M. Kohashi et al., NONTHERMAL EFFECT OF CERAMICS RADIATION ON FOLATE STRUCTURE AND ON XANTHINE-OXIDASE INHIBITION BY FOLATE, Journal of nutritional science and vitaminology, 39(5), 1993, pp. 425-435
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.