No abstract

Induction of tyrosine kinase activity of the insulin receptor (IR) beta-chain is believed to require its autophosphorylation at Tyr1162, Tyr1163, and Tyr1158. However, the mechanism of the initial phosphorylation is poorly understood. We show that treatment of IR-transfected Chinese hamster ovary cells with antioxidants inhibits insulin responsiveness. Conversely, partial inhibition of glutathione biosynthesis by buthionine sulfoximine (BSO) and glutathione reductase by 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU), i.e., procedures that intracellularly induce mildly oxidative conditions, caused a decrease in IR beta-chain sulfhydryl groups and enhanced synergistically the induction of IR tyrosine phosphorylation by insulin. The IR beta-chain from cells treated with BSO/BCNU in the absence of insulin was not detectably tyrosine phosphorylated, but nevertheless was functionally altered, as demonstrated in vitro by a moderate kinase activity at lowATP concentrations (5 nM) and a strong kinase activity at 25 microM ATP. This activity was found to be specific for tyrosine (not for serine or threonine), and tryptic peptide maps indicated that it is more selective than that induced by insulin. Moreover, the kinase activity from BSO/BCNU-treated cells showed a spontaneous decay that was not prevented by the phosphatase inhibitor vanadate. Together, these results suggest that optimal insulin responsiveness may require a process of 'redox priming' of the IR beta-chain that involves structural and functional changes in the absence of detectable tyrosine phosphorylation of the beta-chain.

Signaling by insulin requires autophosphorylation of the insulin receptor kinase (IRK) at Tyr1158, Tyr1162, and Tyr1163. Earlier experiments with (32)P-gamma-ATP indicated that the nonphosphorylated IRK (IRK-0P) is relatively inactive, and crystallographic data indicated that the ATP binding site of IRK-0P is blocked by its activation loop. We now show that phosphocreatine (PCr) in combination with hydrogen peroxide serves as an alternative phosphate donor and that ATP and PCr use distinct binding sites. Whereas phosphorylation of the IRK by ATP is inhibited by the nonhydrolyzable competitor adenylyl-imidodiphosphate, phosphorylation by PCr is enhanced. The IRK mutant Tyr1158Phe showed no phosphorylation with PCr but almost normal phosphorylation with ATP, whereas Tyr1162Phe was phosphorylated well with PCr but less then normal with ATP. 3-Dimensional models of IRK-0P revealed that the conversion of any of the four cysteine residues 1056, 1138, 1234, and 1245 into sulfenic acid produces structural changes that bring Tyr1158 into close contact with Asp1083 and render the well-known catalytic site at Asp1132 and Tyr1162 accessible from a direction that differs from the known ATP binding site. The mutant Cys1138Ala, in contrast, showed relatively inaccessible catalytic sites and weak catalytic activity in functional experiments. Taken together, these findings indicate that 'redox priming' of the IRK facilitates its autophosphorylation by PCr in the activation loop.

Abnormally low plasma cystine levels have been found in the late asymptomatic stage of HIV infection and several other diseases associated with progressive loss of skeletal muscle mass. The phenomenon is commonly associated with a low NK cell activity, skeletal muscle wasting or muscle fatigue and increased rates of urea production. In its extreme form, the negative nitrogen balance leads to overt cachexia and is associated with severe debilitation and psychological stress. The low NK cell activity is in most cases not life-threatening but may be disasterous in HIV infection, because it may compromise the initially stable balance between immune system and virus and trigger disease progression. This review summarizes briefly (i) the role of cysteine in the physiological regulation of body cell mass and the development of skeletal muscle wasting, and (ii) the role of glutathione in the immune system.

