Objective—To compare hepatic metabolism of
pyrrolizidine alkaloids (PAs) between sheep and cattle
and elucidate the protective mechanism of sheep.
Sample Population—Liver microsomes and cytosol
from 8 sheep and 8 cattle.
Procedure—The PA senecionine, senecionine N-oxide
(nontoxic metabolite) and 6,7-dihydro-7-hydroxy-
1-hydroxymethyl-5H-pyrrolizine (DHP; toxic metabolite)
were measured in microsomal incubations. The
kcat (turnover number) was determined for DHP and
N-oxide formation. Chemical and immunochemical
inhibitors were used to assess the role of cytochrome
P450s, flavin-containing monooxygenases (FMOs),
and carboxylesterases in senecionine metabolism.
The CYP3A, CYP2B, and FMO concentrations and
activities were determined, in addition to the role of
glutathione (GSH) in senecionine metabolism.
Results—DHP concentration did not differ between
species. Sheep formed more N-oxide, had higher N-oxide
kcat, and metabolized senecionine faster than
cattle. The P450 concentrations and isoforms had a
large influence on DHP formation, whereas FMOs
had a large influence on N-oxide formation. In cattle,
CYP3A played a larger role in DHP formation than in
sheep. FMO activity was greater in sheep than in cattle.
Addition of GSH to in vitro microsomal incubations
decreased DHP formation; addition of cytosol
decreased N-oxide formation.
Conclusions and Clinical Relevance—Hepatic
metabolism differences alone do not account for the
variation in susceptibility seen between these
species. Rather, increased ruminal metabolism in
sheep appears to be an important protective mechanism,
with hepatic enzymes providing a secondary
means to degrade any PAs that are absorbed from the
rumen. (Am J Vet Res 2004;65:1563–1572)