Porphyrin Photosensitization of Proteins in Cell Membranes as Studied by Spin‐labelling and by Quantification of Dtnb‐reactive Sh‐groups

Human cells of the line NHIK 3025 were exposed to hematoporphyrin derivative (Hpd) and light and analysed with respect to; (i) the mobility of membrane proteins as determined by electron spin resonance measurements of a protein‐bound spin label, (ii) fluorescence excitation spectra, (iii) relative number of DTNB‐reactive SH‐groups on their surface and in sonicated cell homogenates, (iv) survival, and (v) morphologic appearance as seen by ordinary phase contrast microscopy. A significant fraction of the porphyrins bound to the outer cell membrane was in close contact with proteins. 5,5′‐Dithiobis‐2‐nitrobenzoic acid reactive SH‐groups on the outer cell membrane were very sensitive to the treatment with Hpd + light and were degraded according to non‐exponential kinetics. When the cells were irradiated after spin labelling, the labelled proteins became less mobile during the irradiation, indicating protein cross linking. Irradiation before spinlabelling resulted in a selective degradation of low‐mobility proteins.