Ontogenic expression of detoxication enzymes in an Australian marsupial, the brushtail possum (Trichosurus vulpecula)

Xenobiotic metabolism of plant secondary compounds in juniper (Juniperus monosperma) by specialist and generalist woodrat herbivores, genus Neotoma

Mammalian herbivores routinely consume diets laden with often-toxic xenobiotics, yet the manner in which mammalian herbivores detoxify these plant secondary compounds (PSC) is largely unknown. Theory predicts that specialists rely more heavily on functionalization pathways whereas generalists rely on conjugation pathways to metabolize PSC in their diet. We took a pharmacological approach to determine how a specialist (Neotoma stephensi) of juniper foliage (Juniperus monosperma) and a generalist (N. albigula) may process the same dietary PSC. We investigated the xenobiotic metabolizing enzymes of the specialist and generalist on a control diet and a low (25%) juniper diet. We also examined enzyme activities in the specialist on a high (70%) juniper diet. We assayed for cytochrome P450 concentration and biotransformation activities of three specific cytochrome P450 isozymes (CYP1A, CYP2B, CYP3A), NAD(P)H:quinone oxidoreductase, glutathione conjugation, sulfation and glucuronidation. Results provide partial evidence for the hypothesis in that the specialist and generalist consuming juniper at a level similar to their natural diet, differ in the level of conjugation enzyme activity with generalists having higher activity overall than specialists.

Application of Pharmacological Approaches to Plant–Mammal Interactions

The dominant theory in the field of mammalian herbivore-plant interactions is that intake, and therefore tolerance, of plant secondary metabolites (PSMs) is regulated by mechanisms that reduce absorption and increase detoxification of PSMs. Methods designed by pharmacologists to measure detoxification enzyme activity, metabolite excretion, and most recently, drug absorption, have been successfully applied by ecologists to study PSM intake in a variety of mammalian study systems. Here, we describe several pharmacological and molecular techniques used to investigate the fate of drugs in human that have potential to further advance knowledge of mammalian herbivore-plant interactions.

Xenobiotic metabolism in Australian marsupials

The Australian marsupials are significant and unique Australian fauna. Xenobiotic metabolism is the process of enzymatic modification of xenobiotics, which include the chemicals, such as agricultural chemicals and natural dietary toxins, that these animals may be exposed to. Very little is known about the enzymes involved in xenobiotic metabolism in this unique group of animals. Folivore marsupials such as the koala (Phascolarctos cinereus and the brushtail possum (Trichosurus vulpecula) represent unique adaptation which has only been relatively superficially examined to date. We provide an overview of our current knowledge of marsupial xenobiotic metabolism.

Testosterone dehydrogenase activity in koala liver: characterisation of cofactor and steroid substrate differences

We have studied the hepatic microsomal 17beta-hydroxysteroid dehydrogenase (17beta-HSD) capacity of koala (Phascolarctos cinereus) and tammar wallaby (Macropus eugenii). A detailed comparison of the activity in hepatic fractions from koala and rat was made. Hepatic microsomal NADP-supported 17beta-HSD activity was significantly higher in koala (11.64+/-3.35 nmoles/mg protein/min), (mean+/-S.D.) than in tammar wallaby liver (1.52+/-0.79 nmoles/mg protein/min). However, when NAD was utilised as cofactor the activity was similar in both marsupial species (2.83+/-2.03 nmoles/mg protein/min, koala; 0.70+/-0.71 nmoles/mg protein/min, tammar wallaby). Data for rat indicated a cofactor preference for NAD rather than NADP (17.94+/-6.40 nmoles/mg protein/min, NAD; 2.18+/-1.04 nmoles/mg protein/min, NADP). Michaelis-Menten parameters for the kinetics of 17beta-HSD testosterone oxidation by NADP and NAD were determined in the koala. The Km for testosterone was of the order of 10.0-24.0 microM (n = 6) irrespective of the cofactor used, whilst the Km for NADP was 0.28-0.43 microM (n = 2) and for NAD was 13.9-18.5 microM (n = 2). 17beta-estradiol was found to be an inhibitor of both NAD- and NADP- supported 17beta-HSD activity. These findings indicate that NADP-mediated, but not NAD-mediated testosterone dehydrogenation is a major pathway of steroid biotransformation in koala liver; the reaction is less extensive in fractions from wallaby, human and rat. Such species-related differences in cofactor preference may contribute along with species differences in gene expression to observed rates of 17beta-HSD activity in mammals.