title: Methotrexate assay by enzymatic inhibition, with use of centrifugal analyzer.
abstract: We present a method for measuring methotrexate by enzymatic inhibition, with use of the centrifugal analyzer. In the presence of dihydrofolate reductase (EC 1.5.1.3), dihydrofolic acid is converted to tetrahydrofolate. NADPH acts as a coenzyme. Methotrexate inhibits the action of dihydrofolate reductase. Increasing concentrations of methotrexate depress the conversion of NADPH to NADP+, and the reaction may be followed at 340 nm. Precision, accuracy, and sensitivity are satisfactory. A comparison to the radioimmunoassay method (x) showed the following least-squares regression: y = 0.9813x + 5.6 microgram/liter;  r = .9812; Sy = 6.3 microgram/liter. Some interference was noted from bilirubin, lipermia, and hemoglobin. Up to 60 patients' samples per hour may be estimated by this technique.
mesh_terms: D002498:Centrifugation / Q000379:methods; D003201:Computers; D005493:Folic Acid Antagonists; D006801:Humans; D008727:Methotrexate / Q000097:blood* / Q000494:pharmacology; D012044:Regression Analysis
chemical_list: D005493:Folic Acid Antagonists; D008727:Methotrexate