Investigation of the membrane-fluidizing properties of porphyrin-inducing drugs.

The objective of this study was to test the hypothesis that porphyrin-inducing drugs act at least in part by disrupting membrane lipids. The porphyrin-inducing steroids, 3 alpha-hydroxy-5 alpha-pregnane-11,20-dione (alfaxalone) and 3 alpha-hydroxy-5 alpha-pregnan-20-one induce considerably greater fluidity changes in spin-labelled phospholipid-cholesterol bilayers than do the corresponding 3 beta-hydroxy steroids which are also less potent as porphyrin inducers. The steroids did not cause any significant change in spin-labelled vesicles lacking cholesterol. The porphyrin-inducing compound 3,5-diethoxycarbonyl-1,4-dihydro-2,4,6-trimethylpyridine and a series of analogues caused fluidity changes in phospholipid bilayers in the presence and absence of cholesterol. The porphyrin-inducing nonplanar 2,2',4,4'-6,6'-hexachlorobiphenyl caused a significant change in bilayer fluidity in phospholipid bilayers in the presence and absence of cholesterol. Since the planar 3,3',4,4'-tetrachlorobiphenyl, allylisopropylacetamide (AIA), and griseofulvin are potent porphyrin-inducing compounds, but do not fluidize a lipid bilayer, it was clear that the original hypothesis required modification. No evidence could be obtained to support the idea that a subset of porphyrin-inducing drugs exists which are membrane fluidizers and whose common mechanism of action as porphyrin-inducers might be revealed by a common pattern of porphyrin accumulation in chick embryo liver cells. It is suggested that those porphyrin-inducing compounds with membrane-fluidizing properties might fluidize the nuclear membrane, thus facilitating the transfer of an induction specific RNA for delta-aminolevulinic acid synthetase from the nucleus to the cytoplasm.