304 2. ANALOGS OF ENZYME REACTION COMPONENTS 



melanin from tyrosine, was noted by KuU et al. (1954), for example (see 

 following tabulation). Other substitutions in the 4-position usually reduce 



or abolish the inhibitory activity. The formation of melanin involves several 

 steps and the inhibitions observed are not necessarily entirely on tyrosinase. 

 Hydroquinone was found to be a weak inhibitor but the monobenzyl 

 ether of hydroquinone is as potent as resorcinol. This latter substance, 



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Monobenzone (Benoquin) 



known also as monobenzone (Benoquin), is an inhibitor of melanin for- 

 mation in the skin when applied topically, can produce leucoderma in 

 Negroes, and is used in various conditions of melanosis. Some have thought 

 that it releases hydroquinone after penetration into the skin but this is 

 questionable in view of its own inhibitory activity. 



An active tyrosinase occurs in the Hardin-Passey mouse melanoma and 

 is probably responsible for pigment formation. It is competitively inhib- 

 ited by various tyrosine analogs (Lerner et al., 1951). The values of K^ 

 shown in the tabulation were calculated on the basis of a K„, of 0.60 mM 



Inhibitor J^, (mM) 



iV-Acetyl-L-tyrosine 0.140 



iV-Formyl-L-tyrosine 0.177 



3-Amino-L-tyrosine 0.314 



3-Fluoro-L-tyrosine 1.25 



