CORRELATION BETWEEN CYTOCHROME-P450 2B1 INDUCTION BY BARBITURATES AND TOXICITY IN THE CHICK-EMBRYO IN OVO

Using the highly sensitive probe-substrate pentoxyresorufin towards cytochrome P450 2B1 isozymes, the relationship between the induction of this set of hemoproteins by barbiturates and toxicity was studied. Each chemical, allobarbital (AB), amobarbital (AM), barbital (BA), butobarbital (BE), buthetal (BU), cyclobarbital (CB), heptobarbital (HE), hexobarbital (HX), mephobarbital (ME), pentobarbital (PA), pentotal (PT), phenobarbital (PB), secobarbital (SE) and vinbarbital (VB) was injected in the chick embryo at the 16-18 day of embryonic incubation in equivalent doses (50, 25, 12.5, 6.25 and 3.12% of the respective LD(50)) and the mixed function monooxygenase activity examined in purified hepatic microsomes. For each barbiturate, linear regression was performed for both the inductive and toxic slope related to P450 2B1-like activity in order to evaluate the IC100 in mu umol/egg [the dose producing a 100 % increase of pentoxyresorufin dealkylation (PROD) during the induction phase] and the TC100 (the dose producing the same percental induction but during the toxic phase). The ranking of the relative induction potency was PB > PT > PA > AM > BU > BE > HX > AB > HE > VB > CB > MB > BA > SE, with an increase in enzyme activity ranging from 142% for SE to 952% for PB. A regression coefficient of 0.783 (n = 14) between log 1/IC100 vs log 1/TC100 of 0.768 (n = 14) between log 1/IC100 vs log 1/LD(50) and of 0.861 (n = 14) between log 1/TC100 vs log 1/LD(50) was found. These findings provide evidences for a correlation between induction of CYP2B1 P450(s) (confirmed in chick embryo by means of Western immunoblotting analysis using rabbit polyclonal antibody anti-P450 2B1), i.e. PROD induction potencies of specific barbiturates and their relative toxicities.