Lysosomal destabilization via increased potassium ion permeability following photodamage

Isotonic K2SO4 solution protected lysosomes osmotically during a 20 min incubation, but lost its protective effect if the lysosomes were initially photooxidized after sensitization with Methylene blue. Increasing K2SO4 concentration promoted the latency loss of photodamaged lysosomes, but did not impair the integrity of unirradiated lysosomes. The results indicate that the photodamage enhanced lysosomal ionic permeability, with osmotic imbalance over the lysosomal membrane. Out of the decreased latency induced by the photodamage, 32% was prevented by the addition of 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid to the incubation solution, suggesting that electroneutral K+/SO4(2-) co-uptake plays a role in the lysosomal destabilization. The photooxidation increased lysosomal H+/K+ exchange, which was confirmed by monitoring the H+ leakage with the pH sensitive probe p-nitrophenol and examining the K+ entry by membrane potential measurements. Addition of K2SO4 to a lysosomal suspension lowered the delta pH of photodamaged lysosomes, presumably due to an increase in the exchange of internal H+ for external K+. Out of the photodamage-induced lysosomal latency loss, 50-60% was prevented by either lowering the external pH or preincubating the lysosomes with methylamine to elevate their internal pH. The results suggest that the photodamage-promoted K+/H+ exchange plays a major role in lysosomal osmotic destabilization.