In vitro metabolism of isoline, a pyrrolizidine alkaloid from Ligularia duciformis, by rodent liver microsomal esterase and enhanced hepatotoxicity by esterase inhibitors.
Isoline, a major retronecine-type pyrrolizidine alkaloid (PA) from the Chinese medicinal herb Ligularia duciformis, was suggested to be the most toxic known PA. Its in vitro metabolism was thus examined in rat and mouse liver microsomes, and its toxicity was compared with that of clivorine and monocrotaline after i.p. injection in mice. Isoline was more rapidly metabolized by both microsomes than clivorine and monocrotaline and converted to two polar metabolites M1 and M2, which were spectroscopically determined to be bisline (a deacetylated metabolite of isoline) and bisline lactone, respectively. Both metabolites were formed in the presence or absence of an NADPH-generating system with liver microsomes but not cytosol. Their formation was completely inhibited by the esterase inhibitors, triorthocresyl phosphate (TOCP) and phenylmethylsulfonyl fluoride, but not at all or partially by cytochrome P450 (P450) inhibitors, alpha-naphthoflavone and proadifen (SKF 525A), respectively. These results demonstrated that both metabolites were produced by microsomal esterase(s) but not P450 isozymes. The esterase(s) involved showed not only quite different activities but also responses to different inhibitors in rat and mouse liver microsomes, suggesting that different key isozyme(s) or combinations might be responsible for the deacetylation of isoline. Isoline injected i.p. into mice induced liver-specific toxicity that was much greater than that with either clivorine or monocrotaline, as judged by histopathology as well as serum alanine aminotransferase and aspartate aminotransferase levels. Isoline-induced hepatotoxicity was remarkably enhanced by the esterase inhibitor TOCP but was reduced by the P450 inhibitor SKF 525A, indicating that rodent hepatic esterase(s) played a principal role in the detoxification of isoline via rapid deacetylation in vivo.
Tang
Jun
J
Institute of Natural Medicine, University of Toyama, Toyama 930-0194, Japan.
Akao
Teruaki
T
Nakamura
Norio
N
Wang
Zheng-Tao
ZT
Takagawa
Kiyoshi
K
Sasahara
Masakiyo
M
Hattori
Masao
M
D000410:Alanine Transaminase / Q000097:blood; D000818:Animals; D000996:Antitussive Agents / Q000378:metabolism / Q000633:toxicity; D001219:Aspartate Aminotransferases / Q000097:blood; D019659:Asteraceae / Q000737:chemistry*; D001571:Benzoflavones / Q000494:pharmacology; D065607:Cytochrome P-450 Enzyme Inhibitors; D004365:Drugs, Chinese Herbal / Q000378:metabolism / Q000633:toxicity; D004950:Esterases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D005100:Expectorants / Q000378:metabolism / Q000633:toxicity; D008099:Liver / Q000187:drug effects / Q000473:pathology; D008297:Male; D051379:Mice; D008815:Mice, Inbred Strains; D008862:Microsomes, Liver / Q000378:metabolism; D016686:Monocrotaline / Q000378:metabolism / Q000633:toxicity; D010664:Phenylmethylsulfonyl Fluoride / Q000494:pharmacology; D011335:Proadifen / Q000494:pharmacology; D011763:Pyrrolizidine Alkaloids / Q000378:metabolism* / Q000633:toxicity*; D051381:Rats; D017207:Rats, Sprague-Dawley; D014317:Tritolyl Phosphates / Q000494:pharmacology
D000996:Antitussive Agents; D001571:Benzoflavones; D065607:Cytochrome P-450 Enzyme Inhibitors; D004365:Drugs, Chinese Herbal; D005100:Expectorants; D011763:Pyrrolizidine Alkaloids; D014317:Tritolyl Phosphates; C489556:isoline; D010664:Phenylmethylsulfonyl Fluoride; C011512:alpha-naphthoflavone; D016686:Monocrotaline; D011335:Proadifen; D001219:Aspartate Aminotransferases; D000410:Alanine Transaminase; D004950:Esterases; C028677:clivorine; C025541:tri-o-cresyl phosphate