Flow Injection Spectrophotometric Determination of L-Dopa and Carbidopa in Pharmaceutical Formulations Using a Crude Extract of Sweet Potato Root [Ipomoea batatas (L.) Lam.] as Enzymatic Source

Abstract

A flow injection (FI) spectrophotometric method is proposed for the determination of L-dopa and carbidopa in pharmaceutical formulations. After selection of the extraction medium (e.g., buffer-to-tissue ratio, pH, buffer concentration, protective agents and/or stabilizers) and storage conditions, crude extract of sweet potato root [Ipomoea batatas (L.) Lam.] was used as an enzymatic source of polyphenol oxidase (Tyrosinase; catechol oxidase; EC.1.14.18.1) directly in the carrier. This enzyme catalyses the oxidation of these catecholamines to the corresponding dopaquinone. Further, dopaquinone undergoes a rapid spontaneous auto-oxidation to leucodopachrome, which is in turn oxidized to dopachrome; this last compound has a strong absorption at 480 and 360 nm for L-dopa and carbidopa, respectively. For the optimum extraction conditions found the enzyme activity of the crude extract did not vary for at least 5 months when stored at 4 °C and decreased by only 4–5% during an 8 h working period at 25 °C. The results obtained for L-dopa and carbidopa by the proposed enzymatic FI method were in close agreement with the label values (r ₁ = 0.9699 and r ₂ = 0.9999) and also with those obtained using a pharmacopeial method (r ₃ = 0.9675). The throughput was 26 samples h ^{- 1} , and 2.30 ml of crude extract were consumed in each determination, corresponding to only 72 mg of the original sweet potato root. The detection limit (three times the signal blank/slope) was 1.5 ×10 ^{- 5} and 2.0 × 10 ^{- 5} mol l ^{- 1} for L-dopa and carbidopa, respectively; the recovery of L-dopa and carbidopa from three samples ranged from 98.6 to 106.3% of the added amount.