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

A flow injection (FI) spectrophotometric method is proposed for the de termination of L-dopa and carbidopa in pharmaceutical formulations, Af ter selection of the extraction medium (e.g., buffer-to-tissue ratio, pH, buffer concentration, protective agents and/or stabilizers) and st orage conditions, crude extract of sweet potato root [lpomoea batatas (L.) Lam.] was used as an enzymatic source of polyphenol oxidase (Tyro sinase; catechol oxidase; EC.1.14.18.1) directly in the carrier, This enzyme catalyses the oxidation of these catecholamines to the correspo nding dopaquinone, Further, dopaquinone undergoes a rapid spontaneous auto-oxidation to leucodopachrome, which is in turn oxidized to dopach rome; this last compound has a strong absorption at 480 and 360 nm for L-dopa and carbidopa, respectively, For the optimum extraction condit ions found the enzyme activity of the crude extract did not vary for a t least 5 months when stored at 4 degrees C and decreased by only 4-5% during an 8 h working period at 25 degrees C, The results obtained fo r L-dopa and carbidopa by the proposed enzymatic FI method were in clo se agreement with the label values (r(1) = 0.9699 and r(2) = 0.9999) a nd also with those obtained using a pharmacopeial method (r(3) = 0.967 5), 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 th e original sweet potato root, The detection limit (three times the sig nal blank/slope) was 1.5 x 10(-5) and 2.0 x 10(-5) mol l(-1) for L-dop a and carbidopa, respectively; the recovery of L-dopa and carbidopa fr om three samples ranged from 98.6 to 106.3% of the added amount.