Voltammetric investigation of macrolides by an HPLC-coulometric assay.

Voltammograms of macrolides, including anhydroerythromycin A, azithromycin, erythromycin A, erythromycin A enol ether, pseudoerythromycin A enol ether, oleandomycin and tylosin have been investigated using a dual electrode cell in combination with a high-throughput LC method. The half-wave potentials (E(1/2)) of the seven macrolides investigated ranged from 0.734 to 0.866 V, and the current responses reached the maxima at over 1.0 V. The current response of the downstream electrode displayed a non-linear behavior at high potentials over +0.75 V, probably because of polarization of solvent components, e.g., water. The HPLC-coulometric assay was optimized with the potentials of the upstream and downstream electrodes at +0.65 and +0.85 V, respectively. This method is suitable for detection of 14- and 15-membered macrolides (sensitivity<0.05 microg ml(-1)), but not for a 16-membered macrolide, tylosin (sensitivity>0.1 microg ml(-1)). The assay shows interferences from biomatrices in rat's blood plasma and serum, and human urine, but they were effectively removed by a cold acetonitrile extraction method.

A kinetic study on the degradation of erythromycin A in aqueous solution.

The pH is a critical factor determining the rate of the degradation of erythromycin A in aqueous solutions. However, the kinetics of the acid- and base-catalyzed degradation is still uncertain. This study used a sensitive coulometric detection method to determine concentrations of erythromycin A and its degradation products. To determine the buffer-independent rate constants, sodium acetate (0.05-0.2 M) and Tris-HCl (0.1-0.5 M) were used in a pH range of 3.5-5.5 and 7.0-9.0, respectively. In acidic conditions, anhydroerythromycin A appeared to be produced directly through an internal dehydration of erythromycin A-6,9-hemiketal which simultaneously established an equilibrium with erythromycin A enol ether on the other hand. In weakly alkaline conditions, hydroxide ion appeared to catalyze the hydrolysis of the lactonyl ester bond of erythromycin A-6,9-hemiketal by the pseudo-first-order kinetics, and the C13 --> C11 translactonization and internal dehydration reactions subsequently occurred to form pseudoerythromycin A enol ether. We suggest here a predictive model for reasonable interpretation of the kinetics of erythromycin A degradation in aqueous solutions, in which the observed rate constant was expressed by the sum of the partial reaction rate constants for the acid- and base-catalyzed degradation of erythromycin A-6,9-hemiketal as a function of pH in a range of 3.0-10.0.

Mineralization of erythromycin A in aquaculture sediments.

Mineralization of erythromycin A was studied using two differently (14)C-labeled erythromycins A, which were added to aquaculture sediment samples obtained from the two salmon hatchery sites in Washington state. The added erythromycin A did not significantly alter the numbers of the total viable colonies and erythromycin-resistant bacteria. Erythromycin-resistant Pseudomonas species contained a constitutive erythromycin esterase activity contributing to the inactivation of biologically active erythromycin A in aquatic and sediment environments. The initial rate of mineralization of erythromycin A appeared to be governed by the rate of release of soil-sorbed erythromycin A. After a prolonged lag time, the S-curves of erythromycin A mineralization were observed probably because of the increase in the population density metabolizing it. This study suggests that erythromycin A is partially or completely mineralized by the sediment microbial populations.