L-cysteine determination using a polyphenol oxidase-based inhibition flow injection procedure

The use of crude extract of sweet potato root (Ipomoea batatas L. Lam.) as a source of polyphenol oxidase (PPO; tyrosi nase; EC 1.14.18.1) for the det ermination of L-cysteine in pharmaceutical products is reported. This enzym e catalyses the oxidation of catechol to o-quinone with a strong absorption at 410 nm. When L-cysteine solution is inserted into the flow injection (F I) system, its inhibitory effect on the PPO activity was proportional to th e L-cysteine concentration. The analytical curve of absorbance versus conce ntration of L-cysteine obtained was A = 0.851 -963.16[ L-cysteine]; r = 0.9 987 in the L-cysteine concentration range from 6.0 x 10(-5) to 8.0 x 10(-4) mol l(-1) with a detection limit of 4.4 x 10(-6) mol l(-1). Recovery of L- cysteine from three samples ranged from 98.8 to 101.4% and the relative sta ndard deviations (Sr) were less than 1.2% for solutions containing 2.0 x 10 (-4) and 4.0 x 10(-4) mol l(-1) of L-cysteine (n = 10) with stable baseline s. The throughput was 26 determinations per hour and the results obtained f or L-cysteine by the proposed FI procedure were in good agreement with thos e obtained using a pharmacopeial procedure and within an acceptable range o f error. (C) 1999 Elsevier Science B.V. All rights reserved.