Effect of substitution on the inhibition of mushroom tyrosinase by pyridine carboxaldehydes

The effect of pyridine carboxaldehydes on the inhibition of mushroom tyrosinase has been investigated. The results indicated that pyridine carboxaldehydes can inhibit tyrosinase activity reversibly. Pyridine-2-carboxaldehyde (A), pyridine-3-carboxaldehyde (B) and pyridine-4-carboxaldehyde (C) were found to inhibit both monophenolase and diphenolase activities of mushroom tyrosinase. The trend of tyrosinase inhibition obtained was pyridine-3carboxaldehyde > pyridine-2-carboxaldehyde > pyridine-4-carboxaldehyde. The IC50 for diphenolase activity for these molecules was 0.0225 mM, 1.3 mM, and 5 mM respectively. The results showed that these aldehydes can not only form Schiff bases with the amino acids of the protein residue but can also undergo hydrogen bonding interaction and nucleophilic reactions with the enzyme which may play important roles in the inhibition of tyrosinase activity. During inhibition of the monophenolase activity, the lag time was not lengthened, but the steady state was reduced. Pyridine-3-carboxaldehyde, which was the strongest inhibitor, reduced the steady state of monophenolase activity by almost 50%. Kinetic analyses showed that all these pyridine carboxaldehydes were mixed inhibitors of mushroom tyrosinase.