Effects of metals and nucleotides on the inactivation of sea urchin sperm guanylate cyclase by heat and N-ethylmaleimide.

Preincubation of sea urchin sperm guanylate cyclase at 35, 37, 40, or 43 degrees resultedin inactivation. Various metals were able to protect guanylate cyclase against heat inactivation. Estimated binary enzyme-metal dissociation constants for Mn2+, Fe2+, La3+, Ca2+, Ba2+, Mg2+, Co2+, and Ni2+ were 123, 361, 5.5, 692, 984, 335, 79, and 47 muM, respectively. Extrapolated rates of enzyme denaturation in the presence of saturating concentrations of metal divided by the rates of enzyme denaturation in the absence of metal gave values of 0.13, 0.08, minus 0.1, 0.30, 0.59, 0.66, 0.28, and 0.42 for Mn2+, Fe2+, La3+, Ca2+, Ba2+, Mg2+, Co2+, and Ni2+, respectively. GTP, MgGTP, and SrGTP protected the enzyme only slightly against heat inactivation, but CaGTP and MnGTP protected substantially. Neither CaGTP nor MnGTP protected maximally, however, unless the metal concentration exceeded that of GTP. At fixed free Mn2+ or free Ca2+ concentrations, protection curves as a function of MnGTP or CaGTP appeared to be sigmoidal, suggesting multiple nucleotide binding sites. MnATP also protected against heat, but CaATP was virtually ineffective. Sea urchin sperm guanylate cyclase was inactivated by N-ethylmaleimide; CaGTP and MnATP were effective protectants with estimated binary enzyme-Me2+ nucleoside triphosphate dissociation constants of 40 and 170 muM, respectively. MnGTP protected only slightly or not at all against N-ethylmaleimide. These results suggest that: (a) sea urchin sperm guanylate cyclase binds free metal, (b) the binding of free metal is required for protection by nucleotides, and (c) the enzyme contains multiple nucleotide binding sites.