Tyrosinase

Tyrosinase is a type one membrane glycoprotein resident in the melanosome, carrying two copper ions held by histidine residues in its active site. It performs two consecutive reactions, hydroxylating L-tyrosine to L-DOPA and oxidising L-DOPA to dopaquinone, after which the pathway proceeds largely spontaneously toward eumelanin unless cysteine diverts it. Its importance is not theoretical: loss-of-function mutations in the TYR gene cause oculocutaneous albinism type one despite normal melanocyte numbers.

Because it is rate-limiting, most depigmenting chemistry aims at it, though not always as advertised. Hydroquinone, the historical reference agent, does inhibit the enzyme but also generates reactive quinones cytotoxic to melanocytes, a different and less reversible mechanism. Kojic acid and arbutin act as copper chelators or substrate analogues; the resorcinol derivative thiamidol was developed specifically against human tyrosinase and reports far greater potency on it than on the fungal enzyme.

That last comparison is the decision-relevant point. Screening potency is expressed as an inhibitory concentration, and the ranking of candidate inhibitors depends on which enzyme generated it. A number obtained on one organism's tyrosinase cannot be carried across to another and treated as a property of the compound.

The pervasive error follows directly. Most published tyrosinase inhibition data uses soluble mushroom tyrosinase because it is cheap and easy to assay, yet it differs from the human enzyme in sequence, glycosylation, membrane anchoring and substrate handling, and rank orders demonstrably do not transfer. Even a genuine human-enzyme result leaves the harder questions untouched: surviving formulation, crossing the stratum corneum, and reaching the melanosome.