Sensitive, hydrosoluble, macromolecular fluorogenic substrates for human immunodeficiency virus 1 proteinase

Human ras-converting enzyme (hRCE1) endoproteolytic activity on K-ras-derived peptides

A human gene responsible for one of the steps in Ras post-translational modification and membrane localization, hRCE1, encodes a 35-kDa membrane-associated endoprotease. We examined hRCE1 activity using farnesylated 9 aa peptides with the core sequence, KSKTKC(farnesyl)VIM [(farnesyl) = (f)], from the C-terminus of K-Ras. We first demonstrated hRCE1 specificity in cleavage location and endoproteolysis. We then describe a direct fluorescent microtiter plate assay. We demonstrated that hRCE1 protease cleaved KSKTKC(f)VIM peptides between the C(f) and V positions, generating KSKTKC(f) and the corresponding tripeptides as products. We found that the sequence KSKTKC(f)VI was a better substrate for hRCE1 than KSKTKC(f)VIM. We also found that hRCE1 cleaved modified versions of KSKTKC(f)VIM that incorporated either MCA or ABZ fluorescent chromophores at the N-terminus, and quenching-group-containing amino acids at the V or M, but not the I, amino acid positions of VIM. The quenching-group-containing amino acids used were either Q(S) (dinitrophenyldiaminopropionic acid) or Q(L) (lysine epsilon-dinitrophenyl). Cleavage of KSKTKC(f)VIM and modified versions of this peptide by hRCE1 was initially evaluated by HPLC product resolution and quantitation. The hRCE1 cleavage of quenched peptides enabled us to directly monitor proteolytic activity in a 96-well microtiter fluorescent plate assay. The microtiter format assay was validated by its sensitivity to RPI, an inhibitor of prenyl protein protease. A direct fluorescent assay provides an effective tool for further characterization of this enzyme and also for detection of novel inhibitors.

Use of a phosphotyrosine-antibody pair as a general detection method in homogeneous time-resolved fluorescence: application to human immunodeficiency viral protease

A homogeneous time-resolved fluorescence (HTRF) assay has been developed for human immunodeficiency viral (HIV) protease. The assay utilizes a peptide substrate, differentially labeled on either side of the scissile bond, to bring two detection components, streptavidin-cross-linked XL665 (SA/XL665) and a europium cryptate (Eu(K))-labeled antiphosphotyrosine antibody, into proximity allowing fluorescence resonance energy transfer (FRET) to occur. Cleavage of the doubly labeled substrate by HIV protease precludes complex formation, thereby decreasing FRET, and allowing enzyme activity to be measured. Potential substrates were evaluated by HTRF with the best results being obtained using (LCB)K4AVSQNbeta-NapPIVpYA(NH2) and Eu(K)-pY20 where the peptide titrated with an EC50 of 7.7 +/- 0.3 nM under optimized detection conditions. Using these HTRF detection conditions, HIV protease cleaved the substrate in 50 mM NaOAc, 150 mM KF, 0.05% Tween 20, pH 5.5, with apparent first-order kinetics with a Km of 37.8 +/- 8.7 microM and a kcat of 0.95 +/- 0.07 s-1. Examination of the first-order rate constant versus enzyme concentration suggested a Kd of 9.4 +/- 2.7 nM for the HIV protease monomer-dimer equilibrium. The HTRF assay was also utilized to measure the inhibition of the enzyme by two known inhibitors.

Conformational selectivity of HIV-1 protease cleavage of X-Pro peptide bonds and its implications

Kinetic measurements on a fluorescent peptide analog of the p17/p24 cleavage site of the Gag polyprotein demonstrate the conformational selectivity of human immunodeficiency virus, type 1 protease for the trans conformation of the Tyr-Pro bond. A mean cis/trans ratio of 0. 3, and a cis --> trans isomerization rate constant of 0.022 s-1 are determined at T = 22 degrees C. This rate is in excellent agreement with that predicted by 19F NMR studies of structurally analogous peptides containing a fluorine/hydroxyl substitution on the tyrosyl residue. Addition of recombinant human cyclophilin resulted in a significant enhancement of this rate, and it is proposed that this enzyme, which has been shown to be associated with the Gag protein, functions as an auxiliary enzyme for the protease during cleavage in the virion.