Abstract
Rat liver xanthine dehydrogenase exists as NAD-dependent type(type-D) in the freshly prepared sample but through the procedure of purification most of type-D enzyme is converted to O2-dependent type(type-O). This conversion occurs either reversibly through the oxidation of sulfhydryl groups or irreversibly through the proteolysis of enzyme molecule. In the present experiment using purified type-D enzyme 8 moles of sulfhydryl groups per mole of FAD were modified with 4,4′-dithiodipyridine for the complete conversion. 2 moles of 4-thiopyridone were formed by addition of 1 mole of 4, 4′-dithiodipyridine, suggesting that these sulfhydryl groups were located closely to each other. Modification with other sulfhydryl reagents showed that sulfhydryl groups essential for the conversion existed in the hydrophobic environments. This enzyme is known to consist of two identical subunits. Each subunit contains one FAD, two iron-sulfur centers and one molybdo-pterine as cofactors. Anaerobic titration of both types of enzyme showed that the redox properties of FAD of type-D enzyme was different from that of type-O enzyme, but that the former became similar to the latter in the presence of NAD. This suggests that the binding of NAD modulates the reactivity of type-D enzyme. The steady state kinetics of both types of enzyme showed that Km value for O2 of type-D enzyme was considerably higher than that of type-O ana that Vmax of xanthine-O2 activity of type-D enzyme was about one-fourth of that of type-O enzyme.
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Saito, T., Nishino, T. & Tsushima, K. 131 DIFFERENCES IN PROPERTIES BETWEEN THE NAD-DEPENDENT AND O2-DEPENDENT TYPES OF RAT LIVER XANTHINE DEHYDROGENASE. Pediatr Res 24, 133 (1988). https://doi.org/10.1203/00006450-198807000-00155
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DOI: https://doi.org/10.1203/00006450-198807000-00155