Enzymatic determination of vitamin A in pharmaceutical formulations with spectrophotometric detection

An enzymatic method for the determination vitamin A (retinol) is reported using soluble and immobilized alcohol dehydrogenase, isolated from rabbit liver. The reaction is based on the oxidation of retinol and simultaneous reduction of NAD(+) to NADH followed by spectrophotometric detection at 340 nm. The calibration graph was linear over the range of 2.0-10 microM with correlation coefficients of 0.9967 and 0.9992 (n=5) for soluble and immobilized alcohol dehydrogenase respectively, with relative standard deviations (n=3) in the range of 0.5-1.2%. The limit of detection was lower than 1.0 microM. The proposed method was applied to determine vitamin A in pharmaceuticals, and the results obtained were in reasonable agreement with the amount labeled. The results were compared using spectrophotometric reference method, and no significant difference was found between the results of the both methods.