Partial characterization of guanylyl cyclase activity in calf thyroid.

The aim of the present study was to perform the partial characterization of the enzyme guanylyl cyclase (GC) in bovine thyroid. The results obtained showed the presence of two types of GC: one is soluble and comprises around 79% of total activity, while the other is particuiate. Treatment with 1% Triton X-100 increased both activities.When the kinetics of the enzyme was analyzed, using the complex MnGTP as a substrate, the results showed a Michaelis type kinetics for the soluble enzyme, with a Km of 0.037 mM, whereas the particulate GC showed a positive allosteric behavior with a S0.5 of 0.214 mM and a Hill coefficient of 1.9, indicating that the enzyme has at least two binding sites for the substrate. When the influence of different Mn2+ concentrations was studied, a positive allosteric behavior for the soluble GC was found, with a S0.5 of 1.2 mM and a Hill coefficient of 2.2. The kinetics of the particulate enzyme under similar conditions was of Michaelis type, with a Km of 0.752 mM.Although the enzyme ...