Activation of sea urchin sperm motility is accompanied by an increase in the creatine kinase exchange flux

The kinetics of the creatine kinase (CK) reaction were studied in suspensions of quiescent and active, intact sea-urchin spermatozoa in artificial seawater, using 31P-NMR magnetization transfer. In inactive sperm, no CK-mediated exchange flux was detected, whereas in activated motile sperm, the forward pseudo-first-order rate constant was 0.13±0.04 s-1 at 10 °C, corresponding to a steady-state CK flux of 3.1±0.5 mM·s-1. Intracellular pH shifted from 6.6±0.1 to 7.6±0.1 upon activation. The phosphocreatine (PCr)/ATP and PCr/Pi ratios were only marginally reduced in activated sperm, whereas the estimated cytosolic free ADP concentration increased remarkably from 9 µM in quiescent, to 114 µM in activated spermatozoa. The elevation of CK flux upon sperm activation is discussed in the light of the proposition that in sea-urchin spermatozoa, which are fuelled entirely by oxidative phosphorylation, high-energy phosphate transport is mediated by a 'CK/PCr shuttle'.