Milk, xanthine oxidase

Boroxazothienopyridines, 4, 1029-1032 Borsche synthesis, 3, 44 Botrydiplodin mass spectrometry, 4, 585 Boulton-Katritzky rearrangement, 5, 288 Bovine milk xanthine oxidase substrates, 1, 234 Bradsher reaction... 

Another Mossbauer study on molybdenum hydroxylases was performed on a nonenriched sample of milk xanthine oxidase (219), and an unusually large AEq (3.2 mm/s at 175 K) was also observed for the ferrous site of one of the clusters. 

Fridovich, I. (1970). Quantitative aspects of the production of superoxide anion radical by milk xanthine oxidase. J. Biol. Chem. 245, 4053-4057. 

V. Milk Xanthine Oxidase Studies on the Role of Molybdenum. 117... 

Studies bearing on the role of molybdenum in enzymes will be exemplified by a detailed summary of results on the most studied of these enzymes, which is undoubtedly milk xanthine oxidase. To put this in its context, it will be preceded by a review of the general properties of xanthine oxidase. The final section will then be a short account of work on some of the other molybdenum enzymes. 

Since studies bearing on the role of molybdenum in milk xanthine oxidase have depended heavily on the EPR method, it is convenient to precede detailed discussion by a general description of the various molyb-denum(V) EPR signals which have been obtained from the enzyme. 

Fig. 1. Types of Mo(V) EPR spectra from reduced milk xanthine oxidase. All were recorded at about —150 ° on a 9.3 GHz instrument and are shown with magnetic field increasing from left to right. The reducing agents and conditions of reduction (generally at about 20 °) were as follows ...

Table 2. EPR parameters of Mo(V) species from milk xanthine oxidase. Parameters have been measured from spectra of frozen solutions at about —150°, in some cases at both 9 and 35 GHz. Correctness of some of the interpretations was confirmed by computer simulations...

Scheme 2. Species responsible for Mo(V) signals from xanthine oxidase. The diagram illustrates typical treatments which may be used to obtain the various molybdenum EPR signals from milk xanthine oxidase. Nomenclature of the signals is that of reference (78) and of the enzyme species, that of reference (19). Conditions for signal development refer to pH 8.2 and 20—25 0 with about 0.1 mM enzyme unless otherwise stated...

Xanthine dehydrogenase from chicken liver reacts readily with NAD as acceptor (77) while that from Micrococcus lactilyticus is inactive towards this, reacting instead with ferredoxin (18). Both enzymes react only slowly with oxygen. It seems reasonable to assume, however, that for each member of this group of enzymes, reducing substrates all react via molybdenum, as in milk xanthine oxidase. Presumably, different... 

So little is known about molybdenum enzymes other than milk xanthine oxidase that there is little to be said by way of general conclusions. In all cases where there is direct evidence (except possibly for xanthine dehydrogenase from Micrococcus lactilyticus) it seems that molybdenum in the enzymes does have a redox function in catalysis. For the xanthine oxidases and dehydrogenases and for aldehyde oxidase, the metal is concerned in interaction of the enzymes with reducing substrates. However, for nitrate reductase it is apparently in interaction with the oxidizing substrate that the metal is involved. In nitrogenase the role of molybdenum is still quite uncertain. 

Panoutsopoulos GI, Beedham C. Kinetics and specificity of guinea pig liver aldehyde oxidase and bovine milk xanthine oxidase towards substituted benzaldehydes. Acta Biochim Pol 2004 51(3) 649-663. 

Barber, M. J., Siegel, L. M. Proton and electron affinities and magnetic interactions associated with the molybdenum flavin, and iron-sulfur centers of milk xanthine oxidase. In Flavins and flavoproteins (Massey, V., Williams, C. H. eds.) pp. 796-804, New York, Amsterdam, Oxford, Elsevier/North Holland 1982... 

In 1975, the Life Sciences Research Office of the Federation of American Societies for Experimental Biology (Carr et al. 1975), upon an extensive review of the available evidence, concluded that it was doubtful whether XO in homogenized cow s milk was a causal or risk factor for heart disease. More recently, Clifford et al (1983) and Deeth (1983), in critical reviews of the homogenized cow s milk XO hypothesis, have arrived at a similar, if not more definitive, conclusion. As stated by Clifford et al. (1983), experimental evidence has failed to substantiate, and in many cases has refuted, the hypothesis that homogenized bovine milk xanthine oxidase intake or plasmalogen depletion are causal factors in the development of atherosclerosis. And, according to Deeth (1983), there appears to be no unequivocal evidence that the absorbed enzyme has any pathological effects that may contribute to development of atherosclerotic heart disease. ... 

Carr, C. J., Talbot, J. M. and Fisher, K. D. 1975. A review of the significance of bovine milk xanthine oxidase in the etiology of atherosclerosis. Life Science Research Office, Federation of American Societies for Experimental Biology, Bethesda, Md. (Prepared for the Food and Drug Administration, Washington, D. C., Contract No. FDA 223-75-2090.)... 

Clifford, A. J., Ho, C. Y. and Swenerton, H. 1983. Homogenized bovine milk xanthine oxidase A critique of the hypothesis relating to plasmalogen depletion and cardiovascular disease. Am J. Clin. Nutr. 38, 327-332. 

Ho, C. Y., Crane, R. T. and Clifford, A. J. 1978. Studies on lymphatic absorption of and the availability of riboflavin from bovine milk xanthine oxidase. J. Nutr. 108, 55-60. 

Rzucidlo, S. J. and Zikakis, J. P. 1979. Correlation of dairy food intake with human antibody to bovine milk xanthine oxidase. Proc. Soc. Exp. Biol Med. 160, 477-482. 

Zikakis, J. P., Rzucidlo, S. J. and Biasotlo, N. O. 1977. Persistence of bovine milk xanthine oxidase activity after gastric digestion in vivo and in vitro. J. Dairy Sci. 60, 533-541. 

Battelli, M. G., Lorenzoni, E. and Stirpe, F. 1973. Milk xanthine oxidase type D (dehydrogenase) and type 0 (oxidase). Purification, interconversion and some properties. Biochem. J. 131, 191-198. 

Nathans, G. R. and Hade, E. P. K. 1978. Bovine milk xanthine oxidase. Purification by ultrafiltration and conventional methods which omit addition of proteases. Some criteria for homogeneity of native xanthine oxidase. Biochim. Biophys. Acta 526, 328-344. 

Waud, W. R., Brady, F. O., Wiley, R. D. and Rajagopalan, K. V. 1975. A new purification procedure for bovine milk xanthine oxidase Effect of proteolysis on the subunit structure. Arch. Biochem. Biophys. 169, 695-701. 

It is obvious that x-ray cyrstallographic methods will be the final arbiter of the structural features of molybdoproteins, but until such structures are obtained, and even afterwards as far as dynamic features are concerned, spectroscopic methods must be used to gain insight into the nature of these catalysts. Electronic spectroscopy so far has been of little use here since molybdenum complexes in general appear to exhibit broad weak absorptions. In proteins these are always buried under absorptions from hemes, flavins, and iron-sulfur centers. Massey et al., (15) discovered that pyrazolo [3,4-d] pyrimidines will bind Mo (IV) in milk xanthine oxidase that had been reduced with xanthine... 

DlArdenne, S. C., and Edmondson, D. E., 1990, Kinetic isotope effect studies on milk xanthine oxidase and on chicken liver xanthine dehydrogenase. Biochemistry 29 904611 9052. 

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