INFLUENCE OF PH ON SULFHYDRYL-GROUPS AND FLUIDITY OF THE MITOCHONDRIAL-MEMBRANE

Fluidity of the red blood cell membrane decreases as pH changes from 8 to 7.5. In rat liver mitochondrial (RLM) membrane fluidity precipitously declines as pH drops from 7.35 toward 7.0. With dithionitrobenzoate (Nbs2), reaction rates of mitochondrial SH groups from rat liver and heart (RHM) and in beef heart submitochondrial particles are reduced at pH 7.0 as compared to 7.35. Similar results are obtained with the lipophilic fluorescence dye monobromobimane (MB). Bromobimane Q (MQ), which predominantly labels superficially located SH groups, does not detect differences in SH reaction rate between pH 7.35 and 7.0. Oligomycin diminishes the amount of reactive SH groups in RLM titrated with Nbs2 only at pH 7.35, whereas with MB a decrease caused by oligomycin is found at pH 7.35 and pH 7.0. With MQ, an increase in reaction rate is observed for both pH values after pretreatment with oligomycin. Using 4-maleimido-TEMPO mobilization of SH groups is found with oligomycin at pH 7.0, whereas at pH 7.35 they are immobilized. Phosphate significantly stimulates reaction rates of SH groups at pH 7.0 in RHM and RLM. In RHM inhibition of succinate oxidation by oxaloacetate as well as the efflux of NAD(P)H is enhanced at pH 7.0, indicating increased permeability in both directions. Decreases in pH, fluidity, and thiol reactivity are important factors in hypoxic/ischemic membrane damage. © 1990.