The Human Blood Enzymes (activated Coagulation Factor Xiii) and the Guinea Pig Hair Follicle Enzyme

The transfer reaction of [W!]methylamine into the acetylated B chain of oxidized insulin as catalyzed by several transglutaminases has been studied at pH ‘7.5 in the presence of calcium ion. The only radioactive product of the transfer reaction with each transglutaminase was identified as peptide-bound y-glutamic acid methylamide. Initial velocity and product inhibition patterns for the human plasma and guinea pig hair follicle transglutaminases are consistent with a ping-pong mechanism, previously proposed for guinea pig liver transglutaminase (FOLK, J. E. (1969) J. BioZ. Chem. 244, 3707) in which acyl enzyme formed from peptide-bound glutamine may react with water (hydrolysis) or with a primary amine (transfer). Hydrolysis and isotope exchange in the presence of one substrate and one product add substantial support for this mechanism. The pattern of calcium activation of plasma transglutaminase in the transfer reaction is in accord with an equilibrium-ordered activation mechanism in which the first substrate, acetylated B chain of oxidized insulin, adds only to enzyme-metal complex. The similarity in the kinetic constants obtained for human plasma and platelet transglutaminases adds support to recent evidence that the catalytic subunits of the plasma and platelet zymogens and enzymes are closely related (SCHWARTZ, M. L., hzzo, S. V., HILL, R. L., AND MCKEE, P. A. (1971) J. Biol. Chem. 246, 5851). Improved purification procedures for plasma and platelet protransglutaminases are presented.