Mitochondrial porin is a complex channel with many diverse functional properties. Therefore, along with measuring single-channel conductance and selectivity, monitoring VDAC's voltage gating has become the gold standard for assessing mitochondrial porin function.
To understand the mechanisms of gating and ion selectivity, the detergent-solubilized porin was incubated with different concentrations of diethylpyrocarbonate (DEPC). The extent of the reaction of DEPC with histidine residues was monitored by differential absorbance at 242 nm. The resulting modified proteins were loaded onto (CM-cellulose) columns, and the collected fractions were examined for their activity.
The apparent ion selectivity was determined by measuring the membrane potential at zero-current conditions. The reversal potentials obtained for the unmodified-porin and ethoxyformyl-porin eluted from the resin (CMC-52) with the equilibration buffer were, respectively, +7 mV and -6 mV. The activity was slowly restored by incubation with hydroxylamine, and the extent of reversal depended on the reagent's concentration. The histidine residues are non-essential in the voltage-gating process since their modification has no direct impact on porin inactivation. However, the decrease of both the channel conductance and the critical potential needed to close the channel resulted from the covalent modification of one or more lysine residues in the EF-porin fraction, suggesting that lysine residues may contribute, in a unique way, to the gating process.