Evaluation of a method for enzymic radiochemical assay of tobramycin and amikacin in serum.

This enzymic radiochemical procedure for measuring tobramycin and amikacin in serum is based on the transfer of the 14C-acetyl group from [14C]acetyl-coenzyme A to the 6' nitrogen atom of the drug by the enzyme kanamycin 6'-acetyltransferase (EC 2.3.1.55). The transfer, stoichiometric and quantitative, is complete after 10-min incubation at 37 degrees C. The labeled acetylaminoglycoside is adsorbed onto phosphocellulose paper discs, which are washed to removed any unreacted [14C]acetyl-coenzyme A. The radioactivity is then eluted into liquid scintillation counting vials and counted for 1 min each. The assay detects as little as 2 ng of either drug and the standard curve is linear into the toxic range of concentrations. Most of the commonly administered aminoglycosides act as substrates in the assay, except for the C1 component of gentamicin C complex. Neither hemolysis, lipemia, nor icterus interfere with the assay. Results compare favorably with those determined by radioimmunoassay and a microbiological method.