Studies on Xanthine Oxidase: The Function of Catalase

The Inactivation of Human Aldehyde Oxidase 1 by Hydrogen Peroxide and Superoxide

Mammalian aldehyde oxidases (AOX) are molybdo-flavoenzymes of pharmacological and pathophysiologic relevance that are involved in phase I drug metabolism and, as a product of their enzymatic activity, are also involved in the generation of reactive oxygen species. So far, the physiologic role of aldehyde oxidase 1 in the human body remains unknown. The human enzyme hAOX1 is characterized by a broad substrate specificity, oxidizing aromatic/aliphatic aldehydes into their corresponding carboxylic acids, and hydroxylating various heteroaromatic rings. The enzyme uses oxygen as terminal electron acceptor to produce hydrogen peroxide and superoxide during turnover. Since hAOX1 and, in particular, some natural variants produce not only H₂O₂ but also high amounts of superoxide, we investigated the effect of both ROS molecules on the enzymatic activity of hAOX1 in more detail. We compared hAOX1 to the high-O₂ ^.--producing natural variant L438V for their time-dependent inactivation with H₂O₂/O₂ ^.- during substrate turnover. We show that the inactivation of the hAOX1 wild-type enzyme is mainly based on the production of hydrogen peroxide, whereas for the variant L438V, both hydrogen peroxide and superoxide contribute to the time-dependent inactivation of the enzyme during turnover. Further, the level of inactivation was revealed to be substrate-dependent: using substrates with higher turnover numbers resulted in a faster inactivation of the enzymes. Analysis of the inactivation site of the enzyme identified a loss of the terminal sulfido ligand at the molybdenum active site by the produced ROS during turnover. SIGNIFICANCE STATEMENT: This work characterizes the substrate-dependent inactivation of human aldehyde oxidase 1 under turnover by reactive oxygen species and identifies the site of inactivation. The role of ROS in the inhibition of human aldehyde oxidase 1 will have a high impact on future studies.

Similar developmental fluctuations of hepato-renal xanthine oxidoreductase gene expression and xanthine oxidase activity in layer and broiler chicken embryos

1. Xanthine oxidase (XO) has many physiological functions associated with the synthesis of both antioxidant (uric acid: UA) and numerous oxidants (e.g. H₂O₂), which makes it an important regulator of the cellular redox potential involving organogenesis. The ontogenetic study of hepatic and renal XO makes a better understanding of the putative role of this enzyme in the development of these tissues. 2. Developmental changes of gene expression of xanthine oxidoreductase (XOR), XO activity and UA content of liver and kidney tissues in both broiler and layer chicken embryos were examined during incubation d 14-21. 3. In both strains, hepatic XOR gene expression peaked on d 21 while renal XOR gene expression did not change. 4. The XO activity was higher in kidney than liver in both strains. Hepatic XO activity of both strains peaked on d 18 and thereafter was decreased on d 21. Renal XO activity peaked on d 18 and from then on did not show any significant changes until d 21 in both strains. 5. The UA content was higher in kidney vs. liver in both strains. The hepatic and renal UA values of the both strains increased significantly from d 14 to d 21. 6. The present results showed dissimilar behaviour of XOR gene expression, XO activity and UA content of liver and kidney tissues in both broiler and layer chicken embryos.

THE DECOMPOSITION OF HYDROGEN PEROXIDE BY LIVER CATALASE

1. The velocity of decomposition of hydrogen peroxide by catalase as a function of (a) concentration of catalase, (b) concentration of hydrogen peroxide, (c) hydrogen ion concentration, (d) temperature has been studied in an attempt to correlate these variables as far as possible. It is concluded that the reaction involves primarily adsorption of hydrogen peroxide at the catalase surface. 2. The decomposition of hydrogen peroxide by catalase is regarded as involving two reactions, namely, the catalytic decomposition of hydrogen peroxide, which is a maximum at the optimum pH 6.8 to 7.0, and the "induced inactivation" of catalase by the "nascent" oxygen produced by the hydrogen peroxide and still adhering to the catalase surface. This differs from the more generally accepted view, namely that the induced inactivation is due to the H₂O₂ itself. On the basis of the above view, a new interpretation is given to the equation of Yamasaki and the connection between the equations of Yamasaki and of Northrop is pointed out. It is shown that the velocity of induced inactivation is a minimum at the pH which is optimal for the decomposition of hydrogen peroxide. 3. The critical increment of the catalytic decomposition of hydrogen peroxide by catalase is of the order 3000 calories. The critical increment of induced inactivation is low in dilute hydrogen peroxide solutions but increases to a value of 30,000 calories in concentrated solutions of peroxide.

Cytochrome and respiratory enzymes

The object of this paper is the study of the functional relationship between the intracellular hæmatin compounds and the oxidising enzymes such as dehydrases and oxidases. It was shown previously (1925-1927) that aerobic organisms contain a very widely distributed intracellular hæmatin pigment-cytochrome-which can undergo reversible oxidation and reduction without being destroyed. Being the only compound directly visible in the living cell, cytochrome gives us important indications, not only of its own activity but also of that of other components of the respiratory system of the cell. The present paper will first deal with the thermostable peroxidase of yeast and other cells, and with the true thermostable oxidases such as the indophenol oxidase of yeast and muscle cells and the polyphenol oxidase of potato. This will be followed by the study of intracellular hæmatin compounds, and especially of the effects of various factors on the oxidation and reduction of cytochrome. The results of this study will enable us to determine the nature of the relationship between the oxidising enzymes and the intracellular hæmatin compounds, and this will help to elucidate at least one portion of the complicated respiratory mechanism, of the cell.