Photodynamic action of protoporphyrin on resealed erythrocyte membranes: mechanisms of release of trapped markers.

Photoactivation of protoporphyrin IX (PP) bound to resealed human erythrocyte (RBC) ghosts results in membrane damage which is manifested by the release of trapped markers Na+ and glucose-6-phosphate (G6P). Efflux of Na+ was rapid, continuous, and virtually complete before the onset of G6P efflux. The sugar phosphate emerged abruptly after a long lag. The antioxidant butylated hydroxytoluene (BHT) had no effect on the permeation of Na+, but greatly suppressed that of G6P. These results suggest that the markers are emitted via different mechanisms. For G6P, disruption of the bilayer by free radical lipid peroxidation appears to be necessary, inasmuch as BHT inhibited peroxidation as measured by thiobarbituric acid reactivity and appearance of phospholipid and cholesterol hydroperoxides on thin layer chromatograms. It is deduced that non-lipid damage is sufficient for Na+ release. This effect is manifested at low light intensities and low PP concentrations. Protein regulators of passive cation permeability may be the primary targets in this case. When sensitive sulfhydryl groups on these proteins were blocked with p-chloromercuri-benzene-sulfonate, Na+ leaked out rapidly, but G6P was unaffected, thereby mimicking the early stages of membrane photodamage.