Histidine decarboxylase from rat mast cells. Enhanced recovery in cell-free extracts and isotopic labelling

Homology-based molecular modelling of PLP-dependent histidine decarboxylase from Mmorganella morganii

The 3-D structural information is a prerequisite for a rational ligand design. In the absence of experimental data, model building on the basis of a known 3-D structure of a homologous protein is at present the only reliable method to obtain structural information. A homology model building study of the pyridoxal 5'-phosphate (PLP)-dependent histidine decarboxylase from Morganella morganii (HDC-MM) has been carried out based on the crystal structure of the aspartate aminotransferase from Escherichia coli (AAT-EC). The primary sequences of AAT-EC and HDC-MM were aligned by automated alignment procedure. A 3-D model of HDC-MM was constructed by copying the coordinates of the residues from the crystal structure of AAT-EC into the corresponding residues in HDC-MM. After energy-minimization of the resulting 3-D model of HDC-MM, possible active site residues were identified by fitting the substrate (l-histidine) into the proposed active-site. In our model, several residues, which have an important role in the AAT-EC active-site, are located in positions spatially identical to those in AAT-EC structure. The back-bone of the modelled active site pocket is constructed by residues; Gly-92, Gly-93, Thr-93, Ser-115, Asp-200, Ala-202, Ser-229 and Lys-232 together with residues Asn-8, His-119, Thr-171, His-198, Leu-203, His-231, Ser-236 and Ile-238. In the ligand binding site, it appears that the HDC-MM model will position l-histidine (substrate) in the area consisting of the residues; Glu-29, Ser-30, Leu-38, His-231 and Lys-232. The nitrogen atom of the imidazole ring (N2) of the substrate is predicted to interact with the carboxylate group of Ser-30. The alpha-carboxylate of histidine points toward the Lys-232 to have electrostatic interaction with its side chain nitrogen atom (N(Z)). In conclusion, this combination of sequence and 3-D structural homology between AAT-EC and HDC-MM model could provide insight in assigning the probable active site residues.

Recombinant expression of rat histidine decarboxylase: generation of antibodies useful for western blot analysis

Histidine decarboxylase catalyses the formation of histamine, an important biological messenger. In spite of the essential biological functions exerted by histamine the knowledge about the mechanisms involved in the regulation of histidine decarboxylase is rather limited. This is most likely due to the limited supply of suitable tools, including highly specific antibodies. In the present study we describe the production and characterisation of specific antisera against rat histidine decarboxylase using recombinant protein synthesised in a bacterial expression system. The antisera were shown to effectively immunoprecipitate histidine decarboxylase activity in extracts of fetal rat liver as well as to detect the histidine decarboxylase protein by Western blot analysis of COS-7 cells expressing recombinant rat histidine decarboxylase. The results demonstrate the successful production of highly specific antisera to histidine decarboxylase which may become valuable tools in future studies of the structure and function of this enzyme.