Abstract
A kinetic enzymological assay for methotrexate [an antineoplastic agent] in human serum is described; the results are available after 1.5 h. The lower limit of sensitivity is 50 nmol/l; the day-to-day variation coefficients compare favorably with other procedures (about 7% in the 200-600 nmol/l range). The method uses purified dihydrofolic acid reductase [EC 1.5.1.3] from bovine liver. The problem of its instability in the assay is circumvented by several measures, e.g., the use of a reaction rate analyzer.

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