L-Cysteine determination using a polyphenol oxidase-based inhibition flow injection procedure Original Research Article

The use of crude extract of sweet potato root (Ipomoea batatas L. Lam.) as a source of polyphenol oxidase (PPO; tyrosinase; EC 1.14.18.1) for the determination of L-cysteine in pharmaceutical products is reported. This enzyme 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 (FI) system, its inhibitory effect on the PPO activity was proportional to the L-cysteine concentration. The analytical curve of absorbance versus concentration of L-cysteine obtained was A = 0.851 − 963.16[L-cysteine]; r = 0.9987 in the L-cysteine concentration range from 6.0 × 10−5 to 8.0 × 10−4 mol l−1 with a detection limit of 4.4 × 10−6 mol l−1. Recovery of L-cysteine from three samples ranged from 98.8 to 101.4% and the relative standard deviations (Sr) were less than 1.2% for solutions containing 2.0 × 10−4 and 4.0 × 10−4 mol l−1 of L-cysteine (n = 10) with stable baselines. The throughput was 26 determinations per hour and the results obtained for L-cysteine by the proposed FI procedure were in good agreement with those obtained using a pharmacopeial procedure and within an acceptable range of error.