Resolution of the competitive inhibitory effects of lithium and AMPPNP on the beat frequency of ATP-reactivated, demembranated, sea urchin sperm flagella.

About half of the inhibitory effect on the beat frequency of sea urchin spermatozoa flagella by beta, gamma-imido-adenosine-5'-triphosphate (AMPPNP) previously reported by Okuno & Brokaw (1981) was actually the result of inhibition by lithium resulting from the use of a tetralithium salt of AMPPNP. Li+ (or LiATP3-) acts as a competitive inhibitor of beat frequency, with a Ki for Li+ of about 30 mM. Under improved reactivation conditions which reduce the Km for beat frequency to about 0.1 mM, the Ki for lithium-free MgAMPPNP2- is about 2.3 mM, and magnesium pyrophosphate (MgPPi-), which may be an equivalent ligand, gives a Ki of 3.4 mM.