l-Cysteine determination in pharmaceutical formulations using a biosensor based on laccase from Aspergillus oryzae

A new graphite electrode modified with laccase from Aspergillus oryzae was developed for the determination of l-cysteine in pharmaceutical formulations. The biosensor was characterized with respect to phenolic compounds (caffeic acid, catechol, dopamine, hydroquinone, l-DOPA and methyl- l-DOPA), inhibition effects (ascorbic acid, benzoic acid, l-cysteine, sodium sulfite, thiourea and metal ions: Ag +, Zn 2+, Hg 2+), pH dependence, linear range, sensitivity and Michaelis–Menten constant. Measurements were performed in the presence of hydroquinone and l-cysteine in 0.1 mol L −1 phosphate buffer (pH 7.0) at an applied potential of −0.08 V versus Ag/AgCl. The recovery of l-cysteine from three samples ranged from 89.7 to 103.4%, the relative standard deviation was less than 1.0% for a solution containing 4.95 × 10 −4 mol L −1 hydroquinone ( n = 8) and the lifetime of the biosensor was 9 months (at least 950 determinations). Results obtained for the determination of l-cysteine in pharmaceutical formulations using the proposed biosensor and those obtained by a standard method are in agreement at the 95% confidence level.