferine N-oxide, a new alkaloid from Tiliacora funifera (Menispermaceae)

A phytochemical investigation of the roots of Tiliacora funifera (Menispermaceae) resulted in the isolation and identification of funiferine N-oxide, a new bis(benzylisoquinoline) alkaloid.

Metabolic N-oxidation of atropine, hyoscine and the corresponding nor-alkaloids by guinea-pig liver microsomal preparations

Incubation of guinea-pig liver microsomal preparations with atropine or hyoscine resulted in the formation of the corresponding nor-alkaloids and both isomers of atropine N-oxide from atropine and one isomer of hyoscine N-oxide from hyoscine. Separate incubations of guinea-pig liver microsomal preparations with nor-atropine and nor-hyoscine yielded the corresponding hydroxylamines. The N-oxide and hydroxylamine metabolites were identified by comparison of their t.l.c. behavior and m.x. with prepared compounds and also by their reduction to the corresponding tertiary or secondary amines.