Ion Permeability Induced in Planar Lipid Bilayer Membranes by Quinone Pigments Derived from Eukaryotic Microorganisms

Abstract

Numerous quinone and quinoid pigments have been isolated from a variety of eukaryotic microorganisms, including fungi and protozoa. Structural information on many of these quinone compounds suggests that these molecules are amphiphilic in nature, which is indicative of a strong membrane association potential. Therefore, we have extensively investigated the interaction of various quinone pigments with typical biological membrane models, planar lipid bilayer membranes and mitochondrial membranes. Anthraquinone mycotoxins, versicolorin A and averufin are metabolic precursors of aflatoxin B 1 , and have been found to exhibit genotoxic effects in the hepatocyte/DNA repair test. Using planar lipid bilayer membranes, we demonstrated that versicolorin A and averufin greatly increased the proton conductance of bilayer membranes. The concentrations employed are comparable to those used in mitochondrial experiments and thus the effects of both toxins on mitochondrial oxidative phosphorylation might be mediated by increases in the proton permeability of the mitochondrial membrane. On the other hand, it is interesting that averufin increased ionic permeability in addition to proton permeability in planar bilayer membranes. This observation suggests that the mechanism of averufin-induced uncoupling may differ somewhat from that of versicolorin A. Blepharismins are polycyclic quinones found in the pigment granules of the ciliated protozoan, Blepharisma . At cytotoxic concentrations, blepharismins formed cation-selective channels in planar phospholipid bilayer membranes. The channels formed in a diphytanoylphosphatidylcholine bilayer had a K + /Cl − permeability ratio of 6.6:1. Single-channel recordings revealed the conductance to be quite heterogeneous, ranging from 0.2 to 2.8 nS in solutions containing 0.1 M KCl, possibly reflecting different states of aggregation of blepharismin. We also studied the effects of blepharismins on membrane permeability in rat liver mitochondria. The results further substantiated the channel formation by blepharismins in biological membranes. The analyses presented here, in conjunction with other biochemical studies described in this chapter, indicate that planar lipid bilayer membranes offer powerful tools for answering important questions regarding the structure and function of a diverse range of naturally occurring compounds.