Removal of Na+ channels in squid giant axons by perfusion with trypsin

The irreversible effects of the proteolytic enzyme trypsin on ionic and gating currents of voltage-clamped squid axon membranes have been studied. At physiological pH, internal perfusion of the fibre with trypsin was found to be very effective in removing Na+ channels leaving the potassium system almost unaltered. At T = 13 degrees C the rates of channel-cleavage averaged 1/10 min-1 for the Na+ and 1/128 min-1 for the K+ channel, respectively. As estimated by the decrement of peak sodium conductance, the rate of loss of Na+ channels correlates well with the rate of decrease of the total charge associated with the ON component of gating currents, indicating that trypsin probably interacts with an essential proteic portion of the channel whose removal might prevent both the displacement of gating charges and the subsequent opening of the channel. Intracellular pH remarkably influences the action of the enzyme. A plot of the pH-dependence of the rate of cleavage of Na+ channels suggests the involvement of a positively charged group (either lysine or arginine) in the substrate region of the trypsin catalytic reaction.