Lysosomes as photochemical targets

Sulfonated tetraphenyl porphines (TPPS n ) are photosensitizing dyes that localize in lysosomes of NHIK 3025 cells. In order to elucidate the mechanisms of cell inactivation by photochemical treatment with TPPS n , lysosomal enzyme inactivation and release of lysosomal contents were examined after treatment. In cells treated with TPPS 4 , and light, the lysosomal enzymes β‐N‐acetyl‐D‐glucosaminidase (β‐AGA) and cathepsin(L + B) were almost completely inactivated and no enzyme activities were released from the lysosomes. In contrast, a maximum of 30 and 50% of the initial β‐AGA activity was released from lysosomes after treatment with TPPS 1 and TPPS 2a , respectively. Forty per cent of the initial β‐AGA activity was released after treatment with TPPS 2a and a non‐cytotoxic dose of light. After such a treatment only approximately 10% of the initial cathepsin activity was found in the cytosol fraction and in all other cases no cathepsin activity was recovered in the cytosol fraction after photochemical treatment. It was found that the constituents of the cytosol partly inhibited cathepsin activity. This inhibitory effect was not influenced by the photochemical treatment, neither was the colony‐forming ability of photochemically treated cells influenced by pre‐treatment with the cathepsin inhibitor E64. The present results indicate that NHIK 3025 cells are not killed by lysosomal disruption after photochemical treatment. This is partly due to photochemical inactivation of the lysosomal enzymes and to the action of cytosolic cysteine cathepsin inhibitors. The present results also indicate that cells can survive a partial lysosomal disruption.