Translational initiation factor 2 (IF2) is a guanine nucleotidebinding protein that can bind guanosine 3 ,5 -(bis) diphosphate (ppGpp), an alarmone involved in stringent response in bacteria. In cells growing under optimal conditions, the GTP concentration is very high, and that of ppGpp very low. However, under stress conditions, the GTP concentration may decline by as much as 50%, and that of ppGpp can attain levels comparable to those of GTP. Here we show that IF2 binds ppGpp at the same nucleotide-binding site and with similar affinity as GTP. Thus, GTP and the alarmone ppGpp can be considered two alternative physiologically relevant IF2 ligands. ppGpp interferes with IF2-dependent initiation complex formation, severely inhibits initiation dipeptide formation, and blocks the initiation step of translation. Our data suggest that IF2 has the properties of a cellular metabolic sensor and regulator that oscillates between an active GTP-bound form under conditions allowing active protein syntheses and an inactive ppGpp-bound form when shortage of nutrients would be detrimental, if not accompanied by slackening of this synthesis.fast kinetics ͉ GTP ͉ translation regulation ͉ nutritional stress I nitiation factor 2 (IF2) is the only initiation factor that is ribosome-bound throughout the entire translation initiation pathway, participating initially in the formation of the 30S initiation complex (30SIC) and subsequently in the assembly of the 70S initiation complex (70SIC), a process that ultimately results in formation of the first peptide bond (initiation dipeptide) and generates the first ribosomal pretranslocation complex (for reviews, see refs. 1-5). Thus, it could be predicted that IF2 functions are accompanied͞modulated by conformational changes that could be either consequence or cause of the interactions of IF2 with its various ligands (30S and 50S ribosomal subunits, fMet-tRNA, GTP, GDP⅐Pi, and GDP). Crystallographic (6) and NMR (7) studies have, in fact, shown that, depending upon the nature of their ligand (i.e., GTP or GDP), aIF5B, the archaeal homologue of bacterial IF2, as well as isolated IF2G2, the G domain of IF2, undergo large structural changes. On the other hand, chemical probing (8) and cryo-EM (9, 10) have clearly shown that several conformational changes of the factor occur during the early, middle, and late events of translation initiation.Bacterial cells growing under optimal nutritional conditions contain a high (Ͼ1 mM) concentration of GTP and a vanishingly low level of GDP. Thus, IF2 is expected to exist and to bind the 30S subunit almost exclusively in the GTP form, because it displays similar affinity for GTP and GDP, both K d s being in the 10-to 100-M range (11). The IF2⅐GTP was shown to have a higher affinity for the 30S ribosomal subunit than either IF2⅐GDP or free IF2 (11). The adjustment of fMet-tRNA in the P site (ref. 12 and refs. therein) and the release of IF2 from 70SIC (13-15) have been attributed to the IF2-dependent GTP hydrolysis, which is very rapidly triggered by th...

Vascular wall protection can be achieved by preventive attachment to the vascular wall of antioxidants and elimination/neutralization of toxic products after their disproportioning. For this purpose we have prepared covalent conjugates between the vascular wall glycosaminglycan chondroitin sulfate (CHS) and the antioxidant enzymes superoxide dismutase (SOD) and catalase (CAT). The following conjugates were obtained: SOD-CHS, CAT-CHS and SOD-CHS-CAT. Their anti-thrombotic activity was compared in a rat model of arterial thrombosis by measuring the time of occlusion emergence and thrombus mass. It is noteworthy that the effectiveness of single bolus injections of SOD-CHS/CAT-CHS mixture was much lower than that of the bienzymic SOD-CHS-CAT conjugate. The conjugate SOD-CHS-CAT proved to be anti-thrombotically effective in doses two orders of magnitude lower than the native biocatalysts and an order of magnitude lower than SOD-CHS and CAT-CHS derivatives. For effective anti-thrombotic protection in oxidative conditions it is important to maintain the stable connection of SOD and CAT activity on the vascular wall and the large size of these conjugates. Covalent conjugate SOD-CHS-CAT is the best prospect for pharmaceutical development.

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Properties of native and aldehyde dextran-modified hyaluronidase (with surface amino group modification about 98%) were investigated. Optimal endoglycosidase activity of the native enzyme was observed at 0.15 M NaCl and pH 5.5 and electrostatic interactions influenced the enzyme activity. The inhibitory effect of heparin on hyaluronidase activity slightly differed at pH 5.5 (1.5-fold inhibition) and 7.5 (1.2-fold inhibition). Ionic strength of the reaction medium only slightly influenced the effect of heparin. Modification of hyaluronidase with dextran increased hydrophobic interactions and steric hindrance. Conjugation with dextran increased the resistance of hyaluronidase activity to denaturing agents (urea, guanidinium hydrobromide) and extended the optimal conditions for maximal endoglycosidase activity (pH 4.5-6.5, the range of NaCl concentration from 0.1 to 0.3 M). The conjugation also reduced electrostatic effects on the active site of hyaluronidase and efficacy of heparin inhibition. At pH 7.5 the enzyme was almost insensitive to heparin. The resistance of dextran-modified hyaluronidase to heparin points to approaches for subsequent studies of the heparin-binding site of this enzyme and biomedical trial of the stabilized enzyme for the treatment of acute cardiovascular lesions.

The glycosaminoglycan microenvironment of testicular hyaluronidase was simulated by multipoint covalent attachment of the enzyme to glycans as a result of benzoquinone activation. The efficiency of their binding was assessed using gel chromatography, ultrafiltration, titration of surface amino groups of the enzyme, electrophoresis, as well as judging by the value of residual endoglycosidase activity and its inhibition with heparin. Copolymer glycosaminoglycans, such as dermatan sulfate and heparin, inactivated the endoglycosidase activity as a result the C-5 epimerization of hexuronic acid. It was shown that glucuronic acid and, to a lesser extent, N-acetylglucosamine determine the specificity of hyaluronidase. The chondroitin-sulfate microenvironment made the enzyme resistant to heparin inhibition because the equatorial orientation of the OH groups is similar to that in hyaluronic acid. Model experiments with dextran and dextran sulfate showed that sulfation of the glycan chain increased its rigidity, thus hampering the stabilizing effect on hyaluronidase. The effect of chondroitin sulfate on the endoglycosidase activity of hyaluronidase had additive character and did not directly affect the small fragment of the active site of the enzyme located at the bottom of a groove. The glycosaminoglycan microenvironment of hyaluronidase, containing an iduronic acid residue, the alpha1-3 and alpha1-4 glycosidic bond, inactivated the hyaluronidase activity of the enzyme, whereas simple polymers (such as gluco- and galactoaminoglycans) potentiated it due to a similar way of linking--beta(1e-4e) and beta(1e-3e). To understand the nature of these interactions in detail, the effect of oligomeric glycosaminoglycan fragments and their derivatives on hyaluronidase should be studied.

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SummaryThe preparation and some properties of microspheres composed of oxidized polysaccharides and some vinyl polymers are described. The microspheres contain immobilized enzyme and can be slowly solubilized in water solutions, thereby releasing active a enzyme into the surrounding medium. The kinetic characteristics of the immobilized enzyme bound with a fragment of matrix after complete solubilization are unchanged, but the enzyme exhibits high thermostability. These preparations could have a wide range of medical applications, e.g., to form a drug "depot" directly in an affected organ.

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