NONTHERMAL EFFECT OF CERAMICS RADIATION ON FOLATE STRUCTURE AND ON XANTHINE-OXIDASE INHIBITION BY FOLATE

Folic acid was irradiated under cooling at 10 degrees C in a slightly alkaline solution with a ceramics radiator (surface temperature: 68 degrees C). The difference spectra of the folate against the non-irradiated one showed a red shift. The difference absorbance of the peak at the wavelength above 320 nm increased with the radiation time. Affinity of the irradiated folate (FA(+)), which is a competitive inhibitor, to the irradiated xanthine oxidase (XO(+)) more increased than the irradiated substrate with increasing temperature of the reaction, while the affinity to the nonirradiated enzyme (XO(-)) significantly decreased. The difference of the standard entropy change in the FA(+)/XO(-) system to that in the non-irradiated FA(-)/XO(-) system, i.e., Delta(Delta S-s) gave a large negative value, while Delta(Delta S-s) of the FA(-)/XO(+) system gave a large positive value. An increase in the Delta(Delta S-s) of the FA(+)/XO(+) system overcame an increase in the enthalpy difference Delta(Delta H-s) of the system, and then the free-energy difference Delta(Delta G(s)) resulted in the most negative value. This means that the driving force by the reaction system depends on an entropy effect. These results indicate that the red shift of the spectra of FA(+) is due to a solvent ordering effect obtained by the radiation, and that the affinity of the FA(+) to the active center of XO(+) increases when XO has also received the ordering effect.