The effects of nitric oxide (NO) on xanthine oxidase (XOD) activity an
d the site(s) of the redox center(s) affected were investigated. XOD a
ctivity was determined by superoxide (O-2(-)) generation and uric acid
formation. NO reversibly and dose-dependently suppressed XOD activity
in both determination methods. The suppression interval also disclose
d a dose-dependent prolongation. The suppression occurred irrespective
of the presence or absence of xanthine; indicating that the reaction
product of NO and O-2(-), peroxynitrite, is not responsible for the su
ppression. Application of synthesized peroxynitrite did not affect XOD
activity up to 2 mu M. Methylene blue, which is an electron acceptor
from Fe/S center, prevented the NO-induced inactivation. The results i
ndicate that NO suppresses XOD activity through reversible alteration
of the flavin prosthetic site.