| Kinase Assay | Enzyme Assays and Methods. : TS activity is assayed using a spectrophotometric method, which involved monitoring the increase in absorbance at 340 nm resulting from formation of the product, 7,8-dihydrofolate. The assay buffer contains 50 mM N-tris[hydroxymethyljmethyl-2-aminoethanesulfonic acid, 25 mM MgC12, 6.5 mM formaldehyde, 1 mM EDTA, and 75 mM 2-mercaptoethanol, pH 7.4. The concentrations of deoxyuridylate monophosphate, 6R-MTHF, and hIS are 100 μM, 30 μM and 30 nM (1.7 milliunits/mL), respectively. At the 6R-MTHF concentration, an uninhibited reaction and six concentrations of inhibitor are assayed. Ki app values are determined by fitting the data to the Morrison equation using nonlinear regression analysis with the aid of the program ENZFITTER. Ki values are calculated using the equation: Ki app= Ki(1 + [S]/Km), where [S] is equal to 30 μM and Km is equal to 3 μM. DHFR activity is assayed spectrophotometrically by monitoring the dis appearance of the substrates NADPH and 7,8-dihydrofolate at 340 nm. The reaction takes place at 25°C in 0.5 mL of 50 mM potassium phosphate buffer, which contains 150 mM KC1 and 10 nM 2-mercaptoethanol, pH 7.5, and 14 nM (0.34 milliunitlmL) DHFR. The NADPH concentration is 10 μM and 7,8-dihydrofolate is varied at 5, 10, or 15 μM. At each 7,8-dihydrofolate concentration, an uninhibited reaction and seven concentrations of inhibitor are assayed. The ENZFITI'ER microcomputer program is used to obtain Ki app values by fitting the data to the Morrison equation by nonlinear regression analysis. Ki app= Ki(1 + [S]/Km), where [S] is equal to the concentration of 7,8-dihydrofolate used and Km of 7,8-dihydrofolate is equal to 0.15 μM. GARFT activity is assayed spectrophotometrically by monitoring the increase of absorbance resulting from formation of the product 5,8-dideazafolate at 295 nm. The reaction solvent contains 75 mM HEPES, 20% glycerol, and 50 mM a-thioglygerol, pH 7.5, at 25°C.The concentrations of substrates and enzyme used are 10 μM α,β-glycinamide ribonucleotide, 0-10 μM 10-formyl-5,8-dideazafolic acid, and 10 nM (1.9 milliunits/mL) GARFT. Ki values are calculated using the Enzyme Mechanism program of the Beckman DU640 spectrophotometer, which uses nonlinear regression analysis to fit data to the Michaelis-Menten equation for competitive inhibition. |