The measurement of histidine decarboxylase activity

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. 

In vitro Measurements Measurement of carbon dioxide production 

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 . 

The most sensitive method for measuring carbon dioxide production by histidine decarboxylase is that in which histidine labelled with carbon-14 in the carboxyl group is used as substrate. The C02 evolved is trapped in a suitable absorbent medium, and its radioactivity is then determined directly in a scintillation spectrometer. Several variations of this procedure have been described - - - - , and these compare favourably in speed and sensitivity with alternative isotopic methods in which the production of radioactive histamine from ring- C-labelled histidine is measured (see following section). 

Alternatively, the histamine can be determined spectrophotofluorimetri-callyi . With this method, it is essential to remove histidine after the incubation, as the substrate would otherwise contribute to the final fluorescence. 

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