Kinetics of the reactions of nitrogen dioxide with glutathione,cysteine, and uric acid at physiological pH

Nitrogen dioxide (NO₂^•) is a key biological oxidant. It can be derived from peroxynitrite via the interaction of nitric oxide with superoxide, from nitrite with peroxidases, or from autoxidation of nitric oxide. In this study, submicromolar concentrations of NO₂^• were generated in < 1 μs using pulse radiolysis, and the kinetics of scavenging NO₂^• by glutathione, cysteine, or uric acid were monitored by spectrophotometry. The formation of the urate radical was observed directly, while the production of the oxidizing radical obtained on reaction of NO₂^• with the thiols (the thiyl radical) was monitored via oxidation of 2,2′-azino-bis-(3-ethylthiazoline-6-sulfonic acid). At pH 7.4, rate constants for reaction of NO₂^• with glutathione, cysteine, and urate were estimated as 2 × 10⁷, 5 × 10⁷, and 2 × 10⁷ M⁻¹ s⁻¹, respectively. The variation of these rate constants with pH indicated that thiolate reacted much faster than undissociated thiol. The dissociation of urate also accelerated reaction with NO₂^• at pH > 8. The thiyl radical from GSH reacted with urate with a rate constant of 3 × 10⁷ M⁻¹ s⁻¹. The implications of these values are: (i) the lifetime of NO₂^• in cytosol is < 10 μs; (ii) thiols are the dominant ‘sink’ for NO₂^• in cells/tissue, whereas urate is also a major scavenger in plasma; (iii) the diffusion distance of NO₂^• is 0.2 μm in the cytoplasm and < 0.8 μm in plasma; (iv) urate protects GSH against depletion on oxidative challenge from NO₂^•; and (v) reactions between NO₂^• and thiols/urate severely limit the likelihood of reaction of NO₂^• with NO• to form N₂O₃ in the cytoplasm.     

Altered antigenic disposition of peroxisomal urate oxidase in PEX5-defective Chinese hamster ovary cells

Since Chinese hamster ovary (CHO) cells never express urate oxidase (UO), we tried to establish cell lines stably producing UO in order to elucidate the peroxisomal import process. The enzyme is a peroxisome targeting signal 1 (PTS1) protein harboring SKL motif at the carboxy-terminus [Biochem. Biophys. Res. Commun. 158 (1989) 991] and PEX5 protein (Pex5p) is supposed to be involved in the import process [Nat. Genet. 9 (1995) 115; J. Cell Biol. 130 (1995) 51]. We transfected a cDNA encoding rat UO into both wild type and PEX5-defective CHO cells to isolate each cell line stably producing the enzyme. While we examined the import process of UO in mutant cells, we noticed an interesting observation by using polyclonal antibody U1 or U2, which separately recognizes epitopes of UO. U1 antibody mainly interacts with epitopes in the amino-terminal region of UO. On the other hand, U2 antibody reacts with many epitopes distributed in the broad region of UO molecule. When UO produced in cultured cells was stained with U2 antibody, the enzyme was detected in peroxisomes of both wild type and PEX5-mutant cells. Whereas, U1 antibody stained the peroxisomal UO in wild type cells, but not in PEX5-mutant cells. These immunocytochemical observations suggest that the epitopes at the amino-terminal region of UO will be concealed in mutant cells. When the mutant cells were transfected with wild type PEX5 cDNA, U1 antibody came to react with UO in peroxisomes of mutant cells. The restoration indicates that the exposure of N-terminal epitopes of UO will depend upon the functional Pex5p. Immunoelectron microscopic observation showed that the peroxisomal import of UO was partially retarded in PEX5 mutant cells. The observation also supported the fact that UO was mainly localized in the peroxisomal matrix of wild type cells but in the membrane of mutant cells.     

