SPECTROPHOTOMETRIC DETERMINATION OF METHYLDOPA AND DOPAMINE IN PHARMACEUTICAL FORMULATIONS USING A CRUDE EXTRACT OF SWEET-POTATO ROOT (IPOMOEA-BATATAS (L.) LAM.) AS ENZYMATIC SOURCE

A rapid, precise and low cost spectrophotometric method is proposed fo r the determination of methyldopa and dopamine in pharmaceutical formu lations. The crude extract of sweet potato root (Ipomoea batatas (L.) Lam.) was used as an enzymatic source of polyphenol oxidase (PPO; EC.1 .14.18.1). This enzyme catalyses the oxidation of catecholamines to th e corresponding methyldopaquinone and dopaminequinone. Those compounds are converted by a rapid spontaneous auto-oxidation to methyldopachro me and dopaminechrome which have a strong absorption at 480 or 470 nm, respectively. The calibration graphs are linear from 2.0 x 10(-4) to 6.0 x 10(-3) M. The results obtained by the proposed enzymatic method are in close agreement with those obtained using a Pharmacopoeia proce dure and also with the label values. The detection limit (three times the signal blank/slope) was 3.4 x 10(-5) and 3.0 x 10(-5) M for methyl dopa and dopamine, respectively, the recovery of methyldopa and dopami ne from three samples ranged from 97.5 to 102.9% of the added amount. (C) 1998 Elsevier Science B.V. All rights reserved.