THE MODULATION OF CA2-RETICULUM ATPASE BY ATP ANALOGS IS PH-DEPENDENT( BINDING TO SARCOPLASMIC)

Excess ATP is known to enhance Ca2+-ATPase activity and, among other e ffects, to accelerate the Ca2+ binding reaction, In previous work, we studied the pH dependence of this reaction and proposed a 3H(+)/2Ca(2) exchange at the transport sites, in agreement with the H+/Ca2+ count er transport, Here we studied the effect of ADP and nonhydrolyzable AT P analogues on the Ca2+ binding reaction at various pH values. At pH 6 , where Ca2+ binding is monophasic and slow, ADP, adenosine 5'-(beta,g amma-methylene)triphosphate (AMP-PCP), or adenyl-5'-yl imidodiphosphat e (AMPPNP) increased the Ca2+ binding rate constant 20-fold, At pH 7 a nd 8, where Ca2+ binding is biphasic, the nucleotides induce fast and monophasic Ca2+ binding. At pH 7, AMP-PCP accelerated Ca2+ binding wit h an apparent dissociation constant of 10 mu M. At acidic pH, ADP, AMP PCP, or AMPPNP increased the equilibrium affinity of Ca2+ for ATPase, whereas at alkaline pH, these nucleotides had no effect, At pH 5.5, AM PPCP increased equilibrium Ca2+ binding with an apparent dissociation constant of 1 mu M.