Interaction of antibiotic lasalocid A (X537A) with praseodymium(III) in methanol

The binding of lasalocid A (X537A) to Pr(III) in methanol has been studied by lasalocid fluorescence, circular dichroism, and 1H and 13C NMR spectroscopy. It is clear that in addition to a mono complex, bis and tris complexes are also formed. Values of the binding constants and spectral properties of the various complexes have been determined by computer fitting of the binding isotherms. The Pr (III) ion binds only at the salicylic "head" of the lasalocid A, in stark contrast with other known structures. The lasalocid A appears to have an "open" conformation in these complexes. The relevance of these results to the structure and conformation of the Ca(II) complex is discussed. The first order rate constant for the dissociation of the tris complex has also been determined.

Spectrofluorometric determination of the antibiotic lasalocid in blood

A spectrofluorometric assay was developed for the determination of the antibiotic lasalocid in dog blood, based on the intrinsic fluorescence of the compound in ethyl acetate. The assay can measure "total" levels of drug and any metabolites present. The specificity of the assay was verified by TLC separation of the dog blood extract, which indicated the presence of only intact drug. The overall recovery (+/- SD) of lasalocid was 62.0 +/- 3.6% in the concentration range of 1.0-10 mug of compound/ml of dog blood. The sensitivity of the assay is 0.5 mug/ml. The assay was applied to the determination of blood levels of lasalocid in the dog following the intravenous administration of a 5-mg/kg dose.