The
antitumor drug acivicin, L-(alpha S,5S)-alpha-amino-3-chloro-4,5-dihydro-5-isoxazoleacetic
acid, irreversibly inactivated in vivo formylglycinamidine
ribonucleotide synthetase (
FGAM synthetase, EC 6.3.5.3) in transplantable rat
hepatoma 3924A while the activity in host liver remained unchanged. At
acivicin doses of 1.0 and 5.0 mg/kg
body weight,
enzyme activity in the
hepatoma decreased to 26 and 5%, respectively, after 2 hr. The activity of the in vivo inactivated
hepatoma 3924A
enzyme could not be restored by gel filtration or 40 hr of dialysis. In the absence of
L-glutamine,
acivicin in vitro inactivated both liver and
hepatoma FGAM synthetase in a time-dependent fashion, with an inactivation constant Kinact = 66 microM and a minimum inactivation half-time T = 1.0 min. In the presence of
L-glutamine, competitive inhibition was observed with a Ki = 5 microM. Protection against in vitro inactivation was observed in the presence of 1 mM
L-glutamine, suggesting that
L-glutamine concentrations are important in the selective toxicity of
acivicin on
hepatoma cells in vivo. Irreversible inhibition of
FGAM synthetase by
acivicin is consistent with the view that this
antibiotic is an active site-directed affinity analog of
L-glutamine and indicates that this
enzyme is a sensitive target of
acivicin action.