Spiro-acridine inhibiting tyrosinase enzyme: Kinetic, protein-ligand interaction and molecular docking studies

Here, we evaluate spiroacridines as inhibitors of tyrosinase, a key enzyme to melanogenesis. For this purpose, the spiroacridines 3-(acridin-9-yl)-N-benzylidene-2-cyanoacrylohydrazide (AMTAC-01) and 3-(acridin-9-yl)-2-cyano-N-(4-metoxybenzylidene)-acrylohydrazide (AMTAC-02) were synthesized and their enzymatic inhibition types and mechanisms were investigated. In addition, the interaction of these compounds with the enzyme were studied by UV-Vis spectroscopy, spectrofluorimetry, ¹H NMR titration as well as molecular docking. Spectroscopic results reveals that the acridine derivatives interact strongly (K_a ≅ 10⁴ - 10⁵ M^-1) with the mushroom tyrosinase and the enzyme undergoes small structural modifications due to the interaction with AMTAC-01 compound. The interaction studies support the enzymatic inhibition results, which suggests that AMTAC-01 compounds inhibit the enzyme reversibly and follows a noncompetitive type (AMTAC-01) and mixed type (AMTAC-02) of inhibition. Nevertheless, AMTAC-02 (IC₅₀ = 96.29 μM) inhibits the enzyme more effectively than AMTAC-01 (IC₅₀ = 189.40 μM), which suggests a highly relevant role of AMTAC-02's methoxy group to the inhibition activity, which is confirmed by docking studies to mushroom tyrosinase. Docking also indicates this interaction to be absent in human tyrosinase. SIGNIFICANCE: Based on previous results which evidenced the relevant activity of two spiroacridinic compounds for cell growth inhibition against melanoma cells, here we improve our understanding about the spiroacridines in the biological media by exploring the molecular mechanism that govern the activities of these two compounds using mushroom tyrosinase (mTYR) enzyme as molecular target. The paper not only will have a major impact upon molecular mechanism that regulates melanin inhibition by spiroacridinic compounds, but also by guiding the search for enzyme inhibitors and the development of new anti-melanoma prophylaxis.