Does excretion of secondary metabolites always involve a measurable metabolic cost? Fate of plant antifeedant salicin in common brushtail possum, Trichosurus vulpecula

Salicin was administered orally to six brushtail possums by incorporation in food for six days at three dose levels (0.05, 0.5, and 1.5% wet weight), giving mean ± SD daily intakes of 0.31 ± 0.09, 2.76 ± 0.75, and 6.04 ± 1.12 mmol salicin. Metabolites were identified by mass spectrometry and assayed by HPLC. Salicyl alcohol glucuronide accounted for 56–64% of urinary metabolites over the three doses, salicyluric acid 15–26%, salicin 10–18%, and there were smaller amounts of free (2–4%) and conjugated (0–6%) salicylic acid. ,2-Dihydroxyphenylpropionic acid was a minor metabolite. The hydrolysis of dietary salicin enabled reconjugation of its aglycone, salicyl alcohol, with a more polar sugar, glucuronic acid, thus enhancing its renal excretion and resulting in little net loss of substrates for conjugation and a low measurable metabolic cost of excretion.