Publications

We have reported [Correia et al. (1987) Arch. Biochem. Biophys. 258, 436-443] that administration of 3,5-dicarbethoxy-4-ethyl-2,6-dimethyl-1,4-dihydropyridine (DDEP) to untreated, phenobarbital (PB) pretreated, or dexamethasone (DEX) pretreated rats results in relatively selective inactivation of cytochrome P-450 (P-450) isozymes h (CYP2C11), k (CYP2C6), and p (CYP3A). Such inactivation involves destruction of P-450 prosthetic heme predominantly by N-ethylation in untreated and PB-pretreated rats, whereas in DEX-pretreated rats, it also appears to be associated with prosthetic heme alkylation of the apocytochrome presumably at the active site. The cause for this differential course of DDEP-mediated P-450 heme destruction is unclear. Since this process is absolutely dependent on NADPH-mediated DDEP metabolism and can be reproduced in vitro, in search of mechanistic clues, we have examined DDEP metabolism by liver microsomes from the three rat sources as well as by isolated purified rat liver P-450h and P-450k. HPLC analyses of microsomal incubations of DDEP with NADPH, in the presence of an esterase inhibitor, revealed the presence of two major products: deethylated pyridine (DP) and 4-ethylpyridine (4-EDP) with product ratios (DP/4-EDP) of 1.4, 1.4, and 0.7 for reactions catalyzed by liver microsomes from untreated, PB-pretreated, and DEX-pretreated rats, respectively. The corresponding mean product ratios for P-450h- and P-450k-catalyzed reactions were 4.2 and 5.5, respectively. On the other hand, partition ratios (DP formed/P-450 destroyed) ranged from 12.0, 10.5, and 4.8, respectively, for incubations of liver microsomes from untreated, PB-pretreated, and DEX-pretreated rats to 9.5 and 28.9 for purified P-450h- and P-450k-catalyzed reactions, respectively. However, DP formation in all these microsomal systems was comparable, and although 4-EDP formation was greatly stimulated by DEX pretreatment, it does not appear to be a destructive pathway. In view of this, our findings reported herein suggest that the active site environment of P-450's h, k, and p apparently determines not only the pattern of DDEP metabolism but also the differential course of prosthetic heme destruction.

The rex gene of the type I human T-cell leukaemia virus (HTLV-I) encodes a phosphorylated nuclear protein of relative molecular mass 27,000 which is required for viral replication. The Rex protein acts by promoting the cytoplasmic expression of the incompletely spliced viral messenger RNAs that encode the virion structural proteins. To identify the biologically important peptide domains within Rex, we introduced a series of mutations throughout its sequence. Two distinct classes of mutations lacking Rex biological activity were identified. One class corresponds to trans-dominant repressors as they inhibit the function of the wild-type Rex protein. The second class of mutants, in contrast, are recessive negative, rather than dominant negative, as they are not appropriately targeted to the cell nucleus. These results indicate the presence of at least two functionally distinct domains within the Rex protein, one involved in protein localization and a second involved in effector function. The trans-dominant Rex mutants may represent a promising new class of anti-viral agents.