Both soluble and particulate forms of human platelet guanylate cyclase were found to be sensitive to sub-micromolar concentrations of free Ca2+; soluble enzyme activity increased as Ca2+ was increased from 10 nM to 1 microM; particulate enzyme activity showed a biphasic response to Ca2+, with maximal enzyme activity between 1 and 10 nM free Ca2+ and inhibition occurring at higher Ca2+ concentrations. Neither Ca2+-sensitivity appeared to be calmodulin-dependent.
