Metabolism of sea urchin sperm. Interrelationships between intracellular pH, ATPase activity, and mitochondrial respiration.

Mitochondria of sperm of the sea urchin Strongylo-centrotus purpuratus are tightly coupled before induction of the acrosomal reaction. When the sperm are diluted at low external pH (pH 5.5) or in high potassium (200 mM) or in the absence of sodium, their internal pH is acidic (6.2-7.0) as measured by amine accumulation. Under these conditions the internal ATPase activity (primarily the dynein ATPase) is inhibited, sperm are immotile, and mitochondria are in respiratory state 4 (ATP concentration is maximal). When the internal pH is alkalinized, the internal ATPase activity is increased, as estimated either in vivo by measuring the decrease in ATP concentration after addition of oligomycin to prevent ATP synthesis, or in vitro using Triton X-100 permeabilized cells. This increase in ATPase activity correlates with an increase of up to 50-fold in respiratory rates and a mitochondrial transition to state 3. Carbonyl cyanide p-trifluoromethoxyphenylhydrazone-uncoupled respiration is also sensitive to the internal pH for it is inhibited at acidic internal pH. However, since nonmotile, nonrespiring sperm that are obtained when the internal pH is acidic have high concentrations of ATP, we conclude that in vivo the internal pH controls the rate of dynein ATPase and that this ATPase activity is limiting for the respiration of tightly coupled mitochondria. The redox state of the respiratory chain may also be under the direct influence of the internal pH.