Three forms of xanthine: acceptor oxidoreductase in rat heart.

Abstract

The enzyme xanthine: acceptor oxidoreductase found in rat heart equilibrates between three forms differing in electron acceptor specificity. Form D transfers electrons exclusively to NAD+ and accounts for 85% of total oxidoreductase activity. Form O transfers electrons to molecular oxygen and accounts for 8%. The D/O form prefers NAD+, but without NAD+ transfers electrons to oxygen. Interconversion from D to O and O to D forms is catalyzed by sulfhydryl group-modifying reagents: Cd2+, Cu2+, disulfiram, and heating with dithiothreitol. This suggests that sulfhydryl groups participate in the first stage of enzyme conversion. The NADH/NAD+ concentration ratio may regulate the dehydrogenase activity of xanthine:acceptor oxidoreductase (NAD+-dependent activity of D and D/O forms). Accumulating NADH inhibits hypoxanthine hydroxylation. The amount of form O increases during cardiac ischemia, facilitating superoxide radical-ion generation. Also, NADH/NAD+ does not regulate form O, promoting adenylate nucleotide pool depletion, especially in the heart which has low de novo purine nucleotide synthesis.

Identification of agents that reduce renal hypoxia–reoxygenation injury using cell-based screening: Purine nucleosides are alternative energy sources in LLC-PK1 cells during hypoxia

“…As adenosine also provides the cells with large amount of xanthine load it may promote ROS production by the xanthine oxidoreductase during the reoxygenation phase. Xanthine oxidoreductase (also known as xanthine: acceptor oxidoreductase) represents an interconvertible enzyme that is mainly present in the cells as xanthine: NAD + oxidoreductase (xanthine dehydrogenase or D form of the enzyme) and transfers electrons to NAD + , however it is also converted to the more stable xanthine: oxygen oxidoreductase (xanthine oxidase or O type of the enzyme) during hypoxia [55, 56]. The depletion of NAD + and the accumulation of hypoxanthine may also block the xanthine dehydrogenase to some extent, but the lack of oxygen certainly inhibits xanthine oxidase during hypoxia.…”

Xanthine oxidoreductase activity in perfused hearts of various species, including humans.

“…We reported blockade of hypoxanthine breakdown during anoxia by allopurinol. 4 [4][5][6][7][8]18 The conversion of xanthine to urate seems to be faster in guinea pig than in mouse and rat hearts. This should be checked with xanthine as the substrate.…”

The activity of xanthine oxidase in heart of pigs, guinea pigs, rabbits, rats, and humans

“…The average activity of xanthine oxidase and dehydrogenase in rat heart was 19 nmol/rnin/g wet weight as compared to activities around 30 nmol/min/g of tissue reported by Schoutsen et al and by Kaminski et al (11,33). These differences can be explained adequately by the fact that our enzyme assays was performed at room temperature and at pH 6.8, while other investigators performed their assay at higher temperatures and at pH 8.0 (the pH optimum of xanthine oxidase) (14).…”

“…Under physiologic conditions xanthine oxidase is primarily a dehydrogenase which utilizes NAD + as the electron acceptor (D-type xanthine oxidase) but, under certain circumstances, it is converted to a form utilizing oxygen as the electron acceptor (O-type xanthine oxidase) (3,6,11,20). In all tissues examined (except rabbit heart) a fraction of the enzyme was present in the O-form, which most likely reflects the conversion of D-type xanthine oxidase to the O-form by purification procedures and by freezing, as has been described by other investigators (3,6,20).…”