The structure of mouse L1210 dihydrofolate reductase-drug complexes and the construction of a model of human enzyme.

The structure of mouse L1210 dihydrofolate reductase (DHFR) complexed with NADPH and trimethoprim has been refined at 2.0 A resolution. The analogous complex with NADPH and methotrexate has been refined at 2.5 A resolution. These structures reveal for the first time details of drug interactions with a mammalian DHFR, which are compared with those observed from previous X-ray investigations of DHFR/inhibitor complexes. The refined L1210 structure has been used as the basis for the construction of a model of the human enzyme. There are only twenty-one sequence differences between mouse L1210 and human DHFRs, and all but two of these are located close to the molecular surface: a strong indication that the active sites are essentially identical in these two mammalian enzymes.

Receptor-based design of dihydrofolate reductase inhibitors: comparison of crystallographically determined enzyme binding with enzyme affinity in a series of carboxy-substituted trimethoprim analogues.

By the use of molecular models of Escherichia coli dihydrofolate reductase (DHFR), analogues of trimethoprim (TMP) were designed which incorporated various 3'-carboxyalkoxy moieties in order to acquire ionic interactions with positively charged active-site residues. Certain of these compounds have shown exceptionally high affinity for this enzyme. For example, the 3'-(carboxypentyl)oxy analogue was found to be 55-fold more inhibitory than TMP toward E. coli DHFR (Ki = 0.024 nM vs. 1.32 nM for TMP). X-ray crystallographic studies of E. coli DHFR in binary complexes with TMP and two members of this acid-containing series of compounds defined the binding of these inhibitors and showed the carboxyl group of the latter two inhibitors to be ionically bound to Arg-57. These observations were in agreement with postulated binding modes that were based on receptor modeling.

Trimethoprim binding to bacterial and mammalian dihydrofolate reductase: a comparison by proton and carbon-13 nuclear magnetic resonance.

The binding of trimethoprim to dihydrofolate reductase from L1210 mouse lymphoma cells has been studied by measuring the changes in chemical shift of nuclei of the ligand that accompanying binding. The 6- and 2',6'-proton chemical shifts of bound trimethoprim have been determined by transfer of saturation experiments, and the 2-carbon chemical shift has been determined by using [2-13C]trimethoprim. The changes in proton chemical shift are substantially smaller than those accompanying binding to bacterial dihydrofolate reductase [Cayley, P. J., Albrand, J. P., Feeney, J., Robert, G. C. K., Piper, E. A., & Burgen, A. S. V. (1979) Biochemistry 18, 3886]. It is shown that this difference arises largely from the fact that trimethoprim adopts different conformations when bound to mammalian and to bacterial dihydrofolate reductase. The proton chemical shifts are interpreted in terms of ring-current contributions from the two aromatic rings of trimethoprim itself and the nearby aromatic amino acid residues of the enzyme. The latter have been located by using the refined crystallographic coordinates of the Lactobacillus casei and Escherichia coli reductases in their complexes with methotrexate [Bolin, J. T., Filman, D. J., Matthews, D. A. & Kraut, J. (1982) J. Biol. Chem. 257, 13650], under the assumption that, as indicated by the 13C chemical shifts, the diaminopyrimidine ring of trimethoprim binds in the same way as does the corresponding part of methotrexate. With use of these assumptions, the conformation of trimethoprim bound to the dihydrofolate reductases from L. casei, E. coli, and L1210 cells has been calculated.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of angiotensinogen and tetradecapeptide as substrates for human renin. Substrate dependence of the mode of inhibition of renin by a statine-containing hexapeptide.

The kinetic properties of two different substrates for human renin, a synthetic tetradecapeptide and the natural substrate human angiotensinogen, have been compared. While the Vmax was similar for the two substrates, the Km values differed by a factor of 10, i.e., 11.7 +/- 0.7 microM (tetradecapeptide) and 1.0 +/- 0.1 microM (angiotensinogen). The mode of inhibition of renin by a statine (Sta)-containing hexapeptide, BW897C, that is a close structural analog of residues 8-13 of human angiotensinogen (Phe-His-Sta-Val-Ile-His-OMe), was determined for the two substrates. Competitive inhibition was observed when tetradecapeptide was the substrate (Ki = 2.0 +/- 0.2 microM), but a more complex mixed inhibition mode (Ki = 1.7 +/- 0.1 microM, Ki' = 3.0 +/- 0.23 microM) was found with angiotensinogen as substrate. This mixed inhibition probably results from the formation of an enzyme-inhibitor-substrate or enzyme-inhibitor-product complex and reflects the more extensive interactions that the protein angiotensinogen, as opposed to the small tetradecapeptide substrate, can make with renin. We conclude that the mixed inhibition observed when angiotensinogen is used as renin substrate could be important in the clinical application of renin inhibitors because it is less readily reversed by increased concentrations of substrate than is simple competitive inhibition.

Dihydrofolate reductase: low-resolution mass-spectrometric analysis of an elastase digest as a sequencing tool.

An elastase digest of a protein of unknown structure, dihydrofolate reductase, was studied by mass spectrometry. This soluble digest contained a large number of small peptides in different yields, within the ideal molecular-weight range (200-1200) for mixture-analysis mass spectrometry. Sequences of the major component peptides in the digest are reported.

Human renin: a new class of inhibitors.

A new class of human renin inhibitor is described, containing a novel analogue of the peptide bond. High inhibitory potency was observed for octapeptide-length substrate analogues but inhibition progressively weakened as the molecule was shortened from the amino terminal end.

Large-scale purification and characterization of dihydrofolate reductase from a methotrexate-resistant strain of Lactobacillus casei.

Dihydrofolate reductase has been purified from a methotrexate-resistant strain of Lactobacillus casei NCB 6375. By careful attention to growth conditions, up to 2.5 g of enzyme is obtained from a 400 litre culture. The purification procedure, involving poly-ethyleneimine treatment, DEAE-cellulose chromatography and affinity chromatography on methotrexate-aminohexyl-Sepharose, operates on the gram scale, with overall yields of 50-60%. Elution of the affinity column by reverse (upward) flow was used, as it led to recovery of the enzyme in a much smaller volume. The enzyme obtained appears to be more than 98% pure, as judged by gel electrophoresis, isoelectric focusing, and gel filtration. It has a mol.wt. of approx. 17900 and a turnover number of 4s-1 (50mM-triethanolamine/400mM-KCl, pH 7.2, 25 degrees C) with dihydrofolate and NADPH as substrates. The turnover number for folate is 0.02s-1. Michaelis constants for a variety of substrates have been measured by using a new fluorimetric assay (0.36 muM-dihydrofolate; 0.78 muM-NADPH), and binding constants determined by using the quenching of protein fluorescence (dihydrofolate, 2.25 X 10(6)M-1; NADPH, greater than 10(8)M-1). The pH/activity profile shows a single maximum at pH 7.3; at this pH, marked activation by 0.5M-NaCl is observed.