302 2. ANALOGS OF ENZYME REACTION COMPONENTS 



the last example the increased binding of benzoate may be due to its great- 

 er polarizability. In general the substitution of groups on the benzene 

 ring reduces the affinity for the enzyme. This may be due to steric inter- 

 ference with the approach of the ring or to inductive effects on the carboxyl 

 group's interaction with the enzyme. It is rather odd that an oriho chlorine 

 does not disturb binding much while an ortho methoxy group reduces the 

 binding some 4 kcal/mole. 



The forces binding the substrates and inhibitors to these enzymes are 

 thus vague at the present time. It is possible that hydrogen bonds between 

 the OH or COOH groups and the enzyme are important, and it is equally 

 possible that bonds to the copper ions are involved. One might conceive 

 of the inhibitor's COOH group reacting with either the two copper ions, 

 or with a copper ion and a vicinal — NH — group. Copper ions are able to 

 catalyze the oxidation and hydroxylation of phenols nonenzymically, and 

 it might be interesting to study the inhibition of such reactions by some 

 of the compounds active in the enzymic reaction. 



TYROSINE METABOLISM 



Many interesting and practically important examples of analog inhibi- 

 tion are to be found in the general field of amino acid metabolism, and we 

 shall begin the discussion of this subject by considering the inhibitions of 

 the various pathways of tyrosine metabolism. Tyrosine may be hydroxyl- 

 ated to form dihydroxyphenylalanine (dopa), oxidatively deaminated or 

 transaminated to form p-hydroxyphenylpyruvate, decarboxylated to form 

 tyramine, or activated prior to incorporation into proteins; inhibition 

 of all of these reactions by analogs has been reported. The scheme on page 

 303 indicates the major pathways of tyrosine metabolism. Interference 

 with these reactions might be expected to bring about physiological changes 

 due to the acceleration or suppression of active amine synthesis, and also 

 to affect melanin formation. 



Tyrosinase (Phenol Oxidase) 



These enzymes hydroxylate tyrosine in the oriho position to form dopa 

 and further oxidize dopa to dopa-quinone. The enzymes discussed in 

 the previous section generally possess this activity. However, mammalian 

 tyrosinases are much more specific than the plant enzymes and oxidize 

 tyrosine and dopa more rapidly than other phenols. Inhibitions of the en- 

 zymes with tyrosine as the substrate will be considered in this section, 

 but the results with p-cresol or catechol as substrate are probably generally 

 applicable to tyrosinase activity. The high inhibitory potency of 4-chloro- 

 resorcinol on potato tyrosinase, as determined by the rate of formation of 



