Fluorimetric determination of ampicillin by use of non-toxic catalysts. Estimation of beta-lactamase activity and parameters.

Abstract

A fluorescence assay based on the use of biological reducing agents as catalysts rather than heavy metal ions has been developed for estimation of ampicillin concentrations. The assay is based on the conversion of ampicillin to its penicilloate, by treatment with sodium hydroxide, then neutralization, dilution with 0.5 M acetate buffer at pH4 and heating for 30 min at 100 degrees C in the presence of ascorbic acid (0.5 mg) and EDTA (50 microM). Reduced glutathione, cysteine and N-acetylcysteine also catalysed the development of fluorescence. A practical sensitivity range of 0.5-50 microM ampicillin was used. The assay was used to estimate ampicillin in some biological solutions to which the antibiotic has been added. Milk, blood serum, trypticase soy broth and nutrient broth containing 25 microM antibiotic assayed at 18.5, 21.7, 16.5 and 14.7 microM, respectively, with standard deviations between 1.2 and 0.7%. The low results were attributed to binding of some ampicillin by proteins or peptides which were removed by pretreatment. Urine containing 5 mM ampicillin assayed at 4.97 mM with a standard deviation of 3%. A modification of the procedure was used to measure beta-lactamase activity against ampicillin in several organisms in a fixed time assay. Kinetic parameters of a commercial beta-lactamase preparation from Bacillus cereus could also be determined by an additional modification. In both instances a correction was required for the intrinsic fluorescence of ampicillin remaining in the solution. The preparation examined had a Michaelis constant (Km) of 0.32 mM, a maximum velocity (Vmax) of 5398 micromol ampicillin hydrolysed mg(-1) min(-1), an apparent catalytic constant (Kcat, turnover number) of 20,220 s(-1) and a Kcat/Km ratio of 0.63 x 10(7) M(-1) s(-1). The major advantage of using this assay technique is that toxic metals are not used in the development of fluorescence so it is more environmentally acceptable. The technique is useful for examining beta-lactamase activity against ampicillin and might be useful for pharmaceutical products-for determining available therapeutic levels and for monitoring the activity of penicillin acylase against the penicilloate of ampicillin.