Permeability and photoxidative damage of membrane enzymes

The pattern of photodynamic damage of pig erythrocyte and rat brain microsomal ATPases and erythrocyte acetylcholinesterases has been studied, using rose bengal as photosensitizer. Of these enzymes the Na⁺-K⁺-Mg²⁺-ATPase, believed to be associated with active transport, is very much more sensitive to damage than are the Mg²⁺-ATPase and the erythrocyte acetylcholinesterase. Earlier photoxidative studies of the haemolysis of erythrocytes have shown that ion movements occur in two phases which are dependent on the duration of exposure to light and it is suggested that these are correlated with the differential sensitivity of these membrane enzymes. The ATPases of the brain microsomal preparation were more sensitive to photodynamic damage during preincubation at 0°C, i.e. in the absence of substrate. Raising the preincubation temperature to 37°C protected both enzymes, the inactivation of the Na⁺-K⁺-Mg²⁺-ATPase being markedly reduced. The presence of substrate during pre-incubation at 0°C also protects both enzymes, especially the Mg²⁺-ATPase. These interacting effects of temperature and substrate are compared with the known different temperature sensitivities of these two enzymes.