The ribonuclease inhibitors from porcine thyroid and liver are slow, tight-binding inhibitors of bovine pancreatic ribonuclease A

Ribonuclease inhibitors were purified from the latent ribonuclease fractions of porcine thyroid and liver and used to test the hypothesis that their inhibition of bovine pancreatic ribonuclease A is correctly described by tight-binding rather than Michaelis-Menton kinetics. Both proteins were found to act as slow, tight-binding inhibitors of the enzyme. These steady-state velocities also showed that both the thyroid and liver inhibitors were competitive inhibitors of bovine pancreatic ribonuclease A with Ki's of 0.1 and 0.4 nM, respectively. In contrast to interpretations based on Michaelis-Menton assumptions that show non-competitive inhibition, these results suggest that an enzyme:inhibitor:substrate complex does not exist.