Isomeric Composition of N-Alkylated Protoporphyrins Produced by Substituted Dihydropyridines in Vivo and in Vitro

Abstract

Substituted dihydropyridines, like DDC and its 4-ethyl analogue (4-ethyl-DDC)* give rise to N-alkylated porphyrins when given to rats and mice in vivo, by donating their intact 4-alkyl group to one of the pyrrole nitrogens of haem in hepatic cytochrome P-450 (De Matteis et al., 1981; Ortiz de Montellano et al., 1981; Tephly et al., 1981). Because of the asymmetrical arrangement of the two vinyl and propionate side chains in protoporphyrin IX, four structural isomers of N-monoalkylated protoporphyrins are possible, according to which of the four pyrrole nitrogens has been substituted. In a previous paper (De Matteis et al., 1983) we have provided evidence that the isomeric composition of the N-ethyl protoporphyrin produced in vivo by treatment with 4-ethyl-DDC depends on the particular cytochrome P-450 which predominates at the time of treatment, suggesting a role for the apo-cytochrome in directing alkylation preferentially on to one of the pyrrole nitrogens. In this present paper we wish to report additional findings in vivo which are compatible with this interpretation. Production of N-ethyl protoporphyrin is also found in vitro, when isolated microsomes are incubated with 4-ethyl-DDC in presence of NADPH and, although a change in isomeric profile is again found with microsomes containing different cytochrome P-450 enzymes, nevertheless the picture obtained with isolated membranes in vitro is different from that seen in the intact cells in vivo.