AMINOACYL ANALOGS OF CHLORAMPHENICOL - EXAMINATION OF THE KINETICS OFINHIBITION OF PEPTIDE-BOND FORMATION

Two aminoacyl analogs and one peptidyl analog of chloramphenicol (1, C l2CHCO-CA) were prepared. These are 2 (L-Phe-CA), 3 (Gly-CA), and 4 (L -Phe-Gly-CA). The kinetics of inhibition of peptide bond formation by these analogs were examined in a cell-free system which was derived fr om E. coli and used previously for the study of 1 (Drainas; et al. Eur . J. Biochem. 1987, 164, 53-58). In the absence of inhibitor, the reac tion follows first-order kinetics for the entire course of the reactio n. In the presence of the analog the reaction gives biphasic log-time plots. The kinetic information pertaining to the initial slope of the plot is analyzed (initial-slope analysis). This information differenti ates the analogs from the parent compound 1. The parent compound 1 giv es complex inhibition kinetics; increasing the concentration of 1 chan ges the inhibition from competitive to mixed noncompetitive (Drainas; et al. Eur. J. Biochem. 1987,164,53-58). In contrast, the analogs give competitive kinetics even at high concentrations of the inhibitor. Th e following K(i) values have been determined: 18.0 muM for 2, 5.5 muM for 3, 1.5 muM for 4. If we were to assume that compounds 2, 3, and 4 behave as classical competitive inhibitors, we could say that 4 is 12 times more potent than 3 and 4 times more potent than 2. On this assum ption we could also compare 1 with 4 and see that 4 is 2 times weaker than 1. It is suggested that as compared with 1, the two aminoacyl ana logs and the dipeptidyl analog have increased structural similarity to the 3'-terminus of aminoacyl-tRNA or of peptidyl-tRNA and that this s imilarity results in a more pronounced competitive inhibition.