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Presence of Inhibitors and Activators of Xanthine Oxidase in Milk¹

Department of Food Science, University of Illinois, Urbana

ABSTRACT

Xanthine-oxidase activity of milk as determined by the catalytic reduction of triphenyl tetrazolium chloride with xanthine as substrate demonstrated that milk contains activators and inhibitors of this enzyme. Heating milk to 91.4 C for 4.2 sec was sufficient to inactivate the enzyme, but did not destroy activators or inhibitors. Efforts to identify these components revealed that all of the major milk protein fractions are activators or inhibitors, depending upon their concentration in the reaction mixture. A nonionic dialyzable inhibitor was also found in the natural-protein-free milk system. The action of these substances upon xanthine oxidase can be explained in a manner similar to that proposed by Hofstee for substrate inhibition.

To determine the xanthine-oxidase content of the various fractions of cow's milk obtained during fractionation procedures, it is necessary to add an amount of protein equivalent to that in milk. Then the true activity can be determined by measuring the formazan produced for three different amounts of sample, extrapolating to zero a plot of the ratio of the volume of the sample to the micrograms of formazan produced versus the volume of the sample, and comparing the value obtained with that of a standard xanthine-oxidase preparation.

¹ Taken from the thesis of Quei-Shiow Hwang, presented in June, 1966, to the University of Illinois, in partial fulfillment of the requirements for the Degree of Doctor of Philosophy.

² Present address, Department of Pharmacology, Jefferson Medical College, Philadelphia, Pennsylvania.

³ Present address, John Stuart Research Laboratory, The Quaker Oats Co., Barrington, Illinois.