Effect of desialylation on the biological properties of human plasminogen

Sialic acid-containing glycans play a role in the activity of snake venom proteases

Structural variability is a feature of snake venom proteins, and glycosylation is a post-translational modification that contributes to the diversification of venom proteomes. Studies by our group have shown that Bothrops venoms are distinctly defined by their glycoprotein content, and that most hybrid/complex N-glycans identified in these venoms contain sialic acid. Considering that metalloproteases and serine proteases are abundant components of Bothrops venoms and essential in the envenomation process, and that these enzymes contain several glycosylation sites, the role of sialic acid in venom proteolytic activity was evaluated. Here we show that removal of sialic acid by treatment of nine Bothrops venoms with neuraminidase (i) altered the pattern of gelatinolysis in zymography of most venoms and reduced the gelatinolytic activity of all venoms, (ii) decreased the proteolytic activity of some venoms on fibrinogen and the clotting activity of human plasma of all venoms, and (iii) altered the proteolysis profile of plasma proteins by B. jararaca venom, suggesting that sialic acid may play a role in the interaction of proteases with their protein substrates. In contrast, the profile of venom amidolytic activity on Bz-Arg-pNA did not change after removal of sialic acid, indicating that this monosaccharide is not essential in N-glycans of serine proteases acting on small substrates. In summary, these results expand the knowledge about the variability of the subproteomes of Bothrops venom proteases, and for the first time point to the importance of carbohydrate chains containing sialic acid in the enzymatic activities of venom proteases relevant in human envenomation.

Angiostatin's molecular mechanism: aspects of specificity and regulation elucidated

Tumor growth requires the development of new vessels that sprout from pre-existing normal vessels in a process known as "angiogenesis" [Folkman (1971) N Engl J Med 285:1182-1186]. These new vessels arise from local capillaries, arteries, and veins in response to the release of soluble growth factors from the tumor mass, enabling these tumors to grow beyond the diffusion-limited size of approximately 2 mm diameter. Angiostatin, a naturally occurring inhibitor of angiogenesis, was discovered based on its ability to block tumor growth in vivo by inhibiting the formation of new tumor blood vessels [O'Reilly et al. (1994a) Cold Spring Harb Symp Quant Biol 59:471-482]. Angiostatin is a proteolytically derived internal fragment of plasminogen and may contain various members of the five plasminogen "kringle" domains, depending on the exact sites of proteolysis. Different forms of angiostatin have measurably different activities, suggesting that much remains to be elucidated about angiostatin biology. A number of groups have sought to identify the native cell surface binding site(s) for angiostatin, resulting in at least five different binding sites proposed for angiostatin on the surface of endothelial cells (EC). This review will consider the data supporting all of the various reported angiostatin binding sites and will focus particular attention on the angiostatin binding protein identified by our group: F(1)F(O) ATP synthase. There have been several developments in the quest to elucidate the mechanism of action of angiostatin and the regulation of its receptor. The purpose of this review is to describe the highlights of research on the mechanism of action of angiostatin, its' interaction with ATP synthase on the EC surface, modulators of its activity, and issues that should be explored in future research related to angiostatin and other anti-angiogenic agents.

Effect of alcohol on lipids and lipoproteins in relation to atherosclerosis

Several studies indicate that light-to-moderate alcohol consumption is associated with a low prevalence of coronary heart disease. An increase in high-density lipoprotein (HDL) cholesterol is associated with alcohol intake and appears to account for approximately half of alcohol's cardioprotective effect. In addition to changes in the concentration and composition of lipoproteins, alcohol consumption may alter the activities of plasma proteins and enzymes involved in lipoprotein metabolism: cholesteryl ester transfer protein, phospholipid transfer protein, lecithin:cholesterol acyltransferase, lipoprotein lipase, hepatic lipase, paraoxonase-1 and phospholipases. Alcohol intake also results in modifications of lipoprotein particles: low sialic acid content in apolipoprotein components of lipoprotein particles (e.g., HDL apo E and apo J) and acetaldehyde modification of apolipoproteins. In addition, "abnormal" lipids, phosphatidylethanol, and fatty acid ethyl esters formed in the presence of ethanol are associated with lipoproteins in plasma. The effects of lipoproteins on the vascular wall cells (endothelial cells, smooth muscle cells, and monocyte/macrophages) may be modulated by ethanol and the alterations further enhanced by modified lipids. The present review discusses the effects of alcohol on lipoproteins in cholesterol transport, as well as the novel effects of lipoproteins on vascular wall cells.

