A coupled enzymatic assay for salicylate and acetylsalicylate using salicylate hydroxylase and tyrosinase

Salicylic acid determination in cow urine and drugs using a bienzymatic sensor

An enzymatic biosensor was developed for salicylic acid (salicylate ion) determined using a Clark type gas diffusion electrode and two enzymes (tyrosinase and salicylate hydroxylase) entrapped in a cellulose triacetate membrane. After optimization, the method was applied to the determination of salicylic acid in cow urine. Relatively good recoveries were achieved, between about 83% and 109%, using the calibration curve, and acceptable precision (R.S.D. about 8%). The method is now being tested for the determination of salicylic acid contained in commercially available drug specialities or galenic products. So far agreement with nominal values has been found to be between 75% and 110% with a R.S.D. of less than 8%.

Potentiometric determination of acetylsalicylic acid by sequential injection analysis (SIA) using a tubular salicylate-selective electrode

This paper deals with the development of an automated procedure for formulation assays and dissolution tests based on a sequential injection analysis (SIA) system involving an ion-selective electrode as sensing device. Construction of a tubular salicylate (Sal) selective electrode suitable for potentiometric determination of acetylsalicylic acid (Asa) in pharmaceutical formulations is described. The flow-through electrode is formed by a PVC membrane containing 29.2% (w/w) PVC, 5.8% (w/w) tetraoctylammonium salicylate (ionic sensor), 58.5% o-nitrophenyloctylether (plasticizer) and 6.5% (w/w) p-tert-octylphenol (stabilising additive which increases electrode selectivity). The calibration range is 0.05--10 mM Sal, the limit of detection (LOD) is 0.05 mM Sal, the slope is 56.0 mV per decade at 22 degrees C. The R.S.D. is 0.20% (15 readings) when determining 2.5 mM Sal in standard solution. The electrode is used for sensing Asa after its on-line chemical hydrolysis to Sal in a SIA system. The sampling rate is 6 h(-1) but for the dissolution tests the frequency is increased to 20 h(-1). The SIA set-up is employed for the assay of Asa in plain tablets, composed tablets and effervescent tablets and for performing dissolution tests of normal and sustained release tablets. Results obtained by this technique compare well with those required by the US Pharmacopoeia XXIV.

A new enzyme electrode for quantification of salicylic acid in a FIA system

This work presents an amperometric biosensor incorporated into a flow configuration comprising salicylate hydroxylase that catalyses the irreversible hydroxylation of salicylate to catechol in the presence of NADH and molecular oxygen, and tyrosinase that further oxidises catechol giving o-quinone which is electrochemically reduced at -100 mV vs. Ag/AgCl yielding catechol and entering the catalytic oxidation and electrochemical reduction cycling which results in signal amplification and, consequently, low limits of detection. Additional incorporation of glucose dehydrogenase in the enzymatic sequence results in regeneration of NADH provided that glucose is present in the carrier stream and incorporation of a dialysis membrane provides operational stability to the biosensor. The analytical characteristics of this catalytic and electrochemical transduction sequence in a FIA system are: a limit of detection of 3.5 10(-6) M (S/N = 3), a sensitivity of 22.6 nA microM(-1) cm(-2), no loss of response at least after 5 h of continuous operation, and a sample frequency of 15 h(-1). Monitoring of salicylate after ingestion of 500 mg of acetylsalicylic acid has been followed in non-pretreated urine samples and the amount of salicylate in several drugs has been also successfully quantified.

Determination of L-phenylalanine based on an NADH-detecting biosensor

An enzyme carbon paste electrode containing three different enzymes was developed for the determination of L-phenylalanine. This sensor is based on the enzymatic/electrochemical recycling of tyrosinase in combination with salicylate hydroxylase and L-phenylalanine dehydrogenase (PADH). The enzymes salicylate hydroxylase and tyrosinase were coimmobilized first in a carbon paste electrode for the sensitive detection of NADH. The principle of the bienzyme scheme is as follows: the first enzyme, salicylate hydroxylase, converts salicylate to catechol in the presence of oxygen and NADH. The second enzyme, tyrosinase, then oxidizes the catechol to o-quinone, which is electrochemically detected and reduced back to catechol at the electrode at an Eappl = -50 mV vs Ag/AgCl. This results in an amplified signal due to the recycling of the catechol and o-quinone between tyrosinase and the surface of the electrode. Prior to adding PADH, the salicylate hydroxylase-tyrosinase carbon paste electrode was characterized in terms of its sensitivity to NADH, pH dependence, buffer composition, interferences, and stability. Interference from ascorbic acid and uric acid was found to be minimal. Human serum was used to investigate whether this bienzyme system was suitable for the detection of NADH in serum and blood samples. The sensitivity for NADH was increased by a factor of 33 times using the bienzyme amplification scheme (electroreduction of o-quinone at Eappl = -50 mV) as opposed to the salicylate hydroxylase single-enzyme system (at which catechol would have been oxidized at Eappl = +150 mV vs Ag/AgCl). The detection limit for NADH achieved by the bienzyme carbon paste electrode was 1 vs 100 microM for the single-enzyme carbon paste electrode. The salicylate hydroxylase-tyrosinase system was then coupled with phenylalanine dehydrogenase for L-phenylalanine determination. This multienzyme sensor was able to achieve a linear range of 20-150 microM and a detection limit of 5 microM for L-phenylalanine. The sensitivity is sufficient since the reference clinical range for L-phenylalanine is 78-206 microM.