Amino-(3,4-dihydroxyphenyl)methyl phosphonic acid, the phosphonic analogof 3,4-dihydroxyphenylglycine, hadbeen previously reported as a potent inhibitor of tyrosinase. The mechanism of the apparent enzyme inhibition by this com-pound has now been established. Amino-(3,4-dihydroxy-phenyl)methyl phosphonic acid turned out to be a substrate and was oxidized too-quinone, which evolved to a final product identified as 3,4-dihydroxybenzaldehyde, the same as for 3,4-dihydroxyphenylglycine.
