title: The ribonuclease inhibitors from porcine thyroid and liver are slow, tight-binding inhibitors of bovine pancreatic ribonuclease A.
abstract: 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.
mesh_terms: D000818:Animals; D001667:Binding, Competitive; D002417:Cattle; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D010179:Pancreas / Q000201:enzymology; D011485:Protein Binding; D012260:Ribonucleases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D013045:Species Specificity; D013552:Swine; D013961:Thyroid Gland / Q000201:enzymology*
chemical_list: D012260:Ribonucleases