Thymidylate synthetase purified to homogeneity from human leukemic cells

Thymidylate synthetase (5,10-methylenetetrahydrofolate:dUMP C-methyltransferase, EC 2.1.1.45) from a human leukemic cell line has been purified to homogeneity with one-step affinity column chromatography. The purified enzyme has a specific activity of 3.8 micron/min per mg of protein, which corresponds to a turnover number of 250. These are the highest values reported for a thymidylate synthetase from neoplastic tissue. A ratio of 1.7 mol of 5-fluoro-2'-deoxyuridylate binds per mol of enzyme in the presence of 5,10-methylenetetrahydrofolate. The ternary complex so formed migrates intact on denaturing gels and can be precipitated with trichloroacetic acid; however, urea dissociates the ternary complex. The human thymidylate synthetase is composed of two subunits of 33,000 daltons each. It contains more residues of cysteine, glycine, and arginine and fewer of histidine than the well-studied thymidylate synthetase from Lactobacillus casei.

The interaction of liver alcohol dehydrogenase with phenyl adenine dinucleotide, a novel analog of pyridine nucleotide coenzymes

We have synthesized phenyl adenine dinucleotide (P1-adenosine-5')-P2-(beta-D-ribofuranosylbenzene-5')-pyrophosphate (PhAD), a novel analog of pyridine nucleotide coenzymes. This compound contains a planar aromatic ring, as does NAD+, but lacks a positive charge. PhAD is an inhibitor of horse liver alcohol dehydrogenase, competitive with NADH. PhAD is very similar to NAD+ sterically since both compounds have a planar aromatic ring. However, PhAD resembles NADH in binding to the enzyme because the dissociation constants of both compounds show a parallel increase as the pH is raised, whereas those of NAD+ behave in the opposite manner. These observations indicate that the enzyme differentiates between NAD+ and NADH on the basis of the positive charge on the molecule and not the stereochemical orientation of the reduced nicotinamide ring.

Inhibition of hexokinase by multisubstrate analogs

The compounds P1-(adenosine-5')-P3-(glucose-6) triphosphate (Ap3 glucose) and P1-(adenosine-5')-P4-(glucose-6)tetraphosphate (Ap4 glucose) were synthesized as possible transition-state analogs for hexokinase (ATP: D-hexose 6- phosphotransferase, EC 2.7.1.1). Both compounds were inhibitors of this enzyme, competitive against ATP and apparently uncompetitive against glucose. The inhibition constants for Ap3 glucose and Ap4 glucose were 0.43 mM and 0.37 mM, respectively. These results indicate that the inhibitors do not appreciably bind to hexokinase until the glucose binding site is filled. The sugar portion of the inhibitors therefore does not contribute to binding, and the compounds are acting as ATP analogs, Ap3 glucose and Ap4 glucose are also slow substrates for glucose-6-phosphate dehydrogenase.

Synthesis and biological studies of 3-(beta-D-ribofuranosyl)-2,3,-dihydro-6H-1,3-oxazine-2,6-dione, a new pyrimidine nucleoside analog related to uridine

Reaction of the trimethylsilyl derivative of 2,3-dihydro-6H-1,3-oxazine-2,6-dione (2, "uracil anhydride") with protected 1-O-acetylribofuranoses in the presence of stannic chloride gave the corresponding block nucleosides. 3-(2,3-5-Tri-O-2',2',2'-trichloroethoxycarbonyl-beta-d-ribofuranosyl)-2,3-dihydro-6H-1,3-oxazine-2,6-dione (4c) thus prepared from the protected sugar 3c, 1-O-acetyl-2,3,5-tri-O-(2,2,2-trichloroethoxycarbonyl)ribofuranose, gave, on removal of the protecting groups with zinc dust,3-(beta-d-ribofuranosyl)-2,3-dihydro-6H-1,3-oxazine-2,6-dione (1). The structure of 1 was confirmed by uv, ir, NMR, and CD spectral data and was shown to be an N nucleoside. Uracil anhydride, 2, and, to a lesser extent, its ribonucleoside 1 exert a moderate growth inhibition of mouse leukemia L5178Y, HeLa, and Novikoff hepatoma cells i- culture. Both compounds produce weak inhibition of vaccinia viral replication in HeLa cells.

