Abstract
In a cell-free system derived from Escherichia coli,lincomycin produces biphasic logarithmic time plots for inhibition of peptide-bond formation when puromycin is used as an acceptor substrate and AcPhe-tRNA as a donor substrate. In a previous study, initial slope analysis of the logarithmic time plots revealed that the encounter complex CI between the initiator ribosomal complex (C) and lincomycin (I) undergoes a slow isomerization to C*I. During this change, the bound AcPhe-tRNA and lincomycin are rearranged to also accommodate puromycin, and this may account for the mixed noncompetitive inhibition (K i* = 70 μM) established at higher concentrations of the drug. The above-mentioned effect was further investigated by analyzing the late phase of the logarithmic time plots. It was found that C*I complex reacts with a second molecule of I, giving C*I2 complex. However, the logarithmic time plots remain biphasic even at high concentrations of lincomycin, making possible the identification of another inhibition constantK i*′, which is equal to 18 μM. The simplest explanation of this finding is to assume the existence of a second isomerization step C*I2 ⇌ C*I2 ′, slowly equilibrated. The determination of K i*′ enables us to calculate the isomerization constant (K isom = 2.9) with the formula K i*′ =K i*/(1 + K isom). Our results suggest that whenever a fast and reversible interaction of lincomycin with the elongating ribosomal complex C occurs, the latter undergoes a slow isomerization, which may be the result of conformational changes induced by the drug.
Footnotes
- Received April 14, 1999.
- Accepted August 2, 1999.
-
Send reprint requests to: Dr. Dimitrios L. Kalpaxis, Laboratory of Biochemistry, School of Medicine, University of Patras, GR-26500 Patras, Greece. E-mail: Dimkal{at}med.upatras.gr
-
Dedicated to the memory of Professor C. Coutsogeorgopoulos, who established research on protein synthesis in our laboratory.
- The American Society for Pharmacology and Experimental Therapeutics
MolPharm articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|
Log in using your username and password
Pay Per Article - You may access this article (from the computer you are currently using) for 1 day for US$35.00
Regain Access - You can regain access to a recent Pay per Article purchase if your access period has not yet expired.
In this issue
Slow Sequential Conformational Changes in Escherichia coli Ribosomes Induced by Lincomycin: Kinetic Evidence
