Distinctive interactions in the holoenzyme formation for two isoforms of glutamate decarboxylase.

The interactions between glutamate decarboxylase (GAD) and its cofactor pyridoxal phosphate (PLP) play a key role in the regulation of GAD activity. The enzyme has two isoforms, GAD65 and GAD67. A comparison of binding constants, rate constants, and kinetic profiles for the formation of holoenzyme (holoGAD65 and holoGAD67) revealed that the two isoforms interact distinctively with the cofactor. GAD67 exhibits a higher binding constant for PLP binding, making it more difficult to dissociate PLP from holoGAD67 than holoGAD65. Meanwhile, PLP binding occurs at a much slower rate for GAD67 than GAD65, as evidenced by lower rate constants and a slower initial rate of the holoenzyme formation. Job's plots revealed a stoichiometry of 1:1 for PLP binding to GAD65 before and after the saturation level of PLP, while 1:2 for PLP binding to GAD67 prior to the saturation of PLP and 1:1 at the saturation level of PLP. These results suggested that the two binding sites of GAD65 exhibit similar affinities for PLP. In contrast, one binding site of GAD67 exhibits a significantly higher affinity for PLP than the other binding site. Based on these findings, it was proposed that a slower PLP binding to GAD67 than GAD65 and a less ease to dissociate PLP from holoGAD67 than holoGAD65 are important underlying factors. This attributes to GAD67 being more highly saturated by PLP and GAD65 being less saturated by PLP. A larger conformation change constant for GAD67 than GAD65 supported a significant conformational change induced by the initial PLP binding to GAD67, which affects the other binding site affinity of GAD67. The present studies provided valuable insights into distinctive properties between the two isoforms of GAD.

Post-mortem degradation of brain glutamate decarboxylase.

The post-mortem stability of the GABA synthesizing enzyme glutamate decarboxylase (GAD) was studied by using SDS-PAGE and quantitative immunoblotting to measure the rates of degradation of GAD in the cerebral cortex, hippocampus, and cerebellum of rats and mice as a function of time after death. The intact 65- and 67-kDa isoforms of GAD (GAD(65) and GAD(67)) disappeared gradually over a 24-h period. In both rats and mice, the degraded GAD appeared as a band with an apparent molecular mass of 55-57 kDa; no significant amounts of smaller forms were observed. The 55-57 kDa band reacted with antiserum W887, which recognizes a shared epitope at the carboxyl-terminal end of both GADs, indicating that GAD was cleaved near the amino-terminal end of the molecule. GAD(67) was cleaved at a site between the amino-terminus and the epitope for antiserum W883 (located within residues 79-93 of GAD(67)), as antiserum W883 stained a 56-kDa band on the blots. The appearance of degraded GAD paralleled the loss of total GAD (GAD(65)+GAD(67)), and after 24h the 55-57 kDa band accounted for 97, 88, and 59% of the intact GAD lost from rat cerebellum, cerebral cortex and hippocampus. On a percentage basis, GAD(67) was degraded more rapidly than was GAD(65) in all brain regions studied. The loss of GAD activity was greater in rat than mouse brain, even though the percent loss of intact GAD protein was similar.

Evaluation of the RIDAQuick norovirus immunochromatographic test kit.

BACKGROUND: Norovirus infections occur frequently and are widespread throughout the US population causing greater than half of all foodborne gastroenteritis cases. A rapid norovirus assay would be a useful clinical tool for identification of this common virus in gastroenteritis patient samples, thereby identifying outbreaks and facilitating rapid implementation of control measures. OBJECTIVES: To determine the suitability of the RIDAQuick norovirus kit as a clinical tool by determining the specificity and sensitivity of the assay, and its cross-reactivity with other enteric viruses. STUDY DESIGN: Archived stool specimens containing norovirus genogroup I or II or other viruses were tested using the RIDAQuick norovirus assay and results compared to those obtained with real-time RT-PCR. RESULTS: We tested 62 samples: 19 norovirus genogroup I, 25 genogroup II samples, and 18 norovirus negative samples. Compared to PCR results, RIDAQuick assay sensitivity was 61.4%, and specificity was 100%. The low sensitivity was mainly due to poor results with genogroup I specimens; only 11 of 19 were detected. Additionally, samples of four other common enteric viruses all tested negative with the RIDAQuick assay. CONCLUSIONS: The RIDAQuick kit effectively detects norovirus genogroup II strains, but not genogroup I strains. We found no cross-reactivity with several common enteric viruses. As most norovirus cases are currently genogroup II strains, positive results with RIDAQuick can be used for rapid detection of norovirus in a large percentage of cases, thus also aiding in identification of outbreaks. However, final confirmation and negative results require further testing with more sensitive methods.

