TYKOSINE METABOLISM 309 



stance has been thoroughly studied biochemically and pharmacologically 

 during recent years. It is interesting that these analogs have very little 

 inhibitory activity toward tyrosine decarboxylase and that a-methyl- 

 tyrosine does not inhibit dopa decarboxylase strongly, both facts pointing 

 to the importance of the 3-hydroxyl group in the binding to the enzyme. 

 This is also seen by comparing or-methylphenylalanine and its hydroxylated 

 derivatives: The addition of a 4-hydroxyl has little effect, whereas a 

 3-hydroxyl increases the binding energy over 3 kcal/mole. A 3-methoxy 

 group seems to be ineffective. 



The inhibition by a-methyldopa was shown to be pseudoirreversible 

 by varying the enzyme concentration and using the graphic procedure of 

 Ackermann and Potter (1949). At concentrations of 0.01-0.03 mM, the 

 inhibition being 15-25%, the behavior is fairly reversible, but at concen- 

 trations of 0.1 mM or above there is marked nonlinearity of the curves. 

 As pointed out by Sourkes, these data indicate merely that K^ is low and 

 the affinity for the enzyme is high. The binding might be to the apoenzyme, 

 to a great extent through the phenolic groups, or the inhibition could be 

 the result of reaction with pyridoxal phosphate. The former mechanism 

 was favored by Sourkes on the basis of the following evidence against a 

 reaction with the coenzyme. (1) The inhibition is reversible by dialysis. 

 (2) The rate of nonenzymic reaction of a-methyldopa with pyridoxal 

 phosphate is too slow at inhibiting concentrations to be significant. (3) In- 

 crease in pyridoxal phosphate concentration does not alter the inhibition 

 significantly. (4) Analysis for pyridoxal phosphate at the end of inhibition 

 experiments showed no loss. (5) Tyrosine decarboxylase is also a pyridoxal 

 phosphate enzyme and is not inhibited. None of this evidence is completely 

 conclusive and it is possible that a-methyldopa can form a reversible 

 complex with pyridoxal phosphate on the enzyme surface, so that increase 

 in coenzyme concentration would not be effective and analysis for total 

 coenzyme would not detect the small amount combined. 5-Hydroxytryp- 

 tophan decarboxylase is also potently inhibited by a-methyldopa (it is 

 possible that the decarboxylases for dopa, 5-hydroxytryptophan, trypto- 

 phan, tyrosine, and phenylalanine in mammalian tissues represent a single 

 enzyme) and S. E. Smith (1960 a) has investigated the mechanism, using 

 the mouse brain enzyme. Plots of 1/(S) against l/v showed pure competitive 

 inhibition with respect to substrate at higher coenzyme concentrations 

 (above 0.01 mM), but at low coenzjTne concentrations the inhibition be- 

 comes noncompetitive with substrate. In contrast to dopa decarboxylase, 

 increase of coenzyme concentration leads to a reduction in the inhibition 

 (Fig. 2-3). Smith inclines to a coenzyme inactivation mechanism but admits 

 that the inhibition is incompletely explained. If a-methyldopa forms a 

 complex with pyridoxal phosphate on the enzyme surface, which it can do 

 because it is decarboxylated slowly, it might be considered to be an in- 



