Abstract
Publisher Summary Xanthine—oxygen oxidoreductase is an enzyme that contains riboflavin in its molecule in the form of flavine-adenine dinucleotide (FAD). This chapter focuses on the determination of the xanthine oxidase. It is determined aerobically by Warburg manometry and anaerobically by using triphenyltetrazolium chloride (TTC). The TTC method is colorimetric and more readily adapted for measuring very low titers of the enzyme. It can be made more sensitive by using a longer time of incubation. Titers of xanthine oxidase in human milk or in the milk of animals thatis given tungstate are very low but is measured by the TTC method. Xanthine is omitted to obtain blanks. These blanks are easily measureable in cows' milk but are often negligible in goats' milk. Blanks on boiled cows' milk show color because the enzyme is not completely destroyed by boiling. Large blanks can be encountered in internal organs such as liver, because of the well-known nonspecific reduction due to endogenous respiration. The enzyme, in this type of assay is referred to as xanthine dehydrogenase, is measured by the amount of formazan it produces in a given time.
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