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Notes: We have observed large second-order nonlinear optical response from zinc oxide (ZnO) thin films deposited on sapphire substrates by pulsed laser ablation. By comparing the second harmonic signal generated in a series of ZnO films with different crystallinity and thickness, we conclude that a significant part of the second harmonic signal is generated at the grain boundaries and interfaces. © 1998 American Institute of Physics.

Notes: An (AlAs/GaAs/AlAs/AlGaAs)/GaAs(001) double-barrier superlattice grown by molecular beam epitaxy (MBE) is studied by combining synchrotron radiation and double-crystal x-ray diffraction (DCD). The intensity of satellite peaks is modulated by the wave function of each sublayer in one superlattice period. Simulated by the x-ray dynamical diffraction theory, it is discovered that the intensity of the satellite peaks situated near the modulating wave node point of each sublayer is very sensitive to the variation of the layer structural parameters. The accurate layer thickness of each sublayer is obtained with an error less than 1 A(ring). Furthermore, x-ray kinematical diffraction theory is used to explain the modulation phenomenon. © 1996 American Institute of Physics.

Notes: A semiconductor laser whose cavities are "self-formed" due to strong optical scattering in highly disordered gain media is demonstrated. The lasers are made of zinc oxide polycrystalline films grown on amorphous fused silica substrates. Lasing occurs at an ultraviolet wavelength of ∼380 nm under optical pumping. Actual images of the microscopic laser cavities formed by multiple scattering have been captured. These results suggest the possibility of using disordered semiconductor microstructures as alternative sources of coherent light emission. © 1998 American Institute of Physics.

Notes: Abstract— The effects of divalent metal ions, sulfhydryl reagents, carbonyl trapping reagents, substrate analogs, and organic solvents on purified mouse brain 4-aminobutyrate-2-ketoglutarate transaminase (EC 2.6.1.19) and the subunit structure of this enzyme were studied. Of the metal ions tested, Hg2+ was found to be the most potent inhibitor inhibiting the enzyme 50 percent at a concentration of 0-7 μM. The order of decreasing inhibitory potency for the divalent metal ions was: Hg2+± Cd2+± Zn2+± Cu2+± Co2+± Ba2+± Sr2+± Ni2+± Mn2+± Ca2+± Mg2+. p-Chloromercuribenzoale was the most potent inhibitor among the sulfhydryl reagents tested inhibiting the enzyme to the extent of 50 per cent at 0-5 μM 3-Mercaptopropionic acid was found to be a competitive inhibitor for GABA and non-competitive for 2-ketoglutarate. The Ki, value was estimated to be 13 μM. Aminooxyacetic acid was the most potent inhibitor of the carbonyl trapping agents with a K, value of 0-06 μM. being competitive with GABA and non-competitive with 2-ketoglutarate. Hydroxylamine and hydrazine were the next most potent compounds in this group. Of a series of substrate analogs and metabolites tested, only acetic acid, propionic acid, butyric acid, glutamic acid, adipic acid, pimelic acid and 2-ketoadipic acid inhibited the enzyme to a significant extent. Dioxan inhibited the enzyme 50 per cent at a concentration of 5 per cent (v/v) whereas methanol and ethanol only inhibited 5-10 per cent at 10 per cent (v/v) concentration.A spectrum of the native enzyme at pH 7-2 showed maxima at 278 nm. 330 nm and 411 nm. Treatment of the enzyme with aminooxyacetic acid or 3-mercaptopropionic acid caused the maximum at 411 nm to disappear.Sodium dodecyl sulfate polyacrylamide gel electrophoresis of the enzyme revealed two protein bands. The molecular weights of these two subunits were determined to be 53.000 and 58,000, respectively.

