OXYGENASE SIDE REACTIONS OF CARBANION FORMING ENZYMES

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Several, but not all, enzymic reactions that involve carbanionic intermediates have associated oxygen-consuming side reactions that do not appear to require oxygen activation. With the exception of the oxygenase reaction of ribulosebisphosphate carboxylase, these reactions have only recently been discovered and a rigorous determination of the products and mechanisms of these reactions has not been completed. Expression of an oxygenase side reaction clearly involves more than accessibility of a carbanionic reaction intermediate to molecular oxygen. A determination of all products of various oxygenase reactions, evaluation of the mechanism of these reactions, and the variability in levels of activity for the same enzyme from various sources will contribute to an understanding of these reactions. Oxygen toxicity is generally not thought to be associated with triplet oxygen per se, but requires conversion of O2 to its singlet state, superoxide, hydrogen peroxide, hydroxyl radical, or the formation of other radicals. The oxygenase side reactions of acetolactate synthase (Salmonella typhimurium) and glutamate decarboxylase (Escherichia coli) give peracetate and hydrogen peroxide as products, respectively, while that of fructosebisphosphate aldolase (Staphylococcus aureus) gives a product capable of reacting with proteins, hydroxypyruvaldehyde phosphate. The relevance of these reactions to the acute or chronic toxicity (aging- related loss of function) of oxygen in mammals is unclear, especially since the potential oxygenase activities of relevant enzymes have not been examined. To this end the mammalian glutamate decarbexylase will be examined for an oxygenase reaction similar to that recently discovered for the enzyme from the bacterium Escherichia coli. A combination of enzymic and chemical methods will be used to determine hydrogen peroxide and superoxide production by these enzymes. An infrared- sensitive chemiluminometer will be used to detect singlet oxygen. Organic and inorganic products of the oxygenase reactions; oxidative products of the enzymes and cofactors will be assessed by mass spectroscopy. Losses of enzymic activity by these enzymes will be assessed in the presence and absence of oxygen and substrate; in the presence and absence of protectants. Acute oxygen toxicity associated with exposure to hyperbaric oxygen is thought to be a consequence of inactivation of glutamate decarboxylase in the brain. A knowledge of the molecular mechanism by which oxygen inactivates and/or inhibits this enzyme could lead to the development of therapeutic agents to prevent or to treat acute oxygen toxicity. These results could also be relevant to chronic effects of oxygen on neural function (e.g. aging). Similarly, the oxygenase activities of other enzymes may contribute to aging in nonneural mammalian tissues.