Binding of Sudan II and IV to lecithin liposomes and E. coli membranes: insights into the toxicity of hydrophobic azo dyes.

Background: Sudan red compounds are hydrophobic azo dyes, still used as food additives in some countries. However, they have been shown to be unsafe, causing tumors in the liver and urinary bladder in rats. They have been classified as category 3 human carcinogens by the International Agency for Research on Cancer. A number of hypotheses that could explain the mechanism of carcinogenesis have been proposed for dyes similar to the Sudan red compounds. Traditionally, investigations of the membrane toxicity of organic substances have focused on hydrocarbons, e.g. polycyclic aromatic hydrocarbons (PAHs), and DDT. In contrast to hydrocarbons, Sudan red compounds contain azo and hydroxy groups, which can form hydrogen bonds with the polar head groups of membrane phospholipids. Thus, entry may be impeded. They could have different toxicities from other lipophilic hydrocarbons. The available data show that because these compounds are lipophilic, interactions with hydrophobic parts of the cell are important for their toxicity. Lipophilic compounds accumulate in the membrane, causing expansion of the membrane surface area, inhibition of primary ion pumps and increased proton permeability.

Results: This work investigated the interactions of the amphiphilic compounds Sudan II and IV with lecithin liposomes and live Escherichia coli (E. coli). Sudan II and IV binding to lecithin liposomes and live E. coli corresponds to the Langmuir adsorption isotherm. In the Sudan red compounds--lecithin liposome solutions, the binding ratio of Sudan II to lecithin is 1/31 and that of Sudan IV to 1/314. The binding constant of the Sudan II-lecithin complex is 1.75 x 104 and that of the Sudan IV-lecithin complex 2.92 x 105. Besides, the influences of pH, electrolyte and temperature were investigated and analyzed quantitatively. In the Sudan red compounds--E.coli mixture, the binding ratios of Sudan II and Sudan IV to E.coli membrane phospholipid are 1/29 and 1/114. The binding constants of the Sudan II--and Sudan IV- E.coli membrane phospholipid complexes are 1.86 x 104 and 6.02 x 104. Over 60% of Sudan II and 75% of Sudan IV penetrated into E.coli, in which 90% of them remained in the E.coli membrane.

Conclusion: Experiments of Sudan II and IV binding to lecithin liposomes and live E. coli indicates that amphiphilic compounds may be sequestered in the lecithin liposomes and membrane phospholipid bilayer according to the Langmuir adsorption law. Penetration into the cytosol was impeded and inhibited for Sudan red compounds. It is possible for such compounds themselves (excluding their metabolites and by-products)not result directly in terminal toxicity. Therefore, membrane toxicity could be manifested as membrane blocking and membrane expansion. The method established here may be useful for evaluating the interaction of toxins with membranes.