Notes: —l-Glutamate decarboxylase purified from mouse brain was found to be highly sensitive to the sulfhydryl reagents, 5,5-dithiobis (2-nitrobenzoic acid) (DTNB) and p-chloromerburibenzoate (PCMB), which were competitive inhibitors (Ki for DTNB is 1·1 · 10−8m). Iodoacetamide and iodoacetic acid were less effective inhibitors than DTNB and PCMB. The mercapto acids, 3-mercaptopropionic, 2-mercaptopropionic, and 2-mercaptoacetic acids were potent competitive inhibitors with Ki values of 1·8, 53 and 300 μm, respectively. 2-Mercaptoethanol was less effective. Aminooxyacetic acid was the most potent carbonyl-trapping reagent tested inhibiting the enzyme activity completely at 1·6 μm, followed by hydroxylamine, hydrazine, semicarbazide, and d-penicillamine. Carboxylic acids with a net negative charge were strong competitive inhibitors e.g. d-glutamate (Ki 0·9 mm), α-ketoglutarate (Ki, l·2mm), fumarate (Ki,1·8 mm), dl-β-hydroxyglutamate (Ki, 2·8 mm), l-aspartate (ki, 3·1 mm) and glutarate (Ki, 3·5 mm). 2-Aminophosphonobutyric and 2-aminophosphonopropionic acids, phosphonic analogs of glutamate and aspartate, respectively, had no effect at l0mm. γ-Aminobutyric acid, l-glutamine, l-γ-methylene-glutamine, and α,γ-diaminoglutaric acid, amino acids with no net negative charge at neutral pH, had no effect at 5 mm. Glutaric and α-ketoglutaric acids were the most potent inhibitors among the various dicarboxylic and α-keto-dicarboxylic acids tested (Ki, 3·5 and 1·2 mm, respectively). Compounds with one carbon less, succinic and oxalacetic acids, or with one carbon more, adipic and α-ketoadipic acids, were less inhibitory. The monovalent cations, Li+, Na+, NH4+, and Cs+ had no effect on l-glutamate decarboxylase activity in concentrations up to 10mm. Divalent cations, on the other hand, were very potent inhibitors. Among eleven divalent cations tested, Zn2+ was the most potent inhibitor, inhibiting to the extent of 50 per cent at 10μm. The decreasing order of inhibitory potency was: Zn2+ 〉 Cd2+, Hg2+, Cu2+ 〉 Ni2+ 〉 Mn2+ Co2+ 〉 Ba2+ 〉 Ca2+ 〉 Mg2+ 〉 Sr+2, The anions, I−, Br−, Cl− and F− were only weak inhibitors. The Ki value for Cl− was 17mm. The above findings suggest minimally the presence of aldehyde, sulfhydryl and positively charged groups at or near the active site of the holoenzyme. Intermediates of glycolysis had little effect on l-glutamate decarboxylase activity, but intermediates of the tricarboxylic acid cycle, e.g. α-ketoglutarate (Ki= 1·2 mm) and fumarate (Ki= 1·8 mm) were relatively potent inhibitors. The nucleotides, ATP, ADP, AMP, cyclic AMP, GTP, GDP, GMP, and cyclic GMP were weak inhibitors. l-Norepinephrine (Ki= 1·3 mm) and serotonin were potent inhibitors, while acetylcholine, dopamine and histamine were less effective. Ethanol and dioxane inhibited the enzyme activity to the extent of 20-50 per cent at 10 per cent (v/v), while slight activation was observed at low concentrations (0·1-1 per cent) of both solvents. The possible role of Zn2+ and some metabolites in the regulation of steady-state levels of γ-aminobutyric acid also was discussed.

Notes: Choline acetyltransferase (CAT) from catfish brain has been purified to electrophoretic homogeneity by a combination of ammonium sulfate fractionation, gel filtration, and preparative polyacrylamide gel electrophoresis. The purified enzyme solutions showed only one protein band on both 7.5% polyacrylamide gel and gradient (3.5% to 15%) polyacrylamide gel with the enzyme activity coincident with the protein band. Furthermore, when the gel slice from which the enzyme was extracted was applied to a gradient polyacrylamide gel, a single protein band was also obtained. The purified enzyme preparations represented a 1510-fold purification over the crude homogenate. The moiecular weight of the enzyme was estimated as 128.000 and 140,000 from gel filtration and gradient (3.5% to 15%) polyacrylamide gel, respectively. On sodium dodecyl sulfate-gel, the purified enzyme preparations showed two protein bands, with molecular weights of 34,000 and 68,000. Since the native enzyme may be a tetramer, the 68.000 subunit may be a dimer of the smallest subunit. The maximum activity of the enzyme appears at pH 7.2; the values of Km for choline and for acetyl CoA are 400 and 92 μM, respectively, and Vmax was obtained as 2.2 × 10-3μmol/min/mg protein. CAT antibodies were obtained from rabbit that had received six biweekly injections with a total of 120 μg of the purified protein. Double immunodiffusion test with CAT antibodies and a crude enzyme extract from catfish brains showed only a single, sharp precipitin band, suggesting that the antibodies are specific only to CAT and antigen and that the purified CAT preparation is monodisperse.