Sialoforms of dipeptidylpeptidase IV from rat kidney and liver

Dipeptidylpeptidase IV (DPP IV, CD26), a serine-type exo- and endopeptidase found in the cell surface membrane of many tissues, was employed as a model membrane glycoprotein to study the expression of sialoforms on cell surface glycoproteins. Native, enzymatically active DPP IV was purified from plasma membranes of kidney and liver by lectin affinity chromatography in conjunction with crown ether anion exchange chromatography. The enzyme was gradient-eluted in continuous fractions, all showing a single polypeptide band of about 100 kDa when separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under reducing, denaturing conditions. Analysis of the purified DPP IV by isoelectric focusing (IEF) showed that it consists of several polypeptides of different isoelectric points (IP) ranging from 5.5 to 7.0. In vitro- desialylation of the enzyme and subsequent isoelectric focusing revealed that the differences in isoelectric points were due to differences in the degree of sialylation. Differences in the degree of sialylation between the fractions were also demonstrated by SDS-PAGE under nonreducing and nondenaturing conditions. Increased sialylation of the enzyme as demonstrated by isoelectric focusing resulted in increased migration velocity in nonreducing and nondenaturing SDS-polyacrylamide gels. In vitro -desialylation of the enzyme and its resialylation confirmed that sialylation was responsible for this extraordinary migration behavior. The native enzyme was predominantly sialylated via alpha 2, 6-linkage, as shown by lectin affinity blotting employing Sambucus nigra agglutinin (SNA) and Maackia amurensis agglutinin (MAA). These findings demonstrate that a distinct membrane glycoprotein may exist in various sialoforms, distinguished from each other by a different number of sialic acid residues. Moreover, these sialoforms can be individually purified by crown ether anion exchange chromatography.

Effect of plasminogen activators on human recombinant apolipoprotein(a) having the plasminogen activation cleavage site

The serine-proteinase domain in human apolipoprotein(a) [apo(a)] and plasminogen exhibit 89% sequence identity including the catalytic triad. Cleavage of the Arg(561)-Val(562) activation site in plasminogen by either tissue- or urokinase-type plasminogen activator results in formation of the fibrinolytic enzyme plasmin. Apo(a) does not contain measurable amidolytic activity nor can it be activated by plasminogen activators. It has been suggested that the latter finding might be explained by the substitution of the plasminogen Arg-Val activation site by Ser-Ile in apo(a). To investigate if introduction of the Arg-Val activation site in apo(a) might result in sensitivity towards plasminogen activators, we expressed wild-type and Arg-Val mutant recombinant apo(a) [r-apo(a)] in human embryonic kidney and hepatocyte cell lines. Free r-apo(a) and lipoprotein-like particles [r-Lp(a)] were obtained in the culture supernatants of transfected 293 and HepG2 cells, respectively. Incubation of mutant r-apo(a)/r-Lp(a) with plasminogen activators produced neither plasmin-like activity nor cleavage at the Arg-Val activation site, even in the presence of various stimulators of plasminogen activation. Our data suggest that the high selectivity of activators for plasminogen activation requires interactions with regions in plasminogen distant from the activation disulfide loop which are not present in apo(a).

