Development of a spectrophotometric assay for cyclase activity

We describe the development of a rapid colorimetric assay for soluble guanylate cyclase (sGC) activity adapted for a 96-well microplate. The assay greatly decreases the analysis time and cost over traditional methodologies based on radio- and immunoassays and high-performance liquid chromatography (HPLC) separations. The method does not demonstrate any significant interference with chemicals commonly used for sGC purification and reaction kinetics. The assay converts the inorganic pyrophosphate produced in the cyclase reaction to inorganic phosphate, which is then measured using a modified Fiske–Subbarow assay. We used the assay to compare the reaction kinetics of preparations of sGC from a commercial source with those from our lab with Mg2+-guanosine 5′-triphosphate (GTP) or Mn2+-GTP as a substrate. The commercial preparation was found to have a specific activity of around 1.5 μmol/min/mg, which is significantly lower than expected, as was the fold-activation upon addition of nitric oxide (NO). Our laboratory preparation had a higher specific activity that was consistent with results from HPLC assays. We determined that the human isoform of sGC is more active in the basal and NO forms with Mn2-GTP as a substrate than Mg2+-GTP, a feature more similar to rat lung sGC than the more commonly studied bovine lung.