Influence of membrane physical state on the lysosomal proton permeability

Influence of membrane physical state on the proton permeability of isolated lysosomes was assessed by measuring the membrane potential with 3,3' -dipr opylthiadicarbocyanine iodide and monitoring their proton leakage with p-ni trophenol. Changes in the membrane order were examined by the steady-state fluorescence anisotropy of 1,6-diphenyl-l,3,5-hexatriene. Both the membrane potential and proton leakage increased with fluidizing the lysosomal membr anes by benzyl alcohol and decreased with rigidifying the membranes by chol esteryl hemisuccinate. The proton permeability increased to the maximum of 42% by the benzyl alcohol treatment and decreased to the minimum of 38.1% b y the cholesteryl hemisuccinate treatment. Treating the lysosomes with prot onophore CCCP increased the proton permeability by 58%. The effects of the membrane fluidization and rigidification can be reversed by rigidifying the fluidized membranes and fluidizing the rigidified membranes, respectively. The results indicate that the proton permeability of lysosomes increased a nd decreased with increasing and decreasing their membrane fluidity, respec tively. Moreover, the lysosomal proton permeability did not alter further i f the changes, either an increase or a decrease, in the fluidity exceeded s ome amount. The results suggest that the proton permeability of lysosomes c an be modulated finitely by the alterations in their membrane physical stat e.