The effect of Raney nickel on the covalent thymidylate synthetase-5-fluoro-2'-deoxyuridylate-5,10-methylenetetrahydrofolate complex

Raney nickel (Ni(H)) catalyzes a specific reductive cleavage of carbon-sulfur bonds and, therefore, can be used to determine whether compounds are covalently bound to proteins through a sulfide linkage. When the covalent thymidylate synthetase-[3H]5-fluoro-2'-deoxyuridylic acid-[14C]-5,10-CH2H4-folate complex (Langenbach et al. (1972a), Biochem, Biophys. Res. Commun. 48, 1565) was denatured and then shaken with Ni(H) at 25 degrees C, both isotopes were rapidly cleaved from the protein, with identical reaction halftimes of less than 10 min. The liberated radioactivity was filterable through nitro-cellulose filters and comigrated with small molecules on Sephadex G-25. Both labels migrated identically upon paper chromatography. A [3H]5-fluoro-2'-deoxyuridylic acid-[35S]thymidylate synthetase complex was formed with enzyme isolated from Lactobacillus casei grown in the presence of [35S]cysteine. This complex, upon Ni(H) treatment, released both tritium and sulfur-35 at identical rates. Control experiments on amino acids showed that only the sulfur-containing amino acids are degraded by Ni(H). Cysteine was rapidly converted to alanine and methionine to alpha-aminobutyric acid. 5-Carboxymethylcysteine and 5-uracilylcysteine, simple models for the tenary enzyme-5-fluoro-2'-deoxyuridylic acid-5,10-CH2H4-folate complex, were converted to alanine at the same rate that 5-fluoro-2'-deoxyuridylic acid (FdUrd-5'-P) was cleaved from the enzyme. Native ribonuclease, which has a tightly coiled structure, was not affected by the reagent, but carboxymethylated ribonuclease was desulfurized. Amino acid analysis of Ni(H)-treated thymidylate synthetase showed that cysteine was the only amino acid degraded. Gel electrophoresis of the proteins after exposure to Ni(H) showed no breakage of polypeptide chains. These results support a sulfide linkage between FdUrd-5'-P and thymidylate synthetase in the covalent complex.

Growth inhibition of cells in cultures and of vaccinia virus infected HeLa cells by derivatives of trifluorothymidine

The effects of 5-trifluoromethyluracil (F3Thy), 5-trifluoromethyl-2'-deoxyuridine (F3dThd), F3dThd-5'-P, and F3dThd-5'-methylphosphonate on the growth of HeLa cells, Novikoff hepatoma cells, L5178Y mouse leukemia cells, and on the replication of vaccinia virus in HeLa cells have been determined. F3Thy and F3dThd-5'-methylophosphonate were approximately 500-fold and 100-fold less effective, respectively, than F3dThd or F3dThd-5'-P in their inhibition of these cells. F3dThd and F3dThd-5'-P are potent inhibitors of vaccinia viral replication of HeLa cells. The nucleoside and nucleotide were 1,000-fold more inhibitory than the free base. F3dThd was nost inhibitory when added between 1 and 2 h post-infection; however, it was also somewhat inhibitory when added at later times.

Inhibition of phenylalanyl-tRNA synthetase by aromatic guanidines and amidines

Aromatic guanidines and amidines were investigated for their ability to inhibit phenylalanyl-tRNA synthetase from E. coli B. 2-Phenylacetamidine (1), benzylguanidine (2), and N-benzylbenzamidine (3) are competive inhibitors with respect to phenylalanine, binding nearly as well as the substrate, The remainder of the inhibitors was unexpectedly found to be noncompetitive, indication the presence of a secondary binding site on the enzyme. Inhibition by these compounds appears to be specific for phenylalanyl-tRNA synthetase and requires the presence of a phenyl ring as well as the amidine or guanidine moiety.