 Xanthine dehydrogenase

Xanthine dehydrogenase is an enzyme that oxidizes xanthine to urate while reducing the oxidized form of nicotinamide adenine dinucleotide. The IUBMB classifies this activity as EC 1.17.1.4 and gives the overall reactionThe conversion therefore couples a change in the purine substrate to transfer of reducing equivalents to a second molecule.^[ref]

A constructed stoichiometric example starts with $5\mu \mathrm{mol}$ of xanthine and $3\mu \mathrm{mol}$ of oxidized nicotinamide adenine dinucleotide, with sufficient water and no regeneration of the electron acceptor. At most $3\mu \mathrm{mol}$ can undergo the stated conversion. That would produce $3\mu \mathrm{mol}$ each of urate and reduced nicotinamide adenine dinucleotide, leaving $2\mu \mathrm{mol}$ xanthine. The smaller available stoichiometric amount limits this idealized reaction extent; more enzyme cannot supply the missing electron acceptor.

The electron acceptor also distinguishes the named activity from xanthine oxidase, which uses oxygen. The mammalian protein can convert between dehydrogenase and oxidase forms under particular conditions, including changes during purification; the names therefore describe experimentally distinguishable catalytic behavior rather than two entirely unrelated substrate names.^[ref]

A meaningful activity comparison must consequently specify both substrate and acceptor conditions. Loss of the NAD-dependent readout could reflect altered reaction conditions or enzyme form, rather than absence of the protein. Conversely, detecting a protein alone does not demonstrate a particular turnover rate. The chemical equation identifies what must be supplied and measured; enzyme kinetics determines how quickly the conversion proceeds.