Metabolism of pyrrolizidine alkaloids by ruminal microbes Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/rf55z979q

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  • The plant Senecio jacobaea (tansy ragwort) produces several macrocyclic pyrrolizidine alkaloids which cause irreversible liver cirrhosis. All of the pyrrolizidine alkaloids in Senecio jacobaea are macrocyclic diesters of the necine base retronecine, with the two most predominant being seneciphylline and jacobine. Unlike horses and cattle, sheep and goats are generally resistant to chronic pyrrolizidine alkaloid toxicosis due to metabolism of the toxic pyrrolizidine alkaloids by ruminal bacteria. In this study, metabolism of jacobine and seneciphylline by ruminal bacteria was investigated, focusing on two possible metabolic pathways. One pathway involved hydrolysis of jacobine and seneciphylline and the subsequent production of the necine base retronecine. For use in these studies, retronecine was isolated and labeled with deuterium. A method was developed for the determination of retronecine to 0.02 μg /mL. Significant hydrolysis of jacobine and seneciphylline was not observed in either ovine whole rumen fluid or with a mixed culture of anaerobic microbes derived from ovine rumen fluid which metabolizes jacobine and seneciphylline. Direct metabolism of retronecine independent of the macrocyclic pyrrolizidine alkaloids was also not observed. The second metabolic pathway studied involved the production of the necine base modified by the conversion of the 1,2-double bond to an external methylene group. Previously this conversion has been observed during metabolism of pyrrolizidine alkaloids by the organism Peptostreptococcus heliotrinreducens. In this study, P. heliotrinreducens was used to convert the pyrrolizidine alkaloids heliotrine and lasiocarpine to the known 1-methylene-pyrrolizidines. The mixed culture of ovine anaerobic microbes also metabolized heliotrine and lasiocarpine rapidly to identical methylene products. This mixed culture metabolized jacobine and seneciphylline rapidly with production of very low levels of the corresponding 1-methylene compounds. In contrast, metabolism of jacobine or seneciphylline by P. heliotrinreducens was not observed. The mixed culture has demonstrated the ability to metabolize a greater variety of pyrrolizidine alkaloids than P. heliotrinreducens. Although the metabolites of jacobine and seneciphylline were not conclusively identified, it was determined that hydrolysis of jacobine and seneciphylline is not occurring. The second pathway studied appears to be more probable, with the production of 1-methylene compounds as intermediates, although not as end-products.
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