A preliminary investigation : plant cyanogenecity as a possible co-factor in a possum specific toxin : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Biochemistry, Massey University, New Zealand

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Since the introduction of thirty Australian brushtail possums into New Zealand in 1858 to start a fur trade industry the possum population has grown considerably. New Zealand is now 'home' to approximately 70 million possums which wreak devastation on our native forests and wildlife. Current effective strategies for the control of possums in uninhabited areas include the use of 1080, brodifacoum, cholecalciferol, cyanide, and trapping or shooting. However these strategies are relatively non-specific in their mode of action and as such cause non-target species, including native wildlife, to die. The use of non-specific toxins and traps is also inappropriate for situations where people, livestock, or pets are present. There is therefore a demand for alternative strategies of possum control that affect only the target species. Methods presently being researched include the development of contraceptive vaccines, possum repellents and possum specific bait stations. This research investigates the feasibility of developing a cyanogenic bait that is activated by a co-factor within the possum diet. The fast acting hydrogen cyanide poison is present in some plants species in an inactive glycoside form. Upon tissue injury the inactive cyanogenic glycoside is exposed to and hydrolysed by catabolic enzymes within the plant thereby releasing the toxin, hydrogen cyanide, at potentially lethal levels for possums. Some plant varieties within cyanogenic species however, have evolved to be acyanogenic due to the absence of either the cyanogenic substrate, the enzyme, or both. The occurrence of these acyanogenic plants which contain either the substrate or the appropriate enzymes are the target of this research. It is these plants that may provide the necessary co-factor for a cyanogenic possum bait to become lethal. Preliminary analyses involved measuring and maximising the cyanide release from plant species known to be highly cyanogenic. Clover leaves (Trifolium repens), cherries (Prumus avium), and almonds (Prumus amygdalus) were the plant tissues analysed to determine whether levels of cyanide toxic to possums could be liberated. All three plant varieties underwent in vitro analyses in which they were exposed to surplus substrate and/or enzymes at varied temperatures and acidities. The maximum cyanide release was determined for each plant variety and in the case of almonds (Prumus amygdalus) a further in vivo study was performed. Although the clover, cherries and almonds all liberated cyanide after addition of either cyanogenic substrate or enzymes, the almonds were the only plant tissue to liberate sufficient levels of cyanide from the in vitro analyses to be considered toxic to possums. The almonds were found to contain high levels of active β-glucosidase enzymes which when incubated with the cyanogenic substrate, amygdalin, released high levels of cyanide. The in vivo analyses of almond macerates administered with amygdalin however were inconclusive in showing almonds as an effective co-factor for the hydrolysis of amygdalin. Nevertheless, two possums did die from cyanide poisoning after the administration of amygdalin with and without added enzymes. A third possum displayed signs of severe cyanide poisoning after it was gavaged with amygdalin and β- glucosidase enzymes but it later made a full recovery. As a result of the limitation imposed by the small size of the in vivo sample group further experimental trials are recommended to possibly obtain a more accurate set of results.
Hydrocyanic acid, Botanical chemistry, New Zealand, Trichosurus vulpecula -- Control