Factors which modify the histidine decarboxylase activity of tissues

FACTORS WHICH MODIFY THE HISTIDINE DECARBOXYLASE ACTIVITY OF TISSUES 

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 histidine decarboxylase activity of rat lung is considerably depressed by the administration of glucocorticoids whereas the aetivity in the lung of mice or guinea-pigs is virtually unchanged . In the rat, treatment with 

Merely to state that the physiological function of the histidine decarboxylases is the conversion of histidine to histamine is to evade the problem of the function of histamine itself. A compromise will therefore be adopted so that due aecount can be taken of the ways in which the search for possible roles for histamine in physiological processes has shed light on the function of the enzymes by which histamine is formed. 

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 . 

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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. 

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