TYROSINE METABOLISM 305 



obtained from the reciprocal plots. 3-Nitro-L-tyrosine and O-methyl-L- 

 tyrosine are not inhibitory. The effects of 3-substitution may be mediated 

 through inductive effects on the 4-OH group and its interaction with the 

 enzyme, whereas iV-substitution must lead to an altered position of binding 

 to prevent oxidation. L-Phenylalanine and phenylpjTuvate inhibit com- 

 petitively the tyrosinase from melanoma, inhibit the incorporation of ty- 

 rosine-C^* into melanin, and depress the respiration of tumor tissue with 

 tyrosine as the substrate (Boylen and Quastel, 1962). The high concentra- 

 tions of these inhibitors in phenylketonuria might be responsible for the 

 reduced pigment formation in these individuals. 



Tyrosine : a-Ketoglutarate Transaminase 



The effects of numerous analogs on the formation of p-hydroxyphenyl- 

 pyruvate from tyrosine and a-ketoglutarate by a dog liver transaminase 

 were reported by Canellakis and Cohen (1956 b); some of the results are 

 given in Table 2-9. Certain of these analogs are transaminated (e.g., the 

 3-substituted tyrosines) and the inhibitory activity varies inversely with 

 their abilities to act as substrates. The rapid transamination of 3-fluoro- 

 tyrosine may partly explain its toxic effects and inhibition of growth, 

 since fluorofumarate or fluoroacetoacetate may be formed. Comparing the 

 hydroxyl-substituted phenylalanines, it is seen that a m- or p-hydroxyl 

 is necessary for tight binding, the contribution to the binding energy 

 being over 2 kcal/mole. The if,,; for L-tyrosine is 0.71 mM, so that its rel- 

 ative binding energy is at least — 4.47 kcal/mole, and that of m-hydroxy- 

 DL-phenylalanine is at least — 4.75 kcal/mole (— 5.18 kcal/mole if only 

 the L-isomer is active), which may be compared with the — 2.55 kcal/ 

 mole for L-phenylalanine. Hydroxyl groups in the o-positions, on the other 

 hand, do not augment binding very much. The tighter the binding between 

 a basic hydroxyl and an acidic enzyme group, the greater the inhibitory 

 activity; ring substituents modify the electronic character or basicity of 

 the phenolic group. A carboxylate group is necessary for strong binding, 

 as may be seen by comparing tjTosine with tyramine, and p-hydroxyben- 

 zoate with p-cresol. The a-amino group may also be involved in the binding 

 (— AF for p-hydroxyphenylacetate is 2.87 kcal/mole), but lacking data on 

 p-hydroxyphenylproprionate it is not possible to evaluate this accurately. 

 The strong inhibition by epinephrine is rather surprising in the light of 

 the absence of a carboxylate group, but the /?-hydroxyl or iV-methyl group 

 may contribute to make up for this deficiency. A similar study on the rat 

 liver enzyme has been reported by Jacoby and La Du (1964). 



p-Hydroxyphenylpyruvate Oxidase 



The further oxidation of the product of tyrosine transamination is ca- 

 talyzed by an enzyme from dog liver and is inhibited markedly by phenyl- 



