MAJOR FACTORS FOR THE SUSCEPTIBILITY OF GUINEA-PIG TO THE PYRROLIZIDINE ALKALOID JACOBINE

Guinea pigs are generally resistant to the toxicity of pyrrolizidine a lkaloids (PAs). However, the PA jacobine (JB) is unusually toxic to th is species. We now have investigated whether different pathways for JB metabolism in the guinea pig could contribute to the susceptibility o f this species to this PA, To investigate the potential for esterolyti c cleavage of PAs, we have initially purified two forms of hepatic car boxylesterase (GPL1 and GPH1) from guinea pigs, The major form (GPL1) was purified to a specific activity of 486 mu mol/min/mg protein in th e hydrolysis of p-nitrophenyl acetate (NPA), whereas the minor form (G PH1) yielded p-nitrophenol with a specific activity of 86 mu mol/min/m g protein from NPA. The metabolism of the highly toxic PA JB and the l ess toxic PA seneclonine (SN) was studied using H-3-labeled PAs with g uinea pig liver micro-somes and purified guinea pig carboxylesterases. Purified carboxylesterase (GPH1) hydrolyzed [H-3]JB and [H-3]SN at ra tes of 4.5 and 11.5 nmol/min/mg protein, respectively, Carboxylesteras e GPL1, however, had no activity toward these PAs. Liver microsomes co nverted [H-3]JB to the pyrrolic metabolite 7-dihydro-7-hydroxy-1-hydro xymethyl-5H-pyrrolizine (DHP) and JB N-oxide at rates of 0.329 and 0.1 04 nmol/min/mg protein (DHP/N-oxide ratio of 3.16), The same microsome s produced DHP and SN N-oxide from [H-3]SN at rates of 0.460 and 0.865 nmol/min/mg protein, yielding a DHP/N-oxide ratio of 0.53. Therefore, we concluded that the combinations of high pyrrole and low N-oxide fo rmation, together with little hydrolysis, are the major factors respon sible for the ready intoxication of guinea pigs by the PA JB.