Structure of tropinone reductase-II complexed with NADP+ and pseudotropine at 1.9 A resolution: implication for stereospecific substrate binding and catalysis.

Tropinone reductase-II (TR-II) catalyzes the NADPH-dependent reduction of the carbonyl group of tropinone to a beta-hydroxyl group. The crystal structure of TR-II complexed with NADP+ and pseudotropine (psi-tropine) has been determined at 1.9 A resolution. A seven-residue peptide near the active site, disordered in the unliganded structure, is fixed in the ternary complex by participation of the cofactor and substrate binding. The psi-tropine molecule is bound in an orientation which satisfies the product configuration and the stereochemical arrangement toward the cofactor. The substrate binding site displays a complementarity to the bound substrate (psi-tropine) in its correct orientation. In addition, electrostatic interactions between the substrate and Glu156 seem to specify the binding position and orientation of the substrate. A comparison between the active sites in TR-II and TR-I shows that they provide different van der Waals surfaces and electrostatic features. These differences likely contribute to the correct binding mode of the substrates, which are in opposite orientations in TR-II and TR-I, and to different reaction stereospecificities. The active site structure in the TR-II ternary complex also suggests that the arrangement of the substrate, cofactor, and catalytic residues is stereoelectronically favorable for the reaction.

Crystal structures of two tropinone reductases: different reaction stereospecificities in the same protein fold.

A pair of tropinone reductases (TRs) share 64% of the same amino acid residues and belong to the short-chain dehydrogenase/reductase family. In the synthesis of tropane alkaloids in several medicinal plants, the TRs reduce a carbonyl group of an alkaloid intermediate, tropinone, to hydroxy groups with different diastereomeric configurations. To clarify the structural basis for their different reaction stereospecificities, we determined the crystal structures of the two enzymes at 2.4- and 2.3-A resolutions. The overall folding of the two enzymes was almost identical. The conservation was not confined within the core domains that are conserved within the protein family but extended outside the core domain where each family member has its characteristic structure. The binding sites for the cofactor and the positions of the active site residues were well conserved between the two TRs. The substrate binding site was composed mostly of hydrophobic amino acids in both TRs, but the presence of different charged residues conferred different electrostatic environments on the two enzymes. A modeling study indicated that these charged residues play a major role in controlling the binding orientation of tropinone within the substrate binding site, thereby determining the stereospecificity of the reaction product. The results obtained herein raise the possibility that in certain cases different stereospecificities can be acquired in enzymes by changing a few amino acid residues within substrate binding sites.

[Clinical reevaluation of continuous intravenous infusion of 5-fluorouracil--plasma concentrations and clinical dose by continuous intravenous and 60-min infusions].

Daily and intermittent continuous intravenous infusions [by gravity drip, (IVG) or infusion pump, (IVP)] and intermittent short-time intravenous drip infusion of 5-FU were carried out on advanced cancer patients. The MTD and dose-limiting toxicity were investigated in relation to the plasma concentrations of 5-FU determined by HPLC. Responses in eleven patients receiving IVG administration daily at 8-21 mg/kg/day were NC, but those given 5-FU alone showed no adverse reactions. Plasma concentrations were too low to be determined. In 9 patients receiving IVG or IVP administration weekly at 60 mg/kg for 24 hr, 1 of the 5 evaluable patients showed reduced hepatic metastatic lesions. One of 4 patients receiving IVP administration weekly at 120 mg/kg for 48 hr showed a disappearance of metastatic lesions in the skeletal muscle, but bone marrow suppression was observed as dose-limiting toxicity. Pharmacokinetics were more stable in IVP than in IVG with less individual difference in the plasma concentrations. Among the outpatients receiving short-time iv, IVG administration once or twice a week, 2 patients given weekly administrations at 20 mg/kg for 60 min showed slight adverse reactions. In 6 patients given high-dose administrations, bone marrow suppression was observed. When pharmacokinetics in the patients given 5-FU for 60 min were compared between the IVG and IVP groups, there were individual differences in plasma concentrations, but the differences were not significant. It was concluded from above results that the following practical dose schedules would be recommendable: 60 mg/kg for 24hr/week by IVP for inpatients and 20 mg/kg for 60 min/week by IVG for outpatients.