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Guanylate cyclase was purified from rat liver supernatant. Electrophoresis under denaturing conditions revealed one major peptide of Mr approx. 69 000. On the basis of the Stokes radius (4.7 nm) and S20,w (6.4S), the calculated Mr value of the native enzyme was 133 000, i.e. it is apparently a homodimer. Kinetics of inactivation by diamide (which was reversible with dithiothreitol) suggested that oxidation of a single class of thiol sites was involved. In the absence of other additions, cyclase activity assayed with Mn2+ was over 7 times that assayed with Mg2+; maximal effects were observed with approx. 5 mM of each (with 1 mM-GTP). The purified enzyme was markedly activated by nitrosylhaemoglobin. Relative activation was much greater in assays with Mg2+ than with Mn2+, although maximal activities were similar. When assayed with Mg2+, the enzyme exhibited a single Km (0.35 mM) for GTP; with Mn2+, plots of 1/v versus 1/[S] were non-linear. Activator or nitrosylhaemoglobin increased Vmax, but did not alter Km in the presence of either Mg2+ or Mn2+. The enzyme was inhibited by Na3VO4, Na2WO4 and Na2B4O7. Reduction from VV to VIV abolished the inhibitory effect of vanadate. Na2B4O7 (2 mM) inhibited activity with Mn2+, but not with Mg2+. In assays with Mg2+, but not with Mn2+, FMN, NAD+ and NADH (each 0.5 mM) inhibited activation by protoporphyrin IX and nitrosylhaemoglobin. Rotenone (0.6 mM) inhibited activity with protoporphyrin IX to a greater extent than with nitrosylhaemoglobin. Methylene Blue (1 mM) inhibited activation by nitrosylhaemoglobin, protoporphyrin IX and activator. It appears that this enzyme purified from rat liver lacks haem (and perhaps other components) required for activation by NO, and it should be particularly useful for elucidating the mechanism of action of NO, protoporphyrin IX and other activators.

Regulation of activity of purified guanylate cyclase from liver that is unresponsive to nitric oxide