Interaction of papain with derivatives of phenylalanylglycinal: fluorescence studies.

Fluorescence studies have been performed on the interaction of papain with active-site-directed inhibitors of the type mansyl-(Gly)n-Phe-glycinal, where n = 0, 1, 2. It has been found that whereas the mansyl [6-(N-methylantilino)-2-naphthalene sulfonyl] fluorescence of mansyl-Phe-glycinal is greatly enhanced, that of the two longer mansyl compounds is not, although all three are equally effective as inhibitors of papain action. Measurements of fluorescence polarization and rotational relaxation time support the conclusion that the fluorescent probe group of the two longer mansyl compounds protrudes into the solvent to a greater degree than that of mansyl-Phe-glycinal. Considerable energy transfer from papain tryptophan to the mansyl group is evident for all three inhibitors, however, although it is most marked with mansyl-Phe-glycinal. Stopped-flow fluorescence measurements have shown that, after initial rapid interaction, the first-order conformational changes in the active-site region of papain in the complex with mansyl-Phe-glycinal are approximately 1/10(4) those observed with comparable mansyl oligopeptide substrates, and approximately 1/10(2) those with acetyl-Phe-glycinal.