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 . 

The histidine decarboxylase which has its maximum activity in the pH range 6-0 7-0 appears to be substrate-specific, acting only on L-histidine. This enzyme, which will be referred to as specific histidine decarboxylase, has been detected in various tissues, notably in mast cells, in the glandular portion of rat stomach, in rat foetal liver, and in certain tumours. Histidine decarboxylase activity which has been shown to be induced in tissues of various species when the animals are subjected to stressful stimuli also has many of the properties of specific histidine decarboxylase. Some comparative properties of the two types of histidine decarboxylase derived from various mammalian sources are given in Table 4.4. 

There is evidence that the specificity of the histidine decarboxylase of rat foetal liver may be even greater than was originally suspected. Thus, not only is its activity confined to L-histidine, but it appears to be further restricted to one particular ionic form of histidine. Over the pH range regarded as optimal for this enzyme, the substrate, histidine, exists as a mixture of ionic forms (V, VI, VII, VIII), and the concentration of each species present in a given solution can be calculated from the Henderson-Hasselbach equation. When the Michaelis constant for the decarboxylation was measured in terms of the 

Source of histidine decarboxylase pH/or Optimal activity Affinity for histidine (Km)mole/l Substrates Effect of benzene Effect of fx-Me-DOPA Effect of CL-Me histidine References 

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This has been achieved for the production of (146) in D. sphaerica by means of histidine decarboxylase inhibitors. Both a-methylhistidine and a-hydrazinohistidine (inhibitors for mammalian specific histidine decarboxylase) inhibited formation of histamine with the result that more (146) was synthesized at the expense of (144) (based on the radioactivity of the products after feeding [ H]histidine, [ C]isovaleric acid and inactive animomethylimidazole [145]). a-Methyldopa, an inhibitor of mammalian non-specific decarboxylase, was without effect on the proportions of the products formed. 

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. 

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

These observations suggest that the true substrate for the specific histidine decarboxylase of rat foetal liver is the anionic form of histidine (F). It should be noted that this interpretation involves the assumption that the enzyme... 

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. 

The results obtained with inhibitors suggest that it is a specific histidine decarboxylase which is mainly involved in the production of histamine in the rat. Studies on the excretion of histamine in the urine of thyroxine-treated AM-i66 and of totally gastrectomized rats indicate that the specific histidine decarboxylase responsible for this histamine production is located mainly in the mucosa of the glandular portion of the stomach. 

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 . ... 

The histidine decarboxylase activity of tissues can be raised or lowered by changing the hormonal state of the animal or by subjecting it to certain stressful stimuli. Administration of thyroid hormones to rats produces a marked increase in the specific histidine decarboxylase activity of the glandular mucosa of the stomach " , while the activity of the nonspecific enzyme in the liver is lowered . Studies of the action of thyroid hormones on other pyridoxal-dependent enzymes in rat liver suggest that, in this organ at least, these changes arise from corresponding alterations in both pyridoxal phosphate and apo-enzyme synthesis . 

The first question to arise is whether the two main classes of mammalian histidine decarboxylase, the specific and non-specific enzymes, are of equal importance in relation to the physiological function of histamine. Consideration of the values of the Michaelis eonstant, approximately 10 m for the specific, and 10 % for the non-specific, enzyme Table 4.6), indicates that the specific histidine decarboxylase has the greater affinity for histidine. This might be taken to imply that the specific enzyme is the more important source of histamine in the body . 

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. 

Nevertheless, when factors which modify the histidine decarboxylase activity of tissues are considered, it is found that the most striking changes occur in the level of the specific enzyme. We have already seen that this is true of the effect of inhibitors and of certain external factors, but it should be noted that most of this information has been derived from studies in the rat. Various physiological and pathological conditions are now described which also lead to changes in the activity of the specific histidine decarboxylase in the tissues of rats and other animal species. 

The peak of histidine decarboxylase activity in foetal rat liver coincides. approximately with that of haemopoietic activity . Subsequently, the bone marrow of the adult rat was found to contain a specific histidine decarboxylase Table 4.4), thus supporting the possibility of a connection between haemopoiesis and the formation of histamine . Histidine decarboxylase also occurs in the bone marrow of the guinea-pig this enzyme activity, which was shown to be related to the number of basophils, does not, however, support a relationship between histidine decarboxylase activity and growth, since the basophils in the buffy layer of guinea-pig blood have also a considerable histamine-forming capacity, but contain no cells in mitosis . ... 

It has been seen that a specific histidine decarboxylase is present in normal mast cells however, since mitoses are never observed in these cells, it is unlikely that the presence of the enzyme is concerned with growth. Moreover,... 

Kahlson, Rosengren and Steinhardt in studies of the Landschutz I tumour growing as isolated, free cells in the mouse peritoneal cavity, have found that on the first day of tumour growth the specific histidine decarboxylase activity is high and correlates with the frequency of tumour cell mitosis. However, it is possible that some of this enzyme activity may have been contributed by the peritoneal mast cells of the host. 

We have seen that the first indication of a relationship between histidine decarboxylase and growth arose from studies in pregnant rats. Increased specific histidine decarboxylase activity was found in the foetal liver and the... 

Activation of the inducible enzyme becomes detectable 0-5-1 hour after application of the stress, and it persists for periods which depend on the nature and intensity of the stimulus. Under exceptional circiunstances the high rate of induced histamine formatation may be sufficiently prolonged to result in the development of shock . On the other hand, when the degree of activation of histidine decarboxylase is inadequate, tissue damage may occur as has been observed in the kidneys of endotoxin-treated rabbits . If histamine produced by inducible histidine decarboxylase is indeed a mediator of the slow phase of inflammation , inhibitors of this enzyme might possess anti-inflammatory action. It is thus of considerable interest that the ability of one class of anti-inflammatory drugs, the acidic group, to inhibit specific histidine decarboxylase runs parallel to their clinical activity . 

The specific histidine decarboxylase of mast cells apparently produces histamine for local storage within the mast cell itself. This may be the primary function of the mast celP . 

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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