Oxygen-induced seizures and inhibition of human glutamate decarboxylase and porcine cysteine sulfinic acid decarboxylase by oxygen and nitric oxide

Abstract

The recombinant forms of the two human isozymes of glutamate decarboxylase, GAD65 and GAD67, are potently and reversibly inhibited by molecular oxygen (Ki = 0.46 and 0.29 mM, respectively). Inhibition of the vesicle-associated glutamate decarboxylase (GAD65) by molecular oxygen is likely to result in incomplete filling of synaptic vesicles with gamma-aminobutyric acid (GABA) and may be a contributing factor in the genesis of oxygen-induced seizures. Under anaerobic conditions, nitric oxide inhibits both GAD65 and GAD67 with comparable potency to molecular oxygen (Ki = 0.5 mM). Two forms of porcine cysteine sulfinic acid decarboxylase (CSADI and CSADII) are also sensitive to inhibition by molecular oxygen (Ki = 0.30 and 0.22 mM, respectively) and nitric oxide (Ki = 0.3 and 0.2 mM, respectively). Similar inhibition of glutamate decarboxylase and cysteine sulfinic acid decarboxylase by two different radical-containing compounds (O2 and NO) is consistent with the notion that these reactions proceed via radical mechanisms.

Oxygen toxicity from plants to people

“…The hydrogen atom transferred in this mechanism (as H AE ) is shown in bold. The scheme is similar to one proposed by Davis et al (2001) their reactions. Like rubisco, if these enzymes catalyze reactions by radical mechanisms, then there must be some advantage to the radical pathway.…”

“…Further, the irreversible losses of activity that had been seen by Faiman and others could be a consequence of self-inflicted, oxidative inactivation by the enzyme, rather than by an indirect, non-specific oxidation, as envisioned by earlier investigators. Recombinant forms of the two human isozymes of GAD, GAD65 and GAD67, have now been shown to be reversibly inhibited by molecular oxygen (Davis et al 2001). The extent of inhibition is substantial, amounting to 70% inhibition of all enzyme forms (GAD65, GAD67 and E. coli GAD) under one atmosphere of pure oxygen.…”

“…Other enzymes that interact with oxygen include cysteine sulfinic acid decarboxylase (Davis et al 2001), DOPA decarboxylase (Bertoldi et al 1998;Voltattorni 2000, 2001) and ornithine decarboxylase (Bertoldi et al 1999). The common interaction of several pyridoxal phosphate-dependent amino acid decarboxylases with molecular oxygen suggests that these enzymes may, like the thiamine pyrophosphate-dependent enzymes ALS and pyruvate decarboxylase, have substantial radical character to For GAD, R is a carboxyethyl moiety and Pyr is the remaining structure of pyridoxal phosphate, other than its aldehydic carbon.…”

“…O 2 ) the rate of the oxygenase reaction is only about 1% of the rate of c-amino-n-butyric acid (GABA) formation. Inhibition of the GABA vesicle-associated GAD65, leading to underfilling of GABA vesicles and imbalance between glutamatergic and GABAergic synaptic function, is thought to result in oxygen-induced seizures (Davis et al 2001). Interestingly, oxidative damage to GAD65, perhaps self-inflicted, is also thought to be involved in the pathogenesis of type-1 diabetes mellitus (Trigwell et al 2001).…”

Enzyme-Catalyzed Side Reactions with Molecular Oxygen may Contribute to Cell Signaling and Neurodegenerative Diseases

“…The recombinant isozymes of human GAD that are either GABA-vesicle associated (GAD65) or localized in the cytosol (GAD67), are both sensitive to reversible inhibition by O 2 (in the presence of thiols) and by nitric oxide (ÁNO) (under anaerobic conditions) [17]. Unlike O 2 , which is a biradical (i.e., contains two unpaired electrons), ÁNO has only a single unpaired electron.…”