Mechanism of action of sparsomycin in protein synthesis.

Before CI isomerizes to CI, we detect a competitive phase of inhibition (Ki = k5/k4 = 0.05 microM) which eventually, by increasing the concentration of I, becomes linear mixed noncompetitive and involves CI in place of CI. The equilibration of C and I according to reaction 2 is much slower than the equilibration between C and S in reaction 1 (time-dependent inhibition). The inactivation plots obey reaction 2 and allow us to estimate k6 as equal to 2.2 min-1. The isomerized CI, free of excess I, can be studied as a mixture with complex C. From the kinetics of the regeneration of C from CI, in the presence of puromycin, we can estimate k7 to be between 0.22 min-1 and 0.06 min-1. Although the isomerized CI survives after adsorption on cellulose nitrate filter disks, it does not survive after gel chromatography on a Sepharose CL-4B column but is converted quantitatively to complex C containing D of unchanged reactivity. This result does not support the proposed [Flynn, G. A., & Ash, R. J., (1990) Biochem. Biophys. Res. Commun. 166, 673-680] chemical reaction between D and I toward new products. The isomerized CI can be obtained not only from the already-made complex C but also de novo from D, R, and M. In the latter case, the reactions which lead to C are represented by the following hypothetical scheme: D + R + M in equilibrium with DRM or C (binding reaction). When CI is formed de novo, this reaction is coupled to reaction 2 and the ultimate product is a mixture of C and CI.(ABSTRACT TRUNCATED AT 250 WORDS)