Binding of methotrexate to dihydrofolate reductase and its relation to protonation of the ligand.

Stopped-flow spectrophotometry and stopped-flow fluorometry have been used to study the binding of methotrexate (MTX) and 3-deazamethotrexate (3-deazaMTX) to dihydrofolate reductase (DHFR) isoenzymes from Streptococcus faecium and from Lactobacillus casei. The absorbance change and fluorescence quenching that occur when MTX binds to DHFR isoenzyme II from S. faecium (SFDHFR II) are both biphasic and give similar apparent rate constants for both phases. The faster phase has an apparent rate constant that is dependent on MTX concentration and therefore corresponds to the initial binding reaction. From the concentration dependence it has been calculated that the association rate constant is 3.0 X 10(5) M-1 s-1 at 20 degrees C and pH 7.3, and the association constant (equilibrium constant) under these conditions is 5.8 X 10(5) M-1. By examination of the amplitude of the fast-phase absorbance change at various wavelengths, it has been determined that the absorbance change occurring in the fast phase is due to MTX protonation. Within the limits of the method it was thus not possible to detect a difference in the rates of binding and of protonation of MTX. The MTX association rate constant is pH dependent, decreasing 330-fold as the pH is decreased from 5.0 to 9.0. The data fit well to a curve generated by assuming a single ionization with a pKa of 6.0 and a pH-independent association rate constant 1000-fold greater for binding of protonated MTX to SFDHFR II than for binding of unprotonated MTX.(ABSTRACT TRUNCATED AT 250 WORDS)