Metabolites of dietary 1,8-cineole in the male koala (Phascolarctos cinereus)
Boyle, R, McLean, S, Foley, W, Davies, NW, Peacock, EJ & Moore, B 2001, Comparative Biochemistry and Physiology Part C, vol. 129, pp. 385-396.
Koalas detoxify and eliminate 1,8-cineole, a Eucalyptus monoterpene, through oxidation into several metabolites. The average 1,8-cineole intake of six male koalas was 2.4 ± 1.1 mmol/kg, of which 1.3 ± 0.4 and 1.4 ± 0.4 was recovered free and total, respectively, in urine and faeces in various forms. The identities and amounts of seven metabolites were found: 9- and 7-hydroxycineole, 9- and 7-cineolic acid, 7-hydroxy-9-cineolic acid, 9-hydroxy-7-cineolic acid and 7,9-dicineolic acid. Hydroxy-cineolic acids comprised 85% of the metabolic profile, with 7-hydroxy-9-cineolic acid accounting for 77% by itself, followed by 10% 7,9-dicineolic acid.
Collected urine was analysed through gas chromatography-mass spectrometry to identify metabolites. Several of the metabolites could be identified through analogy to previously found compounds in koala waste, including 9- and 7-hydroxycineole, 9- and 7-cineolic acid, and 9-hydroxy-7-cineolic acid. The characterisations of 7-hydroxy-9-cineolic acid and 7,9-dicineolic acid were interpretations of the mass spectral and/or nuclear magnetic resonance data knowing the starting material. 7-hydroxycineole and 7-cineolic acid were only recovered in trace amounts, and 9-hydroxycineole and 9-cineolic acid were only found as 2-3% of the recovered amount. Ninety-four per cent of the total recovered metabolites had gained three or four oxygens during oxidation. No terpenes or their metabolites were detected in the faecal samples collected.
The experiment demonstrated that koalas oxidise then eliminate 1,8-cineole in a similar way to other compounds, such as p-cymene. All metabolites except 7,9-dicineolic acid had been found previously in brushtail possum waste, which helped their identification and demonstrated some similarity between the species’ processing of 1,8-cineole. The metabolic products found had oxidation at carbons 7 and/or 9, with C9 oxidated products being the most predominant, whereas brushtail possums had previously been found to oxidise a greater range of carbons. This suggests that the oxidative enzymes in koalas are have much greater regioselective preference than in brushtail possums. The koalas, however, also had more extensively oxidised metabolites (94% had three or more oxygen atoms) compared to brushtail possums, which only extensively oxidised 60%. Koalas, therefore, prefer to extensively oxidise metabolites to excrete, whereas brushtail possums utilise conjugation pathways to remove the less oxidised metabolites.
An understanding of koala terpene metabolites and, therefore, the processes behind their creation allows assessment of their detoxification capability. This also leads to a greater comprehension of diet and foraging behaviour.
Summarised by Laura Wait
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