Determination of lysine in pharmaceutical samples containing endogenous ammonium ions by using a lysine oxidase biosensor based on an all-solid-statepotentiometric ammonium electrode

A new potentiometric method is proposed to determine lysine in pharmaceutic al samples. This method is based on a lysine biosensor consisting of a chem ically immobilized lysine oxidase membrane attached to an all-solid-state a mmonium electrode. Lysine is degraded in the sensor to release ammonium, wh ich is detected by means of the ammonium electrode. The presence of endogen ous ammonium in the samples interferes with these determinations, since the response measured corresponds to the sum of the ammonium generated enzymat ically and that present in the sample. This is a general drawback for all b iosensors based on the detection of ammonium. Study of samples containing b oth lysine and ammonium showed that concentration ranges exist in which a n ear-logarithmic relationship between potentials measured and lysine concent rations is found. Therefore, within these ranges, lysine can be determined by using the standard addition method, with the subsequent data treatment i nvolving an iterative linearization procedure. Results obtained with the pr oposed potentiometric method are consistent with those given by the standar d method for amino acid analysis. (C) 1999 Elsevier Science S.A. All rights reserved.