Toxicology of Industrial Compounds

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pyruvate oxidase can form this type of binding (Schwuger and Bartnik,
1980). Binding of more surfactant molecules leads to conformational
changes in the protein. It is obvious that conformational changes allow
binding of further surfactant molecules on hydrophobic regions which were
previously not exposed.
According to their different chemical structure (e.g. anionic, cationic,
amphoteric or nonionic) surfactants differ significantly in their ability to
carry out cooperative binding and therefore they differ in their biological
activity. Anionic surfactants form adsorption complexes with proteins due
to polar and hydrophobic interactions. Polar interactions between the
negatively charged hydrophilic group of the surfactant and the positively
charged groups of the protein molecule are the precondition for the
formation of hydrophobic associations between surfactant molecule and
protein molecule (Garcia-Dominguez, 1977; Schwuger and Bartnik, 1980).
In the case of dodecylsulphate and tetradecylsulphate the binding results in
denaturation of the proteins (Makino et al., 1973). Cationic surfactants can
interact by polar and hydrophobic binding as well. Polar interactions result
in electrostatic bonds between the negatively charged groups of the protein
molecule and the positively charged surfactant molecule. For example, the
enzyme, glucose oxidase, is deactivated by hexadecyl trimethyl ammonium
bromide through formation of an ion pair between the cationic surfactant
and the anionic amino acid side chain of the enzyme molecule (Tsuge,
1984). Nonionic or amphoteric surfactants and proteins show either no
interaction at all or interactions that are extremely weak and normally
close to the limits of sensitivity of the analytical methods used. For this
reason, nonionic surfactants will not dissolve sparingly soluble proteins,
denature proteins, or contribute to a swelling of the epidermis. Figure 24.1
shows the solubility of the protein zein, which is almost insoluble in water,
and is more or less solubilized by sodium dodecyl sulphate and alkyl
ethyleneglycol ether sulphates, while the nonionic ethoxylated nonylphenol
is ineffective (Schwuger and Bartnik, 1980). A further reason for the poor
interactions between nonionic surfactants and proteins could be that the
concentration necessary for cooperative binding with the protein is not
attained with nonionic surfactants due to their low critical micelle
concentration c M (Makino et al., 1973).
An important consequence of the interactions between anionic
surfactants and proteins is the swelling of the stratum corneum of the skin.
Hydrophobic interactions between surfactant chains and the protein result
in pendant ionic head groups and subsequently in swelling because of
electrostatic repulsion between them. As the substrate matrix expands and
the tertiary structure is disrupted, hydration occurs which leads to swelling
(Blake-Haskins, 1986).