Short-lived complexes between myelin basic protein peptides and IAk.

Kinetic rate constants and the equilibrium dissociation constant have been determined for the reaction between an affinity-purified class II major histocompatibility complex molecule IAk and a myelin basic protein analogue peptide, fluorescein-labeled Ac(1-14)A4C15. Under the experimental conditions used, the lifetime of the peptide-free IAk molecule with respect to inactivation is 3.1 hr. The equilibrium dissociation constant, 3.3 +/- 1.7 microM, is determined from measurements of the kinetics of peptide inhibition of IAk inactivation. The measured peptide dissociation halftime is relatively short, 30 min, and the deduced association rate is 100 M-1.s-1. The rate constants and the equilibrium constant are similar to those characteristic of kinetic intermediates in reactions of peptides and class II proteins that lead to long-lived terminal complexes.