Computational modeling of a binding conformation of the intermediate L-histidinal to histidinol dehydrogenase.

Histidinol dehydrogenase (HDH) is one of the enzymes involved in the L-histidine biosynthesis pathway. HDH is a dimer that contains one Zn2+ ion in each identical subunit. In this study, we predicted a possible binding conformation of the intermediate L-histidinal, which is experimentally not known, using a computational modeling method and three potent HDH inhibitors whose structures are similar to that of L-histidinal. At first, a set of the most probable active conformations of the potent inhibitors was determined using two different pharmacophore mapping techniques, the active analogue approach and the distance comparison method. From the most probable active conformations of the three potent inhibitors, the common parts of the L-histidinal structure were extracted and refined by energy minimization to obtain the binding conformation of L-histidinal. This predicted conformation of L-histidinal agrees with an experimentally determined conformation of L-histidine in a single crystal, suggesting that it is an experimentally acceptable conformation. The capability in this conformation to coordinate a Zn2+ ion was examined by comparing the spatial relative geometry of its functional groups with those of ligands that coordinate with a Zn2+ ion in Zn proteins of the Protein Data Bank. This comparison supported our predicted conformation.

Discovery of IRL 3461: a novel and potent endothelin antagonist with balanced ETA/ETB affinity.

IRL 3461, N-butanesulfonyl-[N-(3,5-dimethylbenzoyl)-N-methyl-3-[4-(5-+ ++isoxazolyl) -phenyl]-alanyl]-(L)-valineamide, a potent and bifunctional (ETA + ETB) [Ki(ETA) = 1.8 nM, Ki(ETB) = 1.2 nM] antagonist was discovered by structural modification of IRL 2500, an ETB selective antagonist. IRL 3461 was found to be stable on incubation with human, rat, mouse, and guinea pig plasmas.

Crystallization and preliminary crystallographic analysis of cabbage histidinol dehydrogenase.

Recombinant Brassica oleracea histidinol dehydrogenase (HDH) has been crystallized in various space groups using the method of vapour diffusion. The presence or absence of inhibitors and substrates as well as the use of different precipitants has enabled the growth of five different crystal forms. Extensive searches with the first crystal form (A) failed to produce any useful heavy-atom derivatives, mainly because of the instability of the crystals. This provoked the search for further crystal forms in the hope of finding more suitable crystals. At least two of these crystal forms are of interest for further study.

113Cd nuclear magnetic resonance studies of cabbage histidinol dehydrogenase.

Histidinol dehydrogenase (HDH), a dimeric protein, catalyzes two sequential oxidation reactions to yield L-histidine from L-histidinol via L-histidinal. HDH contains 1 mol of Zn(II) per mol of subunit, and removal of this metal abolishes the enzymatic activity. On substitution of Zn(II) with 113Cd(II), the enzyme ([113Cd]HDH) showed similar catalytic activity. The 113Cd NMR spectra of [113Cd]HDH were measured under various conditions. The 113Cd NMR spectrum of [113Cd]HDH showed a resonance at 110 ppm, which indicates that the metal ion is bound to the protein by a combination of nitrogen and oxygen ligands. 113Cd NMR spectra of [113Cd]HDH were measured as complexes with two substrates (L-histidinol and DL-histidinal) and four inhibitors (imidazole, histamine, L-histidine, and DL-4-(4-imidazolyl)-3-amino-2-butanone) in the absence and presence of NAD+. Significant shifts of [113Cd]-HDH resonance in the presence of the ligand indicate that the metal ion is located in the catalytic site of HDH and that substrates and inhibitors interact with the metal ion. The role of the metal ion in the HDH reaction is discussed.

A Novel Class of Herbicides (Specific Inhibitors of Imidazoleglycerol Phosphate Dehydratase).

A new mode of herbicidal action was established by finding specific inhibitors of imidazoleglycerol phosphate dehydratase, an enzyme of histidine (His) biosynthesis. Three triazole phosphonates inhibited the reaction of the enzyme with Ki values of 40 [plus or minus] 6.5, 10 [plus or minus] 1.6, and 8.5 [plus or minus] 1.4 nM, respectively, and were highly cytotoxic to cultured plant cells. This effect was completely reversed by the addition of His, proving that the cytotoxicity was primarily caused by the inhibition of His biosynthesis. These inhibitors showed wide-spectrum, postemergent herbicidal activity at application rates ranging from 0.05 to 2 kg/ha.

[The role of intestinal flora in the pathogenesis of infection and aggravation of experimental acute pancreatitis in rats].

This study was undertaken to determine the role of bacterial translocation in the pathogenesis of infection and aggravation of the course of experimental acute pancreatitis in rats. Acute pancreatitis was induced by the injection of 3% sodium taurocholate. The rats were divided into following 3 groups: acute pancreatitis only (AP), total colectomy + pancreatitis (TCAP), acute pancreatitis with ED preparation (EDAP). The positive rate of bacteriological culture of pancreatic tissue was 40.7% at 24 hours in AP group and 50.0% in EDAP group, but in TCAP group, pancreatic tissue was sterile at 6.12 hours and at 24 hours positive rate was only 6%. There was a significant reduction of bacterial contamination in TCAP group compared with AP and EDAP groups. In the bacteriological culture of the liver, spleen and MLN, bacterial contamination was reduced in TCAP group. Blood endotoxin level elevated gradually compared to the level before induction of acute pancreatitis. At 24 hours, there was a significant difference between TCAP group and EDAP group. In TCAP group, survival rate was improved at 24 hours compared to AP group and EDAP group. Infectious complication during the experimental acute pancreatitis in rats can be explained by the bacterial translocation of intestinal flora, especially colonal bacteria, which may result in aggravation of pancreatitis.

Steady-state kinetics of cabbage histidinol dehydrogenase.

Cabbage histidinol dehydrogenase (HDH) oxidizes L-histidinol to L-histidine through two sequential NAD(+)-linked reactions via an alkaline-labile, L-histidinaldehyde intermediate. The kinetic mechanism of the overall reaction as well as the partial reactions involved in the overall catalysis were investigated at pH 7.2 using L-histidinaldehyde as a substrate. Product inhibition patterns conformed to a Bi Uni Uni Bi Ping Pong mechanism as reported for the HDH from Salmonella typhimurium. Thus, the reaction scheme is ordered with the binding of histidinol first and NAD+ second, and histidine is the last product to be released. The intermediate, L-histidinaldehyde, could be a substrate for both the oxidation and the reduction reactions to produce histidine and histidinol, respectively. L-Histidine was not enzymatically reduced in the presence of NADH, indicating that the reaction to oxidize histidinaldehyde is apparently irreversible. L-Histidinaldehyde exhibited a three times greater binding rate constant than histidinol with a considerably small dissociation constant. These results were in agreement with the observation that histidinaldehyde was not released during the overall reaction. The rate of the reduction of histidinaldehyde to histidinol was almost same as that of the overall oxidation reaction. The overall oxidation from histidinol to histidine proceeded about three times slower than the partial oxidation from histidinaldehyde to histidine, suggesting that the first-half forward reaction is the rate-determining step in the total reaction of cabbage HDH.