Milk, aldehyde oxidase

Aldehyde oxidase milk aldehydes fatty acid approx. 7 0... 

Xanthine oxidase, mol wt ca 275,000, present in milk, Hver, and intestinal mucosa (131), is required in the cataboHsm of nucleotides. The free bases guanine and hypoxanthine from the nucleotides are converted to uric acid and xanthine in the intermediate. Xanthine oxidase cataly2es oxidation of hypoxanthine to xanthine and xanthine to uric acid. In these processes and in the oxidations cataly2ed by aldehyde oxidase, molecular oxygen is reduced to H2O2 (133). Xanthine oxidase is also involved in iron metaboHsm. Release of iron from ferritin requires reduction of Fe " to Fe " and reduced xanthine oxidase participates in this conversion (133). 

Of the mammalian enzymes, the sulphite oxidase of bovine liver has only recently been discovered to contain molybdenum (15). The better known molybdenum enzymes, xanthine oxidase from cows milk (31) and aldehyde oxidase from rabbit liver (16) are closely related to one another as they are to the xanthine dehydrogenases from chicken liver (17) and from bacteria (18). 

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. 

Molybdenum Milk, milk products dried legumes or pulses liver and kidney grains Prosthetic group of enzymes aldehyde oxidase Xanthine oxidase Electron transfer chain enzymes... 

Ho, C. Y. and Clifford, A. J. 1976. Digestion and absorption of bovine milk xanthine oxidase and its role as an aldehyde oxidase. J. Nutr. 106, 1600-1609. 

There are probably more publications relating to xanthine oxidase than to any other enzyme studied, certainly more than those pertaining to aldehyde oxidase. This is presumably because the former enzyme is easily accessible from cow s milk rather than from animal tissue. It is not the purpose of this review to include all the data amassed on xanthine oxidase, as this has been fully covered in recent reviews [8, 12, 13]. Furthermore, most of our own work has been concerned with aldehyde oxidase. Thus, this report compares the properties of the molybdenum hydroxylases, where possible, in terms of distribution, substrate and inhibitor specificity and mechanism of oxidation. 

Table 3 2. COMPARATIVE DISTRIBUTION OF XANTHINE OXIDASE AND ALDEHYDE OXIDASE ACTIVITY IN MILK AND LIVER OF MAMMALIAN SPECIES [92, 93, 95]...

Substrate Bovine milk xanthine oxidase Hepatic aldehyde oxidase ... 

Bovine milk oxidase activity with acetaldehyde = 100/s. b Hog liver aldehyde oxidase activity with acetaldehyde = 7.9 /imol/mg per min. c Human aldehyde oxidase activity, comparative rates only. d Not determined. 

Rabbit liver aldehyde oxidase. b Bovine milk xanthine oxidase. c Rat liver aldehyde oxidase. 

As with FMO, this enzyme has some inherent instability and care should be exercised in processing tissue samples of the presence of this enzyme is an issue. Aldehyde oxidase is a true oxidase, in that it transfers electrons to O2 (to form H2O2). The literature contains numerous reports about (milk) xanthine oxidase subsequent work revealed that this is really xanthine dehydrogenase (Rajagopalan, 1997), which is readily converted to an oxidase by proteolysis or modification of sulfides. 

Milk xanthine oxidase, in addition to its activity with purines and aldehydes, also catalyzes the oxidation of DPNH2 with oxygen or methylene blue. This activity was first associated with a milk flavoprotein isolated by Corran and Green which was later reported to be identical with xanthine... 

Aldehyde oxidase occurs in milk, and requires no co-enzyme aldehyde mutase is absent from milk, but well-represented in muscle and liver, and requires co-dehydrogenase I (co-enzyme I) for its effect. 

A comparison of the substrate specificities of bovine milk xanthine oxidase and rabbit liver aldehyde oxidase revealed both similarities and differences [10]. Both enzymes exhibited a preference for heterocycles which contain a condensed-pyrimidine ring system (Figure 3). Of the unsubstituted ring systems studied,... 

Xanthine oxidase (XO) was the first enzyme studied from the family of enzymes now known as the molybdenum hydroxylases (HiUe 1999). XO, which catalyzes the hydroxylation of xanthine to uric acid is abundant in cow s milk and contains several cofactors, including FAD, two Fe-S centers, and a molybdenum cofactor, all of which are required for activity (Massey and Harris 1997). Purified XO has been shown to use xanthine, hypoxan-thine, and several aldehydes as substrates in the reduction of methylene blue (Booth 1938), used as an electron acceptor. Early studies also noted that cyanide was inhibitory but could only inactivate XO during preincubation, not during the reaction with xanthine (Dixon 1927). The target of cyanide inactivation was identified to be a labile sulfur atom, termed the cyanolyzable sulfur (Wahl and Rajagopalan 1982), which is also required for enzyme activity. 

It has been recognized for about 80 years that milk contains an enzyme capable of oxidizing aldehydes and purines. The enzyme is now generally referred to as xanthine oxidase (XO) milk is a very good source of XO, at... 

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