The role of hydrogen peroxide production and catalase in hepatic ethanol metabolism

Integrated Hypoxia Signaling and Oxidative Stress in Developmental Neurotoxicity of Benzo[a]Pyrene in Zebrafish Embryos

Benzo[a]pyrene (B[a]P) is a polycyclic aromatic hydrocarbon formed by the incomplete combustion of organic matter. Environmental B[a]P contamination poses a serious health risk to many organisms because the pollutant may negatively affect many physiological systems. As such, chronic exposure to B[a]P is known to lead to locomotor dysfunction and neurodegeneration in several organisms. In this study, we used the zebrafish model to delineate the acute toxic effects of B[a]P on the developing nervous system. We found that embryonic exposure of B[a]P downregulates shh and isl1, causing morphological hypoplasia in the telencephalon, ventral thalamus, hypothalamus, epiphysis and posterior commissure. Moreover, hypoxia-inducible factors (hif1a and hif2a) are repressed upon embryonic exposure of B[a]P, leading to reduced expression of the Hif-target genes, epo and survivin, which are associated with neural differentiation and maintenance. During normal embryogenesis, low-level oxidative stress regulates neuronal development and function. However, our experiments revealed that embryonic oxidative stress is greatly increased in B[a]P-treated embryos. The expression of catalase was decreased and sod1 expression increased in B[a]P-treated embryos. These transcriptional changes were coincident with increased embryonic levels of H₂O₂ and malondialdehyde, with the levels in B[a]P-treated fish similar to those in embryos treated with 120-μM H₂O₂. Together, our data suggest that reduced Hif signaling and increased oxidative stress are involved in B[a]P-induced acute neurotoxicity during embryogenesis.

Role of peroxisomes in the swift increase in alcohol metabolism

The Swift Increase in Alcohol Metabolism occurs within 2.5 h after an acute gavage of ethanol causing an increase in hepatic respiration, an increase in alcohol metabolism, and pericentral hypoxia in the perfused liver. Alcohol treatment causes a release of endotoxin, activation of Kupffer cells to produce PGE(2), therefore, stimulating mitochondrial function resulting in an increase in cofactor supply for nicotinamide adenine dinucleotide (NAD)-dependent alcohol metabolism and depletion of glycogen reserves. Additionally, liberation of peripheral fatty acids via activation of an adrenergic response to alcohol provides added substrate for peroxisomes. In this study, rats were treated in vivo with ethanol, methanol or oleate and basal rates of oxygen uptake in perfused liver were significantly increased as compared to untreated controls. Methanol (25 mmol/L), a selective substrate for catalase in rodents, was infused into the liver and rates of methanol metabolism were increased 3-4-fold as compared to controls. Gadolinium chloride blocked the increase in oxygen and alcohol metabolism and inhibited the increase in ketogenesis normally observed after ethanol treatment. Excess fatty acids from oleate treatment in vivo provided additional substrate for peroxisomal alcohol metabolism and an increase in alcohol metabolism occurred even when Kupffer cells were eliminated. These data demonstrate that fatty acid supply either via peripheral reserves or after fatty acid treatment to peroxisomes generate sufficient H(2)O(2) for activation of catalase-dependent alcohol metabolism.

UVB light stimulates production of reactive oxygen species: unexpected role for catalase

In keratinocytes, UVB light stimulates the production of reactive oxygen species (ROS). Lysates of these cells were found to possess a non-dialyzable, trypsin- and heat-sensitive material capable of generating ROS in response to UVB light. Using ion exchange, metal affinity, and size exclusion chromatography, a 240-kDa protein was isolated with ROS generating activity. The protein exhibited strong absorption in the 320-360 nm range with additional soret peaks around 400-410 nm, suggesting the presence of heme. Sequencing using liquid chromatography-ion trap mass spectrometry identified the protein as catalase. Using purified catalases from a variety of species, the ROS generating activity was found to be temperature- and O2-dependent, stimulated by inhibitors of the catalatic activity of catalase, including 3-aminotriazole and azide, and inhibited by cyanide. A marked increase in the production of ROS was observed in UVB-treated cells overexpressing catalase and decreased generation of oxidants was found in UVB-treated keratinocytes with reduced levels of catalase. Our data indicate that catalase plays a direct role in generating oxidants in response to UVB light. The finding that catalase mediates the production of ROS following UVB treatment is both novel and highly divergent from the well known antioxidant functions of the enzyme. We hypothesize that, through the actions of catalase, high energy DNA damaging UVB light is absorbed by the enzyme and converted to reactive chemical intermediates that can be detoxified by cellular antioxidant enzymes. Accumulation of excessive ROS, generated through the action of catalase, may lead to oxidative stress, DNA damage, and the development of skin cancer.