Previous studies on cultured skeletal muscle cells have indicated that the insulin-induced expression of GLUT4 transporter protein is inhibited by nitric oxide (NO). Therefore, we determined the effect of NO on the insulin-induced autophosphorylation of the insulin receptor kinase (IRK), i.e., the first step in the insulin-mediated signal transduction pathway. The experiments showed that the insulin-induced autophosphorylation of the insulin receptor beta-chain is strongly inhibited by the NO donors 1,1-diethyl-2-hydroxy-2-nitrosohydrazine (DEA-NO) or S-nitroso-N-acetylpenicillamine (SNAP). The inhibitory effect was ameliorated in cells depleted of glutathione (GSH), suggesting the possibility that S-nitroso-glutathione may operate as an intermediate NO donor. Complementary experiments with different Cys --> Ala mutant proteins showed, surprisingly, that all mutant proteins were inhibited by DEA-NO. Three-dimensional models of the nonphosphorylated IR beta-chain nitrosylated at the accessible cysteine residues 1056, 1138, 1234, or 1245 revealed that derivatization of any of these four cysteine residues leads essentially to the same structural changes of the IRK domain. These changes involve a movement of the amino-terminal lobe against the carboxy-terminal lobe in a direction opposite to the direction of the "lobe closure" that was previously proposed to facilitate the accessibility for ATP and the expression of catalytic activity. Our findings suggest that the occurrence of several functionally relevant cysteine residues in distinct regions of the IRK protein increases the probability of regulatory redox interactions and thus the redox sensitivity of the IRK.

A series of new 6-(alkylthio)ascorbic acids was synthesized, and their inhibitory effects on lipid peroxidation and the oxidative burst of human neutrophils were tested. Of 12 structurally different lipophilic ascorbic acid derivatives 6-S-n-hexadecyl-2-O-methyl-6-deoxy-6-thio-L-ascorbic acid (7b; B-003) inhibited the Fe2+/ADP-induced lipid peroxidation of rat liver microsomes with an IC50 value of 2 microM. In human neutrophils, 7b most potently inhibited the fMLP-induced oxidative burst in a cell density-dependent manner with an IC50 value of 0.6 microM at 5 x 10(5) cells/mL. Shorter alkyl chain lengths decreased the inhibitory potency for both lipid peroxidation and oxidative burst, but in general no correlation was found between the two parameters. Likewise, 6-S-n-hexadecyl-3-O-methyl-6-thio-L-ascorbic acid (7c; B-015), the regioisomer of 7b, was a potent antioxidant but did not affect the oxidative burst. Since superoxide anions generated by xanthine/xanthine oxidase were not quenched by 7b, it became evident that its target was somewhere between receptor stimulation and NADPH-oxidase activation. By measuring the cellular concentrations of 7b and 7c, an accumulation of the first was found explaining its potency and the dependence on cell density. Expecting a pKa value of 3.3 for 7b and 7.7 for 7c a protonophore action of 7b was likely and could be verified by the drop in intracellular pH (pHi) which did not occur with 7c. Ionophores such as nigericin, CCCP, or propionic acid also lowered the pHi but did not inhibit the oxidative burst, indicating that the pHi drop was not the cause for this inhibition. 7b also strongly inhibited the fMLP-induced secretion of azurophilic (IC50 = 7 microM) and specific (IC50 = 2.5 microM) granules.

