Human platelet heparanase: purification, characterization and catalytic activity

Partial sequence of human platelet heparitinase and evidence of its ability to polymerize

Heparitinase cleaves heparan sulfate, a glycosaminoglycan associated with all nucleated mammalian cells and extracellular matrices. Despite the important physiologic role heparitinase is postulated to play in such processes as tumor metastasis and inflammation, the identity of the enzyme remains a matter of controversy and there is a question of whether heparitinase is CTAP III. We report a 900,000-fold purification of heparitinase from human platelets. A multi-step procedure utilizing chromatography on heparin, DEAE, hydroxyapatite and size exclusion matrices was employed and yielded a single protein as judged by Coomassie staining of protein separated by SDS-PAGE. The purified protein had an apparent molecular mass of 35 kDa by size exclusion chromatography and 55 kDa by SDS-PAGE. During purification, heparitinase activity co-eluted from the hydroxyapatite and size exclusion columns with the 35-55 kDa protein, confirming that the purified protein was indeed heparitinase. The 35-55