Conversion of Tubercidin to Toyocamycin: Some Properties of Tubercidin Derivatives (Nucleosides and Nucleotides. 47)1

Abstract

A facile method of conversion of tubercidin to the 5-methyl derivative and toyocamycin is described. The NMR and CD spectra of 5- and 6-substituted tubercidins are presented. These data show that the 6-substituted tubercidins are in the syn-conformations in solution.     

The oxygen-binding modulation of hemocyanin from the Southern spiny lobster <Emphasis Type="Italic">Palinurus gilchristi</Emphasis>

Arthropod hemocyanins transport and store oxygen and are composed of six subunits, or multiples thereof depending on the species. Palinurus gilchristi hemocyanin is found only as 1 × 6-mers, as normally occurs in spiny lobsters. An alkaline pH and removal of calcium ions induce a wholly reversible dissociation into monomers. The oxygen-binding properties of 1 × 6-meric hemocyanin from P. gilchristi were investigated with respect to pH and modulating effect exerted by calcium, lactate and urate. The oxygen affinity was highly affected by pH in the presence of calcium ions, while in its absence the Bohr coefficient became 60% lower. The protein is insensitive to lactate, but affected by urate which markedly increased hemocyanin–oxygen affinity, acting as the physiological major positive effector. Calcium ions decrease oxygen affinity at low concentration range (0–1 mM), while as concentration becomes higher than 100 mM, the oxygen affinity increases, indicating the presence of two independent types of calcium-binding sites with high and low affinity, respectively. The previous hypothesis, that the presence of high-affinity binding sites in addition to low affinity ones could be a characteristic feature of Palinuran hemocyanins, has been tested by analyzing, with respect to calcium–hemocyanin interaction, three other species belonging to Palinura.     

Urate Oxidation in CSF and Blood of Patients with Inflammatory Disorders of the Nervous System

Urate is largely excluded from the brain under non‐inflammatory conditions (concentration gradient serum:CSF about 10:1), but increases markedly in Guillain–Barré Syndrome and bacterial meningitis. The oxidation product allantoin is normally not passively distributed between blood and cerebrospinal fluid (gradient 3:1) and increases 5‐fold in CSF of patients with meningitis. Patients with multiple sclerosis had normal levels of urate and allantoin in blood and CSF.     

尿酸氧化酶在大肠杆菌中的表达、纯化及活性鉴定

尿酸氧化酶(urate oxidase,Uricase,EC.1.7.3.3)是一种能将尿酸氧化为尿囊素的蛋白酶。合成黄曲霉(Aspergillus flavus)尿酸氧化酶基因,构建表达载体pET43.1a/uox,重组质粒经双酶切鉴定和序列分析,证明插入序列正确,转化到大肠杆菌(Escherichia coli)JM109,菌株经诱导表达尿酸氧化酶蛋白,目的蛋白经过超声破碎,经检测以可溶性蛋白为主;菌体经超声破碎后,上清经过阴离子柱和阳离子柱两步纯化,得到尿酸氧化酶纯品,纯品以分光光度法进行体外酶活性测定。结果显示:尿酸氧化酶在大肠杆菌中获得高效表达,目的蛋白占菌体总蛋白的50%;表达产物经过两步层析柱纯化,获得电泳扫描纯度为95%的纯品;在体外活性测定中具有分解尿酸的能力,在临床检测和治疗中有重要意义。     

Haemocyanin oxygen binding and the physiological ecology of a range of talitroidean amphipods (Crustacea) II. Effect of freezing,inorganic ions,and urate on O2 binding in vitro

