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Fusidic acid inhibits polypeptide chain elongation by binding to the ribosome - elongation factor-G - GDP complex and thereby preventing its dissociation. The experiments reported here quantitate the interaction of the antibiotic [3H]-24,25-dihydrofusidic acid, an active analog of fusidic acid, with the ribosome - elongation factor-G - GDP comples. All components of the complex are essential for [3H]-24,25-dihydrofusidic acid binding. The stoichiometry of the interaction is ca. 1:1, and the Ka apparent, as determined by equilibrium dialysis, is 2.6 times 10-6 M-minus 1. It is further shown that GTP and GDP are equally effective in forming complexes to which the antibiotic may bind, whereas GMP and beta,gamma-methyleneguanosine triphosphate will not form complexes to which the antibiotic may bind. In order to examine the structural basis of the mode of antibiotic action shown by fusidic acid, we have considered two activities of 21 structural analogs of this antibiotic: ability to bind to the aforementioned ternary complex and ability to stabilize this complex. The comparative binding capability of the analogs were extablished through competition experiments with [3H]-24,25-dihydrofusidic acid. The data obtained from these experiments can be summarized as follows. (1) The C17-20 double bond of fusidic acid appears to be critical for both binding and complex stabilization activities. (2) A carboxyl group in the vicinity of the C20 carbon is also essential for both activities. (3) Modifications of other functional groups in the molecule can lead to significantly decreased stabilization of the ternary ribosome complex and/or ability to compete with [3H]-24,25-dihydrofusidic acid for binding to the complex, but do not demonstrate absolute structural requirements for either activity.

The preparation of a series of acyloxymethyl esters of D-a-ammobenzylpenicillin (ampicillin) is described. In vitro these esters are hydrolyzed to ampicillin by nonspecific esterases present in serum and various tissues from rat, dog, and man.Experiments performed with the hydrochloride of the pivaloyloxymethyl ester in laboratory animals and healthy volunteers show that this compound, after oral administration, is absorbed far more efficiently than ampicillin. After the absorption the compound is rapidly hydrolyzed to ampicillin with the result that high blood and tissue levels of the latter are attained.

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