Development of an optical method for the determination of methionine in the presence of various antioxidant compounds

Methionine is not only an essential amino acid that plays a vital role in the organism's growth and vital functions, but also an effective antioxidant that prevents cell damage caused by oxidative stress. Therefore, the sensitive and selective determination of methionine in biological and pharmaceutical samples has become a significant research topic in recent years. In this context, although many methods have been developed in the literature for the determination of methionine, a method that allows the simple, easy and low-cost determination of methionine in the presence of other antioxidants is required. In this study, a new spectrophotometric method was developed for the determination of methionine independently of the effects of other antioxidant compounds. The method is based on the AuRAC (Gold(III) Ion Reducing Antioxidant Capacity) test principle. First, the DPPH reagent was added to the sample to oxidize antioxidants other than methionine and eliminate their interference. Then, as a result of the oxidation of methionine with gold(III) ions, the remaining gold(III) ions in the reaction medium reacted with ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)), and the absorbance of the resulting ABTS•⁺ radical was measured at 412 nm. Using this method, the determination of methionine was achieved with high sensitivity. The limit of detection (LOD) of the developed method was calculated as 14 μM, and the linear working range was found to be between 17–56 μM. The prepared synthetic mixtures gave successful results in terms of accuracy and reliability with high recovery values (96-101). Finally, the method was applied to a synthetic serum sample and a methionine-containing food supplement. The methionine content obtained using the proposed method for the food supplement was compared with the results obtained using high-performance liquid chromatography (HPLC); the similar results demonstrated the validity of the method. The obtained data demonstrate that this newly developed method is an effective, selective, and applicable alternative for the determination of methionine in complex matrices.