The transition metal substituted arsanes Cp(CO),(L)M -AsMe, ( l ad) (M = Mo, W; L = CO, Me3P) react with the tetrahydrofuran adduct of H3B to yield the corresponding 1 : 1 coordination compounds Cp(CO),(L)M -AsMez-BH, (2ad), which are characterized by 'H, "B NMR, and IR spectroscopy. Treatment of 2a with Me,P results both in CO substitution and BH, abstraction to give 2c, 1 a, and Me3P -BH,. Ubergangsmetall-substituierte Arsane des Typs Cp(CO),(L)M -AsMe, (M = Cr, Mo, W; L = COza), Me3P ,b)) sind aufgrund des ausgepragten Donatorvermtigens ihrer Arsenidofunktion starke Nucleophile und ausgezeichnete Komplexliganden. Diese Eigenschaften lassen sich praparativ zur Gewinnung neuartiger Kationkomplexe durch Quartarisierung mit Alkylhalogeniden2), von metallierten Arsinchalkogeniden durch Oxidation mit elementarem Schwefel oder Selen und zum Aufbau von Zwei-und Mehrkernkomplexen 4, sowie von Clusterverbindungen5) nutzen. Wahrend das Koordinationsvermtigen der metallierten Arsane an Ubergangsmetallzentren inzwischen durch zahlreiche Beispiele belegt ist4J), existieren bisher noch keine Komplexe mit Lewisaciden Hauptgruppenelementverbindungen. In ersten Experimenten haben wir uns jetzt fur die Anlagerung des im freien Zustand extrem kurzlebigen Teilchens BH, interessiert 6), weil fur dieses im Prinzip auch die Mtiglichkeit der Koordination an das Lewis-basische Metallzentrum besteht '). Im Falle einer Anlagerung am trivalenten Arsenatom erscheint daruber hinaus analog zur Zersetzungsreaktion von Me2As -AsMe, . BH38) die Einfuhrung einer BH,-Gruppe am Metall unter Me,AsH-Eliminierung mbglich. Die Umsetzung der Ubergangsmetall-substituierten Arsane Cp(CO),(L)M -AsMe, l ad in Benzol oder THF mit dem Tetrahydrofuran-Addukt von BH, liefert nach GI. (1) unter glatter Ubertragung des BH,-Teilchens vom Ethersauerstoff auf die Arsenidofunktion die 1 : 1-Addukte 2 s -d . Die Metallo-arsan-borane werden in 82 -93proz. Ausbeute als ockergelbe (2a) bzw. hellgelbe (2bd), kristallisierte, gegeniiber Luft und Feuchtigkeit extrem bestiindige Feststoffe gewonnen. 2a, b ltisen sich gut in Pentan und Cyclohexan, 2c, d nur noch in den polaren Solventien THF, Acetonitril oder Aceton. Beim Erhitzen in Benzol (7 d, 60°C) zeigen 2a-d weder Dissoziation, noch die erhoffte Spaltungsreaktion. Trotz des sterisch anspruchsvollen Charakters ihrer metallierten Arsandonatoren sind sie damit thermisch wesentlich stabiler als die entsprechenden BH,-261 Im Massenspektrum von 2a, b findet sich M + rnit geringer Intensitat, fur 2c, d IaBt sich als Ion hochster Masse nur noch [M -BH,]' nachweisen. Anhaltspunkte fur das ursprungliche Vorliegen eines BH3-Addukts ergeben sich aber aus einem intensiven Signal rnit m/e = 28 (rel. Int. = 100%) bzw. m / e = 90 (rel. Int. = 63%), das den Teilchen BzH6+ bzw. Me,P -BH: zuzuordnen ist . Die Deutsche Forschungsgemeinschaft und der Fonds der Chembchen Industrie unterstutzte diese Arbeiten rnit Sachmitteln, die Fa. Hochst, Werk Knapsack, und die BASE Aktiengesellschaft mit Chemikalienspenden. Den Herren Dr. W. Buchner und...

The mammalian Ras GTPase-activating protein (p120 Ras-GAP ) interacts with activated members of the Ras superfamily of GTP-binding proteins to accelerate their deactivation by sharply increasing their rates of GTP hydrolysis. Among the Ras-family proteins interacting with p120Ras-GAP is Rap1A/Krev1, whose activity is not affected by p120Ras-GAP but which competes with Ras for p120 Ras-GAP . A second protein that interacts with p120Ras-GAP is p190Rac-GAP , which activates the GTPase of guanine nucleotide-binding proteins of the Rho family (including Rac1 and Rac2). Both these p120Ras-GAP -binding proteins are of interest in connection with the regulation of the respiratory burst oxidase, Rap1A/ Krev1 because it copurifies with cytochrome b 558 , and p190Rac-GAP because it inhibits the Rac2-dependent activation of the respiratory burst oxidase in a cell-free system. Using an 18-mer antisense oligonucleotide, we were able to decrease the expression of p120Ras-GAP in Epstein-Barr virus-transformed B lymphocytes. Under conditions where p120Ras-GAP expression was significantly depressed by antisense oligonucleotides, we observed a 40% increase in protein kinase C-dependent but not receptor-dependent O 2 . production. In contrast, sense and scrambled oligonucleotides had no effect on either p120 Ras-GAP expression or O 2 . production. Our results suggest a role for p120 Ras-GAP as a negative regulator in the protein kinase C-mediated activation of the respiratory burst oxidase.