INACTIVATION OF CHICK-EMBRYO HEPATIC CYTOCHROME-P450 1A, 2H AND 3A FOLLOWING IN OVO ADMINISTRATION OF YCARBONYL-1,4-DIHYDRO-2,6-DIMETHYL-4-ETHYLPYRIDINE AND -(2,4,6-TRIMETHYLPHENYL)THIOETHYL]-4-METHYLSYDNONE

Rat hepatic cytochrome P450 (P450) isozymes 1A1, 2C6, 2C11, 3A1 and 3A 2 are targets for mechanism-based inactivation by the porphyrinogenic compound ycarbonyl-1,4-dihydro-2,6-dimethyl-4-ethylpyridine (4-ethyl D DC). It is of interest to determine whether similar P450 isozymes are targets of porphyrinogenic drugs in the chick embryo liver. The chick embryo expresses P450 2H1/2 isozymes, which are similar to the rat P45 0 2B1/2 isozymes, a polycyclic aromatic hydrocarbon-inducible P450 1A isozyme, and a pregnenolone 16 alpha-carbonitrile-inducible P450 3A is ozyme. We have found previously that chick embryo hepatic P450 1A and 3A isozymes are targeted for in vitro mechanism-based inactivation by 4-ethyl DDC and by the sydnone -(2,4,6-trimethylphenyl)thioethyl]-4-me thylsydnone (TTMS). Marked differences have been observed between the in vitro and in vivo effects of porphyrinogenic drugs on P450 isozymes . Thus, the first objective of this study was to determine whether chi ck embryo hepatic P450 1A and 3A isozymes are subject to in ovo inacti vation by these porphyrinogenic compounds. Our second objective was to determine whether the chick embryo hepatic P450 2H isozyme(s) was sub ject to in ovo and in vitro inactivation by 4-ethyl DDC and TTMS. Usin g hepatic microsomes prepared from beta-naphthoflavone-, dexamethasone -, phenobarbital-, and glutethimide-induced 19-day-old chick embryos, we found that total P450 content was decreased significantly in micros omes prepared from all treatment groups following in ovo administratio n of 4-ethyl DDC and TTMS. Moreover, in ovo administration of both 4-e thyl DDC and TTMS caused a significant decrease of 7-ethoxyresorufin O -deethylase, erythromycin N-demethylase, and benzphetamine N-demethyla se activities, which are selective catalytic markers for chick embryo hepatic P450 1A, 3A and 2H isozymes, respectively. In addition, in vit ro administration of 4-ethyl DDC and TTMS caused mechanism-based inact ivation of benzphetamine N-demethylase activity in microsomes from phe nobarbital- and glutethimide-treated chick embryos, showing that the c hick embryo hepatic P450 2H isozyme is a target for mechanism-based in activation. Therefore, it was concluded that the chick embryo hepatic P450 1A, 2H and 3A isozymes serve as targets for both in ovo and in vi tro mechanism-based inactivation by 4-ethyl DDC and TTMS.