Mammalian systems, alkaloids

Microorganisms, plants, and mammalian systems all contain enzymes capable of catalyzing chemical transformations with alkaloid substrates. Interesting and useful enzyme reactions that may occur with alkaloids include oxidations, reduc-... 

Fio. 1. Some of the possible effects of enzyme transformations on the activities of alkaloids in mammalian systems. 

The concept of microbial models of mammalian metabolism was elaborated by Smith and Rosazza for just such a purpose (27-32). In principle, this concept recognizes the fact that microorganisms catalyze the same types of metabolic reactions as do mammals (32), and they accomplish these by using essentially the same type of enzymes (29). Useful biotransformation reactions common to microbial and mammalian systems include all of the known Phase I and Phase II metabolic reactions implied, including aromatic hydroxylation (accompanied by the NIH shift), N- and O-dealkylations, and glucuronide and sulfate conjugations of phenol to name but a few (27-34). All of these reactions have value in studies with the alkaloids. 

All of the principles and ideas covered in the previous section may be translated directly to the use of microorganisms as tools in the production of compounds of plant biosynthetic or biodegradative importance. Just as one finds microbial systems to be of value in preparing metabolites in mammalian systems, it may be possible to use microbial transformations to prepare derivatives of alkaloids that might be found rarely or only in very small quantities in plants. In this way, abundant prototype alkaloids may be used as microbial transformation substrates to provide a range of metabolites. As in the mammalian case, metabolism studies using plant tissues, tissue cultures, or cell-free extracts may be conducted in parallel with microbial metabolic systems. Metabolites common to both would be prepared in quantity by relatively simple fermentation scale-up methods. 

It seems reasonable to include a mention of the topic of alkaloids in mammalian systems, since these compounds apparently form in mammals by reac-... 

Over 40 norditerpenoid alkaloids have been reported in species of larkspurs. Data on toxicity in a mammalian system have been reported for 25 of these by the Poisonous Plant Research Laboratory (reviewed in Panter et ah, 2002). The commonality among all the wild larkspur species is the presence of norditerpenoid alkaloids, which are responsible for poisoning livestock. 

A review of isoxazole compounds (covering the period 1963—1977) discusses their synthesis, reactivity, and physiological and physicochemical properties naturally occurring and biologically active isoxazoles are included.27 Some Saudi Arabian plants have been screened for alkaloids.28 Aristolochic acids I (28 R = H) and D (28 R = OH), which are cytotoxic to mammalian systems, are also cytotoxic to non-tumorous plant cells.29... 

The toxicity of plants that contain pyrrolizidine alkaloids has been discussed in an earlier volume of this series (163), and the relationship between the toxic nature of the plants and the metabolism of their alkaloids by the victim has been reviewed (164). Since the toxicity of the pyrrolizidine alkaloids in mammals seems to be due to their metabolites rather than to the alkaloids themselves (164), considerable effort has been expended in the identification of the metabolites produced both in vivo and in vitro by mammalian systems. The material summarized in Table V (165-172) is supplementary to that discussed in reference 164. 

The subject of metabolism of tetrahydroisoquinolines and related alkaloids in mammalian metabolic systems has recently been reviewed (205, 206). The formation of biogenic amine-derived alkaloids in mammals and their transformation by O-methylation reactions have been described. 

Biotransformations of morphinan alkaloids have been reported for plant, fungal, and mammalian enzymatic systems with emphasis on rather specific reactions such as the reduction of ketones, N- and O-demethylation, and perox-idative transformations. Furuya et al. used immobilized tissue culture cells of Papaver somniferum to accomplish the selective reduction of codeinone (135) to codeine (136) (207) (Scheme 30). Suspension cultures of a well-established cell line of P. somniferum were grown for one week as a source of cell mass for immobilization in calcium alginate. The cells continued to live in the alginate matrix for 6 months maintaining their biological activity. The reduction of co-... 

Monocrotaline (170) has been the subject of extensive metabolic study with mammalian and microbiological systems. Pyrrolizidine alkaloids such as monocrotaline require metabolic activation to the corresponding pyrrole derivatives or dehydro alkaloids before they are capable of forming covalent bonds with critical macromolecules within the cell. The X-ray structure of dehydromonocrotaline has recently been determined (226), and the ability of dihydroretronicine derived from monocrotaline to react with deoxyguanosine has been demonstrated in vitro (225). 

A Summary of Alkaloid Transformations Catalyzed by Microbial, Mammalian, and Plant Enzyme Systems... 

In an attempt to correlate pharmacological activity with the rate of biotransformation, Beckett and Morton have studied the metabolism of a series of indole alkaloids and related model systems by a variety of enzyme preparations of mammalian origin (39, 40, 57-59). Of the systems studied, rabbit liver microsomes were effective in performing O-demethylation liver microsome preparations from rat and guinea pig were not very effective in transforming alkaloids 4-7 and 9-19 (but vide infra) (40). In the case of corynantheidine (4), the product of metabolism was identified as the 0-( 17)-demethyl compound 8 by TLC comparison with authentic material. 

Metabolism of the cyclic diesters retrorsine (166), monocrotaline (169), and crispatine (171) by a variety of mammalian liver microsome preparations and by Peptococcus heliotrinreducans has been reported. Mattocks and co-workers have studied the metabolism of retrorsine by liver microsome preparations from several sources 167-169) and have demonstrated the conversion of this alkaloid to the corresponding iV-oxide 167 and a pyrrolic metabolite formulated as 168. The formation of 168 via 167 and dehydration is mitigated against by the observation that retrorsine TV-oxide (167) does not give rise to a pyrrolic metabolite on incubation with rat liver microsomes 167), even though the enzyme system responsible for the production of 168 from retrorsine has many of the properties of the mixed-function oxygenases capable of N-oxidation 167,174). The metabolites of retrorsine... 

Not all oxidations of alkaloids can be ascribed to microsomal P-450 systems. Mammalian liver contains a flavoprotein oxidase, first discovered in rabit liver and called quinine oxidase . It is also an aldehyde oxidase, and its mechanism of action on heterocyclic nitrogen compounds probably involves addition of hydroxide ion to a suitable ring position, followed by dehydrogenation. Thus, the introduced oxygen atom comes from water rather than from molecular oxygen, in contrast to the P-450 oxygenases. [2]. 

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