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 de grees C). The difference spectra of the folate against the non-irradia ted one showed a red shift. The difference absorbance of the peak at t he wavelength above 320 nm increased with the radiation time. Affinity of the irradiated folate (FA(+)), which is a competitive inhibitor, t o the irradiated xanthine oxidase (XO(+)) more increased than the irra diated substrate with increasing temperature of the reaction, while th e affinity to the nonirradiated enzyme (XO(-)) significantly decreased . The difference of the standard entropy change in the FA(+)/XO(-) sys tem to that in the non-irradiated FA(-)/XO(-) system, i.e., Delta(Delt a 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(D elta S-s) of the FA(+)/XO(+) system overcame an increase in the enthal py difference Delta(Delta H-s) of the system, and then the free-energy difference Delta(Delta G(s)) resulted in the most negative value. Thi s means that the driving force by the reaction system depends on an en tropy effect. These results indicate that the red shift of the spectra of FA(+) is due to a solvent ordering effect obtained by the radiatio n, and that the affinity of the FA(+) to the active center of XO(+) in creases when XO has also received the ordering effect.