[Molecular biology of L-histidine decarboxylase]

Serum histamine and acetylcholine variations as new noninvasive biochemical markers in staging of experimental hepatocellular carcinoma

Angiogenesis is a major prerequisite for hepatocellular carcinoma (HCC) development and progression. The present study aims to assess the potential role of two endogenous regulators of angiogenesis histamine (His) and acetylcholine (Ach), as possible biochemical markers for staging of HCC. Five groups of rats were used in this study: a control healthy group (I), another 4 intoxicated groups used for the induction of HCC with a high dose of diethyl nitrosamine (DENA, 200 mg/kg, single I.P. dose), (II, III, IV, and V). Groups II, III, IV, and V were killed following 8, 16, 24, and 32 weeks after DENA injection, respectively. Serum level of His and Ach was estimated using high-performance liquid chromatography technique coupled with diode array detector (HPLC-DAD), and alpha-fetoprotein (AFP) was measured using ELISA technique along with liver histological examination for all groups. Progression of HCC was estimated by histopathological examination. The results exhibited prominent increase in serum His and Ach levels during the early stages of HCC in group II, III in comparison with the control, and then His serum level declined to the normal level during the last stage of HCC development (group V).However, Ach elevation continued. AFP serum level showed marked increase, till 32 weeks after hepatocarcinogenesis. The decreased histamine level, combined to elevated AFP, indicates an early stage, while continued elevation of Ach with decreased His levels indicates a later stage of HCC. The combination of these two neurotransmitters to AFP may contribute to a noninvasive biochemical staging for HCC.

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.