Notes: —L-Glutamic acid decarboxylase (GAD) from brain of the channel catfish (Ictalurus punctatus) has been purified to homogeneity by a combination of ammonium sulfate fractionation, gel filtration, calcium phosphate gel and preparative polyacrylamide gel electrophoresis. The purity of the enzyme preparation was established by showing that on both 7.5% regular and 3.7–15% gradient polyacrylamide gel electrophoresis the enzyme migrated as a single protein band which contained all the enzyme activity. The molecular weight of the purified GAD was estimated by gel filtration and gradient polyacrylamide gel to be 84,000 ± 2000 and 90,000 ± 4000, respectively. SDS-polyacrylamide gel electrophoresis revealed three major proteins with molecular weights of 22,000 ± 2000, 40,000 ± 5000 and 90, 000 ± 6000 which may represent a monomer, dimer, and tetramer. Antibodies against the purified enzyme were obtained from rabbit after four biweekly injections with a total of 80 μg of the enzyme. A double immunodiffusion test using these antibodies and a crude extract from catfish brains showed only a single, sharp precipitin band which still retained the enzyme activity, suggesting that the precipitin band was indeed a GAD-anti-GAD complex. In an enzyme inhibition study, a maximum inhibition of 60–70% was obtained at a ratio of GAD protein/anti-GAD serum of about 1:1.6. Furthermore, the precipitate from the GAD-anti-GAD incubation mixture also contained the enzyme activity, suggesting that the antibody was specific to GAD and that the antigen used was homogeneous. Advantages and drawbacks of the purification procedures described here and those used for mouse brain preparations are also discussed.

Notes: Purified homogenous glutamic acid decarboxylase (GAD) from mouse brain and rabbit antiserum prepared to partially purified GAD gave only one sharp precipitin band in the Ouchterlony double diffusion test. GAD activity was inhibited partially by incubating with the antiserum. The maximal extent of inhibition was approximately 50 per cent. In the presence of antiserum all enzyme activity could be precipitated. The precipitates formed by GAD and antiserum had about 50 per cent of the enzyme activity and the Km values for both glutamic acid and pyridoxal phosphate were significantly higher than those of the control system. Pyridoxal phosphate protected GAD from inhibition only slightly, even at very high concentrations. The results suggest that the antibodies may not react with the catalytic site, but rather that the inhibition of enzyme activity is attributable to indirect effects.

Notes: Electrophoretically and ultracentrifugally homogeneous glutamic acid decarboxylase purified from mouse brain showed multiple protein bands after electrophoresis in SDS polyacrylamide gel. The positions and intensities of the multiple bands were constant despite different treatments of the enzyme with various concentrations of SDS, β-mercaptoethanol, and urea at different temperatures. The major band had an apparent molecular weight of approximately 60,000 daltons and there were three minor bands of molecular weights, about 120,000, 90,000, and 75,000 daltons, respectively. The molecular weights of almost all bands were approximately integral multiples of 15,000. The possible subunit structure of this enzyme has been discussed in the light of the latter data and data previously reported from ultracentrifugation and gel filtration studies. We suggest that this enzyme may be a hexamer consisting of 15,000-dalton sub-units and that dissociation of these sub-units in SDS is accompanied by reassociation into a variety of aggregates, the probability of whose formation is determined by structural features that are more important than the differences encountered under the environmental conditions employed in these studies.