Skin depigmentation by hydroquinone: a chemical and biochemical insight

The skin depigmenting agent hydroquinone (HQ) is shown to inhibit melanogenesis in vitro by acting as an alternate substrate of tyrosinase and preventing conversion of tyrosine to melanin. Upon the action of tyrosinase, and in the presence of catalytic dopa, HQ is oxidatively metabolized to give eventually highly colored quinonoid materials, by way of HBQ, the hydroxylation-dehydrogenation product of HQ, and p-benzoquinone (BQ). Overall, the chem. depicted in this study is compatible with, and would lend support to most of the currently accepted theories on the mol. mechanism of action of HQ. The ability of the compd. to compete with tyrosine for tyrosinase oxidn., without inducing significant inactivation of the enzyme, is consistent with both the selectivity and reversibility of HQ to undergo hydroxylation and dehydrogenation to yield highly reactive quinones, such as HBQ, BQ and their dimerization products, provides for the first time a convincing chem. basis to the postulated generation within melanocytes of cytotoxic species disrupting fundamental cell processes. Such species may be involved in redox-exchange, radical generating reactions as well as covalent modifications interfering with the melanization pathway and causing a focal degrdn. of melanosomes.