Identification and characterization of a sialidase released by the salivary gland of the hematophagous insect Triatoma infestans

Sialidases (EC 3.2.1.18) are commonly found in viruses, bacteria, fungi, protozoa, and vertebrates, but not in invertebrates. We have previously reported the presence of a new sialidase activity in the gut of exclusively hematophagous insects of the Triatoma genus, which transmit Chagas' disease (Amino, R., Acosta, A., Morita, O. M., Chioccola, V. L. P., and Schenkman, S. (1995) Glycobiology 5, 625-631). Here we show that this sialidase is present in the salivary gland of Triatoma infestans, and it is released with the saliva during the insect bite. The sialidase was purified to homogeneity (>5000 times) to a specific activity of more than 20 units/mg. It elutes from a gel filtration column with a volume corresponding to the size of 33 kDa, and it migrates as a single 26-kDa band in SDS-polyacrylamide gel electrophoresis, which is unusually smaller when compared with other known sialidases. T. infestans sialidase hydrolyzes preferentially alpha2-->3-linked sialic acids at pH 4-8, with maximal activity between pH 5.5 and 6.5, which is compatible with the optimal pH of secreted sialidases. The sialidase is competitively inhibited by 2-deoxy-2, 3-dehydro-N-acetyl-neuraminic acid (Ki = 0.075 mM) and differently from many sialidases, with exception of Salmonella typhimurium sialidase, it is inhibited competitively by HEPES (Ki = 15 mM). The fact that T. infestans sialidase is released with the saliva and can hydrolyze sialyl-LewisX blood groups, which are the ligands for selectins, suggests that it might have a role in the blood feeding.

Rapid analysis of O-acetylated neuraminic acids by matrix assisted laser desorption/ionization time-of-flight mass spectrometry

N-Acetylneuraminic acid (a sialic acid) occurs mainly as a terminal substituent of oligosaccharides of glycoconjugates. Derivatives of neuraminic acid occur widely, substituted in the amino and hydroxy side chains, as well in the C-9 carbon skeleton. These derivatives are responsible for specific functions of sialic acids during cell-cell, cell-substrate, or cell-virus interactions. The study of O-acetylated neuraminic acids is difficult, because only small amounts are extractable from natural sources and they are generally unstable to acids and bases. We report a new method for the rapid analysis of O-acetylated neuraminic acids, using a combination of reversed phase HPLC and MALDI-TOF mass spectrometry. A mixture of neuraminic acids from bovine submaxillary gland mucins was analysed, as well as neuraminic acids variously substituted in the amino and hydroxy side chains with acetyl and glycolyl groups, respectively.

Sialic acid content of plasminogen 2 glycoforms as a regulator of fibrinolytic activity. Isolation, carbohydrate analysis, and kinetic characterization of six glycoforms of plasminogen

Six glycoforms of plasminogen 2 were isolated using a combination of lectin affinity chromatography and chromatofocussing, and the sialic acid content of each glycoform was determined. The kinetics of activation of each glycoform by tissue-type plasminogen activator were analyzed on a fibrin surface and in solution. The second-order rate constant (measured on a fibrin surface) decreased from 1.65 x 10(6) M-1 s-1 to 3.77 x 10(4) M-1 s-1 as the sialic acid content of the glycoforms increased from 1.3 mol/mol of protein to 13.65 mol/mol of protein. A similar correlation was noted for activation in solution. Each glycoform was converted to plasmin, and the inhibition constants for the reaction between alpha 2-antiplasmin and plasmin glycoforms were determined. All overall Ki values, reflecting the final essentially irreversible complex, were in the picomolar range. Sialic acid does not affect inhibition of plasmin by alpha 2-antiplasmin; however, hypersialylated plasmin does not appear to have a kringle-dependent component to inhibition.

Regulation of proteolytic activity in tissues

Degradation of tissue proteins is controlled by multiple means. These include regulation of the synthesis of proteinases, activation of the zymogen forms, the activity of the mature proteinase, and the degradation of these enzymes and the substrates. Mature proteinases can be controlled by pH, calcium ions, ATP, lipids and the formation of complexes with other proteinases, proteoglycans, and inhibitors.