Evidence that GAD65 mediates increased GABA synthesis during intense neuronal activity in vivo.

In this study we tested the hypothesis that the 65-kDa isoform of glutamate decarboxylase (GAD(65)) mediates activity-dependent GABA synthesis as invoked by seizures in anesthetized rats. GABA synthesis was measured following acute GABA-transaminase inhibition by gabaculine using spatially localized (1)H NMR spectroscopy before and after bicuculline-induced seizures. Experiments were conducted with animals pre-treated with vigabatrin 24 h earlier in order to reduce GAD(67) protein and also with non-treated controls. GAD isoform content was quantified by immunoblotting. GABA was higher in vigabatrin-treated rats compared to non-treated controls. In vigabatrin-treated animals, GABA synthesis was 28% lower compared to controls [p < 0.05; vigabatrin-treated, 0.043 +/- 0.011 micromol/(g min); non-treated, 0.060 +/- 0.014 micromol/(g min)] and GAD(67) was 60% lower. No difference between groups was observed for GAD(65). Seizures increased GABA synthesis in both control [174%; control, 0.060 +/- 0.014 micromol/(g min) vs. seizures, 0.105 +/- 0.043 micromol/(g min)] and vigabatrin-treated rats [214%; control, 0.043 +/- 0.011 micromol/(g min); seizures, 0.092 +/- 0.018 micromol/(g min)]. GAD(67) could account for at least half of basal GABA synthesis but only 20% of the two-fold increase observed in vigabatrin-treated rats during seizures. The seizure-induced activation of GAD(65) in control cortex occurs concomitantly with a 2.3-fold increase in inorganic phosphate, known to be a potent activator of apoGAD(65)in vitro. Our results are consistent with a major role for GAD(65) in activity-dependent GABA synthesis.

Glutamate decarboxylase: loss of N-terminal segment does not affect homodimerization and determination of the oxidation state of cysteine residues.

Glutamate decarboxylase (GAD) produces GABA, the main inhibitory neurotransmitter in adult mammalian brain. The physical characteristics of GAD were studied using mass spectrometry and partial protein digests. The N-termini of the two main isoforms, GAD65 and GAD67, were processed by removal of the initial methionine residues and acetylation of the penultimate alanines. Native recombinant GAD65 and GAD67 exist as homodimers that can be dissociated with non-reducing methods, indicating that homodimerization does not involve intermolecular disulfide bonds. Truncation of the N-terminal segment with trypsin digestion did not affect homodimerization but increased activity by decreasing the Km of GAD67 and increasing the Vmax of both isoforms. Of the 15 cysteines in GAD65, the six found in the N-terminal segment can form disulfide bonds and of the 13 cysteines in GAD67, cysteines 32 and 38 can form a disulfide bond. The in vitro formation of disulfide bonds in the N-termini, and the removal of the termini with relatively low amounts of trypsin, indicate that the N-terminal segments of GAD65 and GAD67 are exposed and flexible. The formation of a disulfide bridge between cysteines 30 and 45 of GAD65 suggests that alteration of normal redox conditions could affect GAD targeting.

Kinetic differences between the isoforms of glutamate decarboxylase: implications for the regulation of GABA synthesis.

Glutamate decarboxylase (GAD) exists as two isoforms, GAD65 and GAD67. GAD activity is regulated by a cycle of activation and inactivation determined by the binding and release of its co-factor, pyridoxal 5'-phosphate. Holoenzyme (GAD with bound co-factor) decarboxylates glutamate to form GABA, but it also catalyzes a slower transamination reaction that produces inactive apoGAD (without bound co-factor). Apoenzyme can reassociate with pyridoxal phosphate to form holoGAD, thus completing the cycle. Within cells, GAD65 is largely apoenzyme (approximately 93%) while GAD67 is mainly holoenzyme (approximately 72%). We found striking kinetic differences between the GAD isoforms that appear to account for this difference in co-factor saturation. The glutamate dependent conversion of holoGAD65 to apoGAD was about 15 times faster than that of holoGAD67 at saturating glutamate. Aspartate and GABA also converted holoGAD65 to apoGAD at higher rates than they did holoGAD67. Nucleoside triphosphates (such as ATP) are known to affect the activation reactions of the cycle. ATP slowed the activation of GAD65 and markedly reduced its steady-state activity, but had little affect on the activation of GAD67 or its steady-state activity. Inorganic phosphate opposed the effect of ATP; it increased the rate of apoGAD65 activation but had little effect on apoGAD67 activation. We conclude that the apo-/holoenzyme cycle of inactivation and reactivation is more important in regulating the activity of GAD65 than of GAD67.