Inhibitor Report for: NBD-AP-MPF

The inhibition rates and spectral characteristics of 2 probes specific for the active-site serine residue of proteases were examined for evidence of conformational change of the proteases upon their binding to alpha 2-macroglobulin (alpha 2M). Elastase, chymotrypsin, trypsin, and plasmin were reacted with (7-nitrobenz-2-oxa-1,3-diazole) aminoethyl- and aminopentyl methylphosphonofluoridate. The inhibition rate constants depend on the chain length of the aminoalkyl moiety of the probe and range from 10(5) to 10(4) M-1 min-1 for elastase and chymotrypsin. They are significantly modified when the proteases are stoichiometrically bound to alpha 2M. The absorption maximum of the chromophore appears in the range of 460-470 nm and 475-480 nm for the aminoethyl- and aminopentyl- conjugates, respectively. The fluorescence emission is maximal around 530 nm with a low quantum yield of about 3%. These spectral characteristics are altered in different ways by the covalent or non-covalent binding mode of the protease to alpha 2M. Finally, the CD spectrum of the NBD aminoethyl and aminopentyl elastase and chymotrypsin conjugates exhibits intense optical activity in the absorbing band of the NBD-moiety. These chiral properties are greatly altered upon binding of the protease to alpha 2M. All these results strongly suggest a conformational change in the protease at its active center upon its binding to alpha 2M; this conformational change could be taken into account to explain the alteration of the catalytic properties of the alpha 2M-bound proteases.

The synthesis, kinetic, and spectral characterization of (7-nitrobenz-2-oxa-1,3-diazole)aminoethyl and (7-nitrobenz-2-oxa-1,3-diazole)aminopentyl methylphosphonofluoridate are described. These homologous organophosphorous agents contain the environmentally sensitive 7-nitrobenz-2-oxa-1,3-diazole chromophore. They inhibit acetylcholinesterase from Torpedo at rates exceeding 10(7) M-1 min-1 to form long-lived conjugates with one chromophore/80-kilodalton subunit. The intensity, position, and line width of the absorption spectra of the conjugates and reactivation kinetics in the presence and absence of the bisquaternary oxime 1,1'-trimethylene-bis(4-formylpyridinium bromide) dioxime indicate that these agents form conjugates in which the NBD-aminoalkyl moieties experience distinctive microscopic environments within the active center. NBD-aminoethyl methylphosphono-acetylcholinesterase undergoes oxime-induced as well as spontaneous reactivation at rates that are 3.6 and 35 times faster, respectively, than the corresponding rates measured for the NBD-aminopentyl conjugate. Hence, reactivation exhibits a marked dependence on structure of the methylphosphonate. Fluorescence emission at wavelengths greater than 520 nm is highly quenched and exhibits quantum efficiencies of less than 5%. Absorption maxima for the covalent NBD-aminoethyl methylphosphono-acetylcholinesterase appear at 475-480 nm while those for the corresponding NBD-aminopentyl methylphosphono-acetylcholinesterase appear at 485-490 nm. Bandwidths of the absorption maxima are substantially broader for the acetylcholinesterase adduct with NBD-aminoethyl methylphosphonofluoridate (3870 cm-1) than for the enzyme adduct with NBD-aminopentyl methylphosphonofluoridate (2870 cm-1). The CD spectrum of NBD-aminopentyl methylphosphono-acetylcholinesterase shows optical activity coincident with the shape and position of the absorption spectrum. In contrast, in addition to optically active transitions at the absorption maxima, the CD spectrum of NBD-aminoethyl methylphosphono-acetylcholinesterase shows intense optical activity at 430 nm, a wavelength region coincident with the region of spectral broadening. The spectral properties of alpha-chymotrypsin conjugates formed by reaction with the two probes are different, and the respective spectra differ also from those observed for the acetylcholinesterase conjugates. These results indicate that there is a reciprocal relationship between the structure of the probe and the structure of the active center.