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Abstract

The metabolic fate of two commonly occurring dietary terpenes in Australian
marsupial eucalypt folivores has been studied. The detoxification mechanisms
employed by these folivores were considered from an ecological perspective.
Interactions between herbivores and their respective diets has received considerable
attention over the last quarter of a century. A popular hypothesis by Freeland and
Janzen (1974) states that feeding behaviour of herbivores is influenced by limitations
of the body's mechanisms for detoxifying plant secondary metabolites (PSM).
However, very little is understood about the physiological processes and limitations
of detoxification in wild animals. This study provides an insight into these processes
in marsupial eucalypt folivores from a range of feeding niches.
An interspecies comparison was made of the metabolism of a bolus dose of the
monoterpene p-cymene in a generalist herbivore, the brushtail possum (Trichosurus
vulpecula), and three specialist folivores, the greater glider (Petauroides volans), the
ringtail possum (Pseudocheirus peregrinus) and the koala (Phascolarctos cinereus),
as well as in the laboratory rat (Rattus norvegicus) which provided a direct
comparison to published data. Each animal was dosed, intragastrically, with single
doses of p-cymene. Urine and faeces were collected pre- and post-dose.
Chronic ingestion of 1,8-cineole, emulating its natural occurrence in the diet of
brushtail possums and koalas, was also studied. Possums were fed an artificial diet in
which 1,8-cineole concentrations were manipulated to cover a range of
concentrations while koalas were fed Eucalyptus cephalocarpa, which has a terpene
profile dominated by 1,8-cineole.
Metabolites were identified by extraction, gas chromatography and mass
spectrometry. Standards for many metabolites were isolated from urine to allow
quantitation. A novel p-cymene metabolite, 2-p-carboxyphenylpropan-1,2-diol, was
identified in the koala. Twelve.novel 1,8-cineole metabolites were identified from
brushtail possum and koala urine. Eleven of these were isomers of hydroxy cineolic
acid. The isomeric structure and partial stereochemistry for 7-hydroxy-9-cineolic
acid were determined by NMR and mass spectrometry.
Observational data from chronic ingestion of 1,8-cineole in possums demonstrated a
threshold in maximum daily intake in most possums between 3 and 4 g/kg. The
pattern of metabolites excreted in the brushtail possum during chronic ingestion
demonstrated induction of enzymes responsible for producing the more oxidised
metabolites during the first days of ingestion. There was no evidence of saturation of
metabolic pathways associated with the larger intakes of 1,8-cineole. For both terpenes, species-specific patterns of metabolite excretion were evident and
reflected the natural occurrence of eucalypt leaves in the different diets. The number
and degree of oxidation of metabolites, as well as the role of conjugation, varied
depending on the natural feeding behaviour of the animals.
To facilitate studying the pattern of metabolism, terpene metabolites were grouped
according to the total number of oxygen atoms (up to four) acquired during oxidation. Overall there was a progressive increase in the proportion of more extensively
oxidised metabolites; from the rat, to the generalist and through to the specialist
folivOres. The generalist utilised a multiplicity of non-specific oxidative pathways
producing an array of metabolites covering all degrees of oxidation (9 metabolites of
p-cymene and 18 metabolites of 1,8-cineole). On the other hand, the specialists had
high capacity and specific oxidative pathways resulting in relatively few metabolites,
all of which were radically oxidised.
Glucuronidation was important in the generalists, compensating for their inability to
excrete the same degree of radically oxidised metabolites as specialists.
Approximately 40 - 50 % of p-cymene metabolites and up to 60 % of 1,8-cineole
metabolites were conjugated with glucuronic acid in brushtail possums. No
significant conjugation of metabolites was observed in the specialists.
Increased polarity, whether achieved by glucuronidation or extensive oxidation,
presumably results in the same overall enhanced capacity to excrete metabolites. It is
proposed that, for specialists, oxidative efficiency reduces the necessity for
subsequent conjugation, conserving glucuronic acid, a valuable resource in a
nutritionally limited diet. The brushtail possum, however, consumes a varied and
arguably better quality diet and can afford to excrete glucuronic acid in the
detoxification of terpenes. Given that PSMs, such as terpenes, are always present in
the eucalypt leaf diet of these folivores, these different strategies indicate that
adaptation of detoxification mechanisms vary with dietary specialisation.

Item Type:	Thesis - PhD
Authors/Creators:	Boyle, Rebecca Reinga
Keywords:	Plant metabolites, Herbivora, Herbivora, Terpenes
Copyright Holders:	The Author
Copyright Information:	Copyright 1999 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).
Additional Information:	Thesis (Ph.D.)--University of Tasmania, 1999. Includes bibliographical references
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