Histidine decarboxylase specificity

Histamine is synthesized from the amino acid histidine via the action of the specific enzyme histidine decarboxylase and can be metabolized by histamine-TV-methyl transferase or diamine oxidase. Interesting, in its role as a neurotransmitter the actions of histamine are terminated by metabolism rather than re-uptake into the pre-synaptic nerve terminals. 

Histamine is synthesised by decarboxylation of histidine, its amino-acid precursor, by the specific enzyme histidine decarboxylase, which like glutaminic acid decarboxylase requires pyridoxal phosphate as co-factor. Histidine is a poor substrate for the L-amino-acid decarboxylase responsible for DA and NA synthesis. The synthesis of histamine in the brain can be increased by the administration of histidine, so its decarboxylase is presumably not saturated normally, but it can be inhibited by a fluoromethylhistidine. No high-affinity neuronal uptake has been demonstrated for histamine although after initial metabolism by histamine A-methyl transferase to 3-methylhistamine, it is deaminated by intraneuronal MAOb to 3-methylimidazole acetic acid (Fig. 13.4). A Ca +-dependent KCl-induced release of histamine has been demonstrated by microdialysis in the rat hypothalamus (Russell et al. 1990) but its overflow in some areas, such as the striatum, is neither increased by KCl nor reduced by tetradotoxin and probably comes from mast cells. 

There are two distinct pools of HA in the brain (1) the neuronal pool and (2) the non-neuronal pool, mainly contributed by the mast cells. The turnover of HA in mast cells is slower than in neurons it is believed that the HA contribution from the mast cells is limited and that almost all brain histaminergic actions are the result of HA released by neurons (Haas Panula, 2003). The blood-brain barrier is impermeable to HA. HA in the brain is formed from L-histidine, an essential amino acid. HA synthesis occurs in two steps (1) neuronal uptake of L-histidine by L-amino acid transporters and (2) subsequent decarboxylation of l-histidine by a specific enzyme, L-histidine decarboxylase (E.C. 4.1.1.22). It appears that the availability of L-histidine is the rate-limiting step for the synthesis of HA. The enzyme HDC is selective for L-histidine and its activity displays circadian fluctuations (Orr Quay, 1975). HA synthesis can be reduced by inhibition of the enzyme HDC. a-Fluoromethylhistidine (a-FMH) is an irreversible and a highly selective inhibitor of HDC a single systemic injection of a-FMH (10-50 mg/kg) can produce up to 90% inhibition of HDC activity within 60-120 min (Monti, 1993). Once synthesized, HA is taken up into vesicles by the vesicular monoamine transporter and is stored until released. 

Figure 6.1 Histamine synthesis and metabolism in neurons. L-histidine is transported into neurons by the L-amino acid transporter. Once inside the neuron, L-histidine is converted into histamine by the specific enzyme histidine decarboxylase. Subsequently, histamine is taken up into vesicles by the vesicular monoamine transporter and stored there until released. In the absence of a high-affinity uptake mechanism in the brain, released histamine is rapidly degraded by histamine methyltransferase, which is located postsynaptically and in glia, to telemethylhistamine, a metabolite that does not show any histamine-like activity.

Specific enzymes control histamine synthesis and breakdown 253 Several forms of histidine decarboxylase may derive from a single gene 254... 

Watanabe, T., Yamatodani, A., Maeyama, K. and Wada, H. Pharmacology of a-fluoromethylhistidine, a specific inhibitor of histidine decarboxylase. Trends Pharmacol. Sci. 11 363-367,1990. 

The most smdied enzyme is histidine decarboxylase from Lactobacillus 30a. There are pyruvate residues at the amino terminals of each of 5 of the 10 subunits in this enzyme. When the organism is grown on [ C] serine, the specific radioactivity of the pymvate is the same as that of serine incorporated into the protein and much greater than that of free lactate or pyruvate in the culture medium This suggests that pyruvate arises by postsynthetic modification of a serine residue. 

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. 

Matsuki Y, Tanimoto A, Hamada T, et al. Histidine decarboxylase expression as a new sensitive and specific marker for small cell lung carcinoma. Mod Pathol. 2003 16 72-78. 

The activity of histidine decarboxylase preparations can be measured by determining either of the decarboxylation products of histidine, i.e. histamine or carbon dioxide. Most of the methods are in vitro techniques using organ slices, minces or cell-free extracts, and they have the usual limitations of in vitro measurements - - . In vivo methods have also been used, but these give information primarily about the decarboxylation of histidine in the body as a whole rather than about the distribution of the enzyme in specific organs. 

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... 

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... 

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... 

It has recently been reported that extracts of foetal rat liver can decarboxy-late not only histidine but also DOPA, 5-HTP and other aromatic L-amino acids i. This may mean that the specifie histidine decarboxylase has a wider substrate specifieity than was originally believed, or that the extract contains a mixture of the specific and non-specific enzymes. The rat glandular stomach provides an example of a tissue in which the presence of both enzymes has now been demonstrated . 

Histidine decarboxylase (HD) activity is expressed in arbitrary units t = low activity 2 = moderate activity 3 = high activity. The probable nature of the enzyme is indicated by N s=s non-specific HD, S = si cific HD where no letter is given the nature of the eozymc is in doubt. 

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. 

The use of tissue slices for experiments on histidine decarboxylation introduces the additional problem of the access of substrate, co-enzyme and inhibitors into the cells. In this connection, it should be noted that in practice the specificity of an enzyme within a cell may be increased by the specificity of the substrate-transporting system. Similar considerations apply to the in vivo inhibition of histidine decarboxylases there is, however, the additional possibility of modifying production of the apo-enzyme either by restricting the supply of amino acids or by altering the hormonal state of the animal. 

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 . 

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