“In … tetrahydropterines can inhibit melanogenesis in several ways: i) by non-enzymatic inhibition involving …-dopaquinone to L-dopa, ii) by acting as substrates which compete with L-tyr and L-dopa, since they are substrates of tyrosinase; and iii) … enzymatic forms met-tyrosinase and deoxy-tyrosinase in anaerobic conditions.”