Toxicology of Industrial Compounds

340 TOXICOLOGY OF SURFACTANTS
Interactions with membranes
Due to their ability to absorb at interfaces, surfactants can interact with
biological membranes. This interaction depends on the concentration of the
surfactant and can be described in the following sequence (Helenius and
Simons, 1975). In the first instance the monomeric surfactant molecule
adsorbs onto the membrane. For a low surfactant/membrane ratio this
changes the permeability of the membrane and leads to cell lysis at higher
concentrations. At even higher surfactant concentrations, the lamellar
structure of the membrane is lost and it is solubilized. A further increase in
surfactant concentration results in the separation of the phospholipids from
the protein. This allows surfactant molecules to adsorb on previously
hidden regions of the protein molecule. For the solubilization of integral
membrane proteins the formation of micelle/protein complexes seems to be
a prerequisite. A significant solubilization of these proteins is possible only
if the critical micelle concentration c M is exceeded. This is indicated by the
fact that the microsomal membrane bound enzyme arylsulphatase-C could
only be extracted from the membrane with retention of the biological
activity after micelles were formed (Chang et al., 1985).
As a consequence of these interactions, surfactants are able to influence
the metabolism of membrane components (DeLeo, 1989). This has been
demonstrated by studies on the pathophysiology of surfactant-mediated
skin irritation. In vitro cultured corneocytes showed an increased release of
cholin metabolites after incubation with anionic surfactants. This effect
was less pronounced after treatment with nonionic surfactants. In
conclusion, these investigations demonstrated that the release of
metabolites is correlated with the irritation potential of surfactants.
Interactions with proteins
Depending on the structure of the surfactant the interactions with proteins
are based on polar or hydrophobic interactions. The binding of surfactants
to protein molecules is a function of the concentration of free surfactant in
equilibrium with the protein. The binding is affected by the pH,
temperature and ionic strength of the solution. These factors can lead to
conformational changes of proteins and thereby increase or decrease the
number of available binding sites. Natural bovine albumin, for example,
has 10 binding sites for decyl glucoside at 10°C and 13 at 25°C
(Wasylewski and Kozik, 1979). According to a theory developed by Jones
(1975), surfactants adsorb onto proteins in multiple equilibria. Only a few
surfactant molecules (