Fluorescent analogs of methotrexate: characterization and interaction with dihydrofolate reductase

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Abstract
The dansylated derivatives of lysine and ornithine analogs of methotrexate [MTX] exhibit fluorescence properties characteristic of the dansyl moiety, with an excitation at 328 nm and an emission maximum at 580 nm in aqueous media. As in the case of dansyl amino acids, the fluorescence emission is dependent on the polarity of the medium. In solvents of low dielectric constant, there is an enhancement of the dansyl fluorescence intensity as well as a shift to shorter wavelengths. The dansylated analogs show a reduction in the quantum yields as compared to N.epsilon.-dansyl-L-lysine and 5-(N,N-dimethylamino)-1-naphthalenesulfonic acid. The absorption spectra of the 2 dansyl analogs are similar to the spectra of the parent basic amino acid precursors, but with reduced molar extinction values. The 2 fluorescent analogs of MTX were potent inhibitors of purified dihydrofolate reductases from Lactobacillus casei and from chicken liver. The binding of these fluorescent analogs to either dihydrofolate reductase resulted in 10- to 15-nm blue shift of the ligand emission maxima and a 2- to 5-fold enhancement of the emission. These fluorescent properties of the bound ligands indicate a possible interaction of the dansyl moiety with a region on the enzyme molecule which is more hydrophobic relative to the surrounding solvent.

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