Considerations of toxicologic interactions in developing new chemicals

Stereospecific, mechanism-based, suicide inhibition of a cytochrome P450 involved in ecdysteroid biosynthesis in the prothoracic glands of Manduca sexta

The first required step in ecdysteroid (molting hormone) biosynthesis, dietary cholesterol (C) conversion to 7-dehydrocholesterol (7dC) via 7,8-dehydrogenation, is mediated by a microsomal cytochrome-P450 monooxygenase specific to the larval prothoracic gland. A subsequent series of unknown "black-box" oxidations of 7dC result in the unusual ring geometry (cis-A/B) and functionality (6-keto-7-ene-14-alpha-ol) of the ecdysteroids and has been thought to involve the initial formation of alpha-5,6-epoxy-7-dehydrocholesterol (alpha epo7dC). Pharmacological studies indicated that conversion of C to 7dC in prothoracic gland homogenates was strongly and equally inhibited by the isomeric cholesterol substrate analogues alpha- and beta-5,6-epoxycholesterol (alpha- and beta epoC) and alpha- and beta-5,6-iminocholesterol (alpha- and beta iminoC). With respect to the conversion of C to ecdysteroids by disrupted glands, however, the two alpha-isomeric substrates were 10-fold more inhibitory than were their beta-analogues. Indeed, alpha amino C was as active as the non-specific pyrimidyl cytochrome-P450 monooxygenase inhibitor fenarimol that shows moderate toxicity in many insect species. All four cholesterol analogues competitively inhibited cholesterol 7,8-dehydrogenation, but only alpha epoC and possibly alpha iminoC were desaturated to delta 7-products. Although the KmS (and KiS) for all the substrates were similar (1.7-6.0 x 10(-5) M), the Vmax for alpha epoC dehydrogenation was eight-fold higher than that of C, making it a superior substrate for following this reaction in ecdysteroidogenic tissues rich in endogenous C. The 7,8-dehydrogenation of alpha epoC and alpha iminoC by prothoracic glands would produce the potentially reactive intermediates, alpha epo7dC and alpha imino7dC, respectively. They, in turn, could then undergo facile, acid-catalyzed ring-opening to the allylic-stabilized carbo-cation electrophiles. These very reactive, transient species, if formed in the active site of the monooxygenase, would then alkylate either the heme group or the apoprotein of the cytochrome or both, leading to the irreversible inhibition of the enzyme. The present data show that alpha epoC and probably alpha iminoC are mechanism-based suicide inhibitors of the enzyme catalyzing cholesterol 7,8-dehydrogenation and may be the prototypes of a new class of selective insect control agents.

The oxidative inactivation of cytochrome P450 in monooxygenase reactions

Possible mechanisms of cytochrome P450 self-inactivation during catalytic turnover have been considered. Two ways of hemoprotein inactivation are so far known. The first, studied extensively by many authors, is the formation of active substrate intermediates, capable of modifying heme and apoenzyme. The second way, revealed quite recently and resulting from uncoupled cytochrome P450-catalyzed monooxygenase reactions, is yet to be clarified. Briefly, it involves formation of hydrogen peroxide in the hemoprotein active center, which interacts with the enzyme associated Fe2+, thereby generating hydroxyl radicals that bleach the heme and modify the apoenzyme. This mechanism operates with substrates and cytochrome P450 forms with partially coupled monooxygenase reactions, thus causing the formation of hydrogen peroxide as a byproduct.

Anticonvulsant and neurotoxic activities of twelve analogues of valproic acid

Twelve racemic analogues of the antiepileptic drug valproic acid (VPA) were tested and compared with VPA for anticonvulsant activity by the subcutaneous pentylenetetrazol (PTZ) seizure threshold test and for neurotoxicity by the rotorod test. Four compounds produced maximal anticonvulsant activity (100% protection) in equimolar doses (1.5 mmol/kg) to VPA and two compounds showed a similar effect with lower doses (1.0 mmol/kg). Four compounds produced lower activity (38-80% protection), and two compounds showed no anticonvulsant activity at the dose used (1.5 mmol/kg). Two of the 12 compounds, (+/-)-2-n-propyl-4-hexynoic acid (11) and (+/-)-4-methyl-2-n-propyl-4-pentenoic acid (12), showed no sedation at doses that produced the maximum anticonvulsant effect. For the first time we succeeded to develop two compounds with higher protective index and safety ratios than VPA. Compound 11 had a longer duration of action and higher protective index but a lower safety ratio than 12. Comparisons of the anticonvulsant and minimal neurotoxic effects of these compounds with their calculated lipophilicity (C log P) revealed that compounds with the desired high anticonvulsant activity and minimal neurotoxicity showed C log P values between 1.84 and 2.64 and had nine carbon atoms (in contrast to eight carbon atoms for VPA).