Molecular regulation of rat histidine decarboxylase
Abstract: Histamine is involved in a variety of physiological processes like gastric acid secretion, inflammation and neurotransmission. The production of histamine is catalysed by the enzyme histidine decarboxylase (HDC), which is a strongly regulated enzyme. Mammalian HDC mRNA encodes a protein with a molecular mass of 74 kDa, which is greater than that reported for the subunit of purified mammalian HDC, which is 54 kDa. The aim of the present thesis was to analyse whether this discrepancy in size may reflect a posttranslational processing and activation of the enzyme. This was performed by examining the expression of rat HDC in vitro and in vivo. Full-length HDC was essentially inactive when produced in various expression systems, whereas C-terminally truncated HDC corresponding to 54 kDa gave rise to high HDC activity when expressed in the same systems. To generate specific antisera against HDC, the enzyme was expressed in large amounts in bacteria and then purified. The antisera produced were able to immunocytochemically detect HDC in gastric mucosa from a variety of species as well as in rat hypothalamus. A Western blot technique for analysis of HDC was generated which revealed the presence of three different forms of HDC, corresponding to about 74-, 63- and 53 kDa, in rat gastric mucosa and fetal liver. The expression of the multiple forms of HDC in the rat gastric mucosa was studied in experimental situations known to induce HDC activity. It was demonstrated that induction of HDC by hypergastrinemia was reflected in a large increase in the amount of the intermediate form, which also was the most abundant form. Furthermore, the 63 kDa form was shown to have a turnover rate corresponding to that of the enzyme activity, indicating that this form was an enzymatically important form. In summary, the results strongly support the theory that HDC is produced as an inactive proenzyme which is posttranslationally processed into a shorter active form of 54 kDa and/or 63 kDa.
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