Abstract
At pH 7.0, in air, 20% of the total electron flux through xanthine oxidase can be accounted for in terms of the univalent reduction of oxygen. The fraction of the total flux of electrons which traversed the univalent pathway to oxygen was increased by raising the pH and by raising the oxygen tension. It was further shown that at any given pH and oxygen tension, the amount of univalently reduced oxygen, which was detectable in terms of the reduction of cytochrome c, rose as the turnover rate of the enzyme was decreased by decreasing the concentration of xanthine. This effect of xanthine was more pronounced at pH 7.0 than at pH 10.0. Another reflection of this same phenomenon was a difference in Km for xanthine measured in terms of urate production as compared to Km for xanthine measured in terms of cytochrome c reduction. Here too the differences were diminished as the pH and the oxygen tension were raised. The quantitative aspects of these phenomena are presented as well as an explanation which is consistent with all of the observations and which was, in fact, predictive of several of them.
Highlights
Effect of Cytochrome c on 02 Uptake at pH lO.O-When xanthine oxidase (5.3 x low[9] M) was allowed to act upon xanthine (4.2 x 10m4 M) at pH 10.0, oxygen was depleted at a linear rate of 9.4 x low6 ~/lo[0] sec
We conclude that under these conditions about 55% of the total flux of electrons from xanthine to oxygen goes by the univalent route which results in the release of 02. from the enzyme
This indicates that raising the oxygen concentration should increase the percentage of the total electron flux which proceeds by the univalent pathway
Summary
At pH 7.0, in air, 20% of the total electron flux through xanthine oxidase can be accounted for in terms of the univalent reduction of oxygen. This effect of superoxide dismutase would constitute an important control since it eliminates the possibility that cytochrome c might inhibit oxygen uptake by some direct effect on the turnover rate of xanthine oxidase Both of these methods have been applied and the fraction of the total oxygen reduction which is accomplished by the univalent pathway has been assessed at pH 7.0 and at pH 10.0. In the course of this work it was observed that variation in the concentration of xanthine had a definite effect upon the percentage of the total electron flux which occurred by the univalent pathway This led to the proposal that the univalent reduction of oxygen by xanthine oxidase occurs within a space which is not accessible to cytochrome c and which is large enough to allow some spontaneous dismutation of 0~ in the time required for this radical to diffuse out of this space.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
