Non-specific histidine decarboxylase

When the development of more sensitive techniques and the introduction of new inhibitors had made more detailed studies possible, it became clear that mammalian histidine decarboxylases could themselves be divided into two sub-classes, one having optimum activity in the range pH 6-7, and the other in the range pH 8-0-9-5 . Other differences between these two enzymes then became apparent, the most notable being in their substrate specificity . Solutions of the enzyme having the lower pH optimum were found to act only on L-histidine. Solutions of the other enzyme were capable of decarboxylating a number of amino acids structurally related to histidine. These enzymes will be referred to as specific and non-specific histidine decarboxylase, respectively. 

Apart from the similar distribution of the decarboxylase activities discussed above, further evidence that various aromatic L-amino acids are all decar-boxylated by a single enzyme is based on (a) failure to dissociate the activities during progressive purification of the enzyme (b) the occurrence of competitive substrate inhibition (r) the fact that all the decarboxylations are inhibited by the same inhibitors (d) under conditions which lead to changes in the ability of a tissue to decarboxylate one substrate, parallel changes occur in the ability to decarboxylate the other substrates. In practice, most of this evidence has been obtained by the use of histidine, DOPA and 

The decarboxylation of DOPA by a rabbit kidney extract was found to be competitively inhibited by 5-HTP, and the decarboxylation of 5-HTP by the same extract was competitively inhibited by DOPA . Kinetic studies of the mutual inhibitions enabled the respective Michaelis and inhibitor constants, and ff,-, to be calculated, thus providing quantitative measurements of the affinities of these substances for the two enzymes Table 4.1). 

DOPA decarboxylase 3,4-dihydroxy-L-phenylalanine carboxy-lyase (EC4.1.1.26) 

Aromatic L-amino acid decarboxylase Aromatic L-amino acid carboxy-lyase. 

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Table 4.3. The substrate spectrum of non-specific histidine decarboxylase... 

Thus there is much evidence to suggest that the histidine decarboxylase having its maximum activity in the pH range 8-0-9-5 is a single enzyme which can decarboxylate not only L-histidine, but also L-/S-(3,4-dihydroxyphenyl)-alanine and L-5-hydroxytryptophan. Enzyme preparations which decarboxylate one or more of these three compounds have been found also to decarboxylate the substances listed in Table 4.3, thus providing support for the existence of a general aromatic amino acid decarboxylase. It is this enzyme which will be referred to as the non-specific histidine decarboxylase. 

In some instances the results obtained by different groups of workers have been sufficiently at variance, particularly where weak substrates have been studied, for doubt to be cast on the existence of a general aromatic amino acid decarboxylase. Thus it has been claimed that some preparations which contain DOPA and 5-HTP decarboxylase activities do not decarboxylate histidine - . In these instances, the sensitivity or specificity of the analytical procedures are open to doubt, and the results require confirmation. In view of conflicting reports in the literature, further experiments should also be carried out to determine whether the mono- and dihydroxyphenylserines > are indeed substrates of non-specific histidine decarboxylase. The status of /)-tyrosine also requires clarification formerly it was not considered to be a substrate " , but recent evidence suggests that it may, in fact, be decarboxylated . 

When purified, the DO PA decarboxylase of rat liver has an absorption spectrum similar to that of other pyridoxal-dependent enzymes. In this case, the co-enzyme seems to be very tightly bound to the apo-enzyme, but addition of an excess of pyridoxal phosphate still causes an increase in the enzyme activityii . It was therefore suggested that pyridoxal phosphate is a prosthetic group of this enzyme, and that when present in excess it acts as a co-enzyme. The 5-HTP decarboxylase of rat kidney was found to be potentiated by pyridoxal phosphate, but the effect was shown only when the tissue had been repeatedly frozen and thawed. These observations provide some evidence that pyridoxal phosphate is the co-enzyme for non-specific histidine decarboxylase. 

Carbonyl reagents, including cyanide, hydroxylamine, semicarbazide, hydrazine and substituted hydrazines inhibit non-specific histidine decarboxylase by combining with the co-enzyme pyridoxal phosphate. Such compounds, of course, inhibit other pyridoxal-dependent enzymes. A list of these and other compounds which inhibit non-specific histidine decarboxylase has been compiled by Schayer . 

We have seen that the non-specific histidine decarboxylase may be identical... 

Inhibition of Specific Histidine Decarboxylase The effect of inhibitors on specific histidine decarboxylase differs in certain important respects from their effect on non-specific histidine decarboxylase. In particular, the specific enzyme, unlike the non-specific enzyme, is scarcely affected by a-methyl-DOPA " >i i i . Conversely, the specific enzyme is subject to moderate inhibition by a-methylhistidine at concentrations which... 

The abilities of various compounds to inhibit the specific histidine decarboxylase of rat hepatoma and the non-specific enzyme of guinea-pig kidney have been compared . Some of these results are given in Table 4.8. Of the a-methylamino acids tested, DL-a-methyI-5-HTP was the most potent inhibitor of the specific enzyme. Hydrazine was a strong inhibitor, and the various hydrazides were moderately effective. The 0-substituted hydroxylamines (XXII, XXIII, and XXIV) and the substituted hydrazine (XXV), which we have seen to be potent inhibitors of non-specific histidine decarboxylase, were similarly effective inhibitors of the specific enzyme. The potencies of these compounds may be due, at least in part, to their ability to react with pyridoxal phosphate. 

There is considerable doubt at present concerning the physiological significance of the non-specific histidine decarboxylase . Nevertheless, the possibility remains that some of the compounds which have been found to inhibit the formation of dopamine and 5-HT may also be useful inhibitors of histamine formation. Comparative potencies, in vitro andf vivo, of various substances as inhibitors of the non-specific decarboxylase with DOPA as substrate have been recorded in the literature . ... 

Such considerations do not, however, exclude the participation of the nonspecific enzyme as a source of body histamine. The non-specific histidine decarboxylase of guinea-pig kidney is known to have a high affinity for DOPA and 5-HTP, but a low affinity for histidine and phenylalanine . At first sight, then, it would appear that this enzyme is more likely to produce dopamine and 5-hydroxytryptamine than to form histamine or / -phenyl-ethylamine. It must be remembered, however, that the substrates DOPA and 5-HTP are not normally detectable in blood or tissues, while histidine and phenylalanine are present in amounts which compensate for the low affinity of the enzyme for these two amino acids. In terms of the capacity to form the corresponding amines, therefore, there is no reason to suppose that the decarboxylation of histidine is a less important function of the non-specific enzyme than is the decarboxylation of its other substrates. 

The function of the non-specific histidine decarboxylase of rabbit- or guinea-pig liver and kidney remains to be clarified. However, in view of its wide substrate specificity [Table 4.3), this enzyme may rather be a general aromatic L-amino acid decarboxylase, the purpose of which is to produce other physiologically important amines in addition to histamine. 

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