Summary The effect of variations in [K], [Ca], [Mg], [NaCl], and [urate] on the in vitro O₂ binding properties of haemocyanin (Hc) from three talitroidean species, viz. the aquatic Apohyale pugettensis, the semi-terrestrial Megalorchestia californiana, and the semi-/euterrestrial Traskorchestia traskiana were studied. Freezing altered the cooperativity of Hc from A. pugettensis and M. californiana but not T. traskiana. Variations in [NaCl], [K], and [Mg] had no effect on cither O₂ affinity or cooperativity of the Hc except for A. pugettensis Hc where an increase in [Mg] resulted in an increase in both O₂ affinity and cooperativity. Increasing [Ca] or [urate] increased O₂ affinity of both A. pugettensis and M. californiana but not T. traskiana Hc. These effects were most marked in A. pugettensis. The results suggest a negative correlation between sensitivity to Hc effectors and the degree of terrestrial adaptation of a particular amphipod species.Abbreviations Hc haemocyanin - STR Stock traskorchestia ringer     

Reconstitution of hepatic uricase in planar lipid bilayer reveals a functional organic anion channel

Rat renal proximal tubule cell membranes have been reported to contain uricase-like proteins that function as electrogenic urate transporters. Although uricase, per se, has only been detected within peroxisomes in rat liver (where it functions as an oxidative enzyme) this protein has been shown to function as a urate transport protein when inserted into liposomes. Since both the uricase-like renal protein and hepatic uricase can transport urate, reconstitution studies were performed to further characterize the mechanism by which uricase may function as a transport protein. Ion channel activity was evaluated in planar lipid bilayers before and after fusion of uricase-containing proteoliposomes. In the presence of symmetrical solutions of urate and KCl, but absence of uricase, no current was generated when the voltage was ramped between ±100 mV. Following fusion of uricase with the bilayer, single channel activity was evident: the reconstituted channel rectified with a mean slope conductance of 8 pS, displayed voltage sensitivity, and demonstrated a marked selectivity for urate relative to K⁺ and Cl^–. The channel was more selective to oxonate, an inhibitor of both enzymatic uricase activity and urate transport, than urate and it was equally selective to urate and pyrazinoate, an inhibitor of urate transport. With time, pyrazinoate blocked both its own movement and the movement of urate through the channel. Channel activity was also blocked by the IgG fraction of a polyclonal antibody to affinity purified pig liver uricase. These studies demonstrate that a highly selective, voltage dependent organic anion channel is formed when a purified preparation of uricase is reconstituted in lipid bilayers.This work was supported in part by the G. Harold and Leila Y. Mathers Charitable Foundation (E.L.P. and R.D.L.), the Irma T. Hirschl Trust (R.D.L.), National Institutes of Health grant DK08419 (B.A.K.) and a Grant-in-Aid from the American Heart Association, N.Y.C. Affiliate (R.G.A.).     

Functional Analysis and Molecular Modeling of a Cloned Urate Transporter/Channel

Recombinant protein, designated UAT, prepared from a cloned rat renal cDNA library functions as a selective voltage-sensitive urate transporter/channel when fused with lipid bilayers. Since we previously suggested that UAT may represent the mammalian electrogenic urate transporter, UAT has been functionally characterized in the presence and absence of potential channel blockers, several of which are known to block mammalian electrogenic urate transport. Two substrates, oxonate (a competitive uricase inhibitor) and pyrazinoate, that inhibit renal electrogenic urate transport also block UAT activity. Of note, oxonate selectively blocks from the cytoplasmic side of the channel while pyrazinoate only blocks from the channel's extracellular face. Like oxonate, anti-uricase (an electrogenic transport inhibitor) also selectively blocks channel activity from the cytoplasmic side. Adenosine blocks from the extracellular side exclusively while xanthine blocks from both sides. These effects are consistent with newly identified regions of homology to uricase and the adenosine A1/A3 receptor in UAT and localize these homologous regions to the cytoplasmic and extracellular faces of UAT, respectively. Additionally, computer analyses identified four putative α-helical transmembrane domains, two β sheets, and blocks of homology to the E and B loops of aquaporin-1 within UAT. The experimental observations substantiate our proposal that UAT is the molecular representation of the renal electrogenic urate transporter and, in conjunction with computer algorithms, suggest a possible molecular structure for this unique channel. Received: 13 October 1998/Revised: 28 January 1999