Inhibitory Effects of a Palladium Complex on the Activity, Stability, and Structure of Tyrosinase Enzyme

Tyrosinase, as a copper-containing enzyme, is widely distributed in different levels of life span. It is also a key enzyme in melanin biosynthesis, which plays a crucial role in determining the color of mammalian skin and hair. The current study aimed to determine the effect of a palladium complex on cresolase and catecholase reactions of mushroom tyrosinase (MT). The MT kinetics parameters were obtained from double reciprocal plots of Lineweaver-Burk and the inhibition constants (Ki) were determined by the secondary plots. Thermodynamic parameters were obtained from thermal and chemical denaturation of the tyrosinase with and without the presence of palladium complex. The tertiary and secondary structures of tyrosinase were detected by fluorescent and Circular Dichroism (CD) techniques. The inhibition modes of palladium complex were competitive in both activities of the enzyme with Ki values of 3.74 and 10.55 μM for cresolase and catecholase activities, respectively. In thermal denaturation, the melting points (Tm) of the enzyme were 59.4˚C and 51˚C for the sole enzyme and its treatment by palladium, respectively. In chemical denaturation, the magnitudes of half denaturant concentration (Cm) were 1 μM vs. 1.36μM and the free energy of Gibss (ΔGH2O) were calculated 9.3 vs. 7.5 kJ/M for the sole enzyme and its treatment by palladium, respectively. In overall the palladium complex acted as a good inhibitor of tyrosinase and induced the enzyme thermodynamic and conformational instability, therefore it can be considered in the hyper expression of tyrosinase in melanoma cancer.