Determination of the structure of an N-substituted protoporphyrin isolated from the livers of griseofulvin-fed mice

Abstract

Feeding mice with griseofulvin, a widely used anti-fungal agent which induces protoporphyria as a side-effect, leads to the formation in the liver of two green pigments which have been shown to be porphyrin adducts. In this work, the major porphyrin adduct isolated from the livers of griseofulvin-fed mice has been characterized structurally using one- and two-dimensional NMR spectroscopy. The adduct was shown to be an N-alkylated protoporphyrin IX in which the whole of griseofulvin (less a hydrogen atom) is attached to a pyrrole ring nitrogen of the porphyrin. It was shown that the drug-to-porphyrin linkage is an -O-CH2-N(pyrrole) = linkage, to either the 4- or 6-position of ring a of griseofulvin. In an attempt to identify which pyrrole nitrogen is involved in this linkage, the 1H spectra of the free base and zinc complex of the adduct were compared with the corresponding spectra of the four regioisomers of N-methylprotoporphyrin. These comparisons indicated that the adduct isolated from the livers of griseofulvin-fed mice is either the N(C) or the N(D) regioisomer, although a clear distinction between these two could not be made on the available evidence. The mechanism of formation of the adduct and its relation to griseofulvin-induced protoporphyria are discussed.