Sparsomycin and its analogues: a new approach for evaluating their potency as inhibitors of peptide bond formation.

The ability of several sparsomycin analogues to inhibit peptide bond formation was studied in vitro. Peptide bonds are formed between puromycin (S) and the acetylPhe-tRNA of acetylPhe-tRNA/70 S ribosome/poly(U) complex (complex C), according to the puromycin reaction: [formula: see text] It was shown that the sparsomycin analogues, like sparsomycin itself, inhibit peptide bond formation in a time-dependent manner; they react with complex C according to the equation [formula: see text] where CI is a conformationally altered species in which I is bound more tightly than in CI. The determination of the rate constant k(7) for the regeneration of complex C from the CI complex allows evaluation of these analogues as inhibitors of peptide bond formation. According to their k7 values, these analogues are classified in order of descending potency as follows: n-pentyl-sparsomycin (4) > n-butyl-sparsomycin (3) approximately n-butyl-deshydroxy-sparsomycin (6) > benzyl-sparsomycin (2) > deshydroxy-sparsomycin (5) approximately sparsomycin (1) > n-propyl-desthio-deshydroxy-sparsomycin (7). The analogues with an aromatic or a larger hydrophobic side chain are stronger inhibitors of the puromycin reaction than are those with a smaller side chain or those lacking the bivalent sulfur atoms; replacement of the hydroxymethyl group with a methyl group does not affect the position of the compound in this ranking; compare the positions of compounds 1 and 3 with those of 5 and 6. In the case of compound 7, CI adsorbed on cellulose nitrate disks was not sufficiently stable to allow examination by the method applied to the other analogues, probably due to a relatively large value of k7. This analogue showed also time-dependent inhibition, but after the isomerization of CI to CI, the kinetics of inhibition become complex, and CI interacted further with puromycin, either as CI or after its dissociation to C*.