Histidine decarboxylase activity measurement

As is the case for most enzyme activities measured in vitro, there is some doubt whether the histidine decarboxylase activities determined as above in various organs truly reflect the contribution of these organs to histidine decarboxylation in the intact animal. In vivo measurements give an overall picture of histidine decarboxylation in the living animal, but they can give little indication of the contribution made by individual organs. Moreover, the interpretation of such measurements is rendered difficult by bacterial decarboxylation of histidine in the gut, by metabolic destruction of histamine, and by the release of histamine from storage sites. Nevertheless, such measurements have provided much useful information, and they are particularly suited to the study of the effectiveness of histidine decarboxylase inhibitors in intact animals. As with in vitro methods the in vivo measurements can, in theory, be made either on the carbon dioxide or on the histamine formed in the decarboxylation. 

Of these procedures for measuring histidine decarboxylase activity in vitro... 

The relationship of the histamine content of a tissue to its histidine decarboxylase activity has been the subject of several investigations . Apart from the difficulty already mentioned of correlating the results of histidine decarboxylase measurements made by different groups of workers. 

The above considerations emphasize that the rate at which histamine is formed by a tissue is of greater significance than the amount of histamine actually present in that tissue. Thus, the determination of histidine decarboxylase activity represents a dynamic approach to the elucidation of the physiological function of histamine, in contrast to the earlier static approach based on measurements of the histamine content of tissues . 

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. 

While the carbon dioxide produced by decarboxylation of histidine can be measured by the standard Warburg manometer technique, the utility of the method is severely limited by the very small degree to which decarboxylation occurs with most mammalian histidine decarboxylase preparations even at high substrate concentrations. In practice, only bacterial histidine decarboxylases have proved sufficiently active to be measured conveniently by the manometric method . Small amounts of carbon dioxide can, however, be determined by the sensitive micro-diffusion technique of Conway , and this has been used successfully for measuring the activity of mammalian histidine decarboxylases . 

In rats treated with the liver carcinogen diethylnitrosamine (DENA), the mean activities of histidine decarboxylase (measured at pH 8 0), DOPA decarboxylase and 5-HTP decarboxylase in the liver were significantly lower than the corresponding mean values in the livers of control rats. In both the control and DENA-treated series, however, a positive correlation was found between the activities of these three enzymes . When rats were maintained on a tryptophan-deficient diet, it was found that the mean activities of histidine decarboxylase (pH 8-0), DOPA decarboxylase and 5-HTP decarboxylase in the livers of the tryptophan-deficient animals were significantly... 

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... 

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