Involvement of a macromolecular activating factor in activity of guanylate cyclase partially purified from supernatant of a pig lung extract.

A guanylate cyclase preparation partially purified from supernatant of a pig lung extract was subjected to affinity chromatography on an Agarose-GTP column. The major portion of the cyclase activity was adsorbed on the column and then eluted with 50 mM EDTA and 0.5 M KCl, whereas the fractions non-adsorbed on the column contained a factor which enhanced the cyclase activity. Addition of the activating factor to a cyclase reaction mixture increase the enzyme activity without a time lag, and this enhancement by the factor was dose-dependent. With concomitant presence of cyclase and the factor in the reaction mixture the apparent Km value for GTP-Mn2+ of the enzyme was 56 microM, this value being the same as in absence of the factor, however, here the maximum velocity increased 4-fold. The factor was nondiffusable, heat-labile, partially sensitive to trypsin, and resistant to acid or alkali. As estimated by gel filtration, this factor had an apparent molecular weight of 85 000.