Comment on “Natural and synthetic flavonoid derivatives as new potential tyrosinase inhibitors: a systematic review” by R. Obaid, E. Mughal, N. Naeem, A. Sadiq, R. Alsantali, R. Jassas, Z. Moussa and S. Ahmed, RSC Advances, 2021, 11, 22159

A review article has been published recently (RSC Advances, 2021, 11, 22159–22198) describing flavonoids as inhibitors of tyrosinase. However, many compounds included in this review have been previously shown to act as substrates of this enzymes or antioxidants reducing tyrosinase-generated o-quinones. Products of their oxidation absorb light in a range different than dopachrome, the oxidation product of l-tyrosine or l-dopa, whose concentration is measured spectrophotometrically in the standard enzymatic assay to monitor the activity of this enzyme. This effect is interpreted as enzyme inhibition, which, in fact, is only apparent and results from inadequate methodology.