Effects of the pyrrolizidine alkaloids senecionine, retrorsine and seneciphylline on aminopyrine N-demethylase activity on the rat liver S-10 fraction.

The effects of individual pyrrolizidine alkaloids on the mixed-function oxidase (MFO) enzyme aminopyrine N-demethylase were determined in rat liver 10 000 X g supernatant. The pyrrolizidine alkaloids, senecionine, seneciphylline and retrorsine were obtained from Senecio vulgaris. Senecionine and seneciphylline were found to be linear mixed-type inhibitors while retrorsine was found to be a competitive inhibitor of aminopyrine N-demethylase. The average Ki's +/- S.E. for senecionine, seneciphylline and retrorsine were 0.18 +/- 0.02, 0.33 +/- 0.06 and 0.92 +/- 0.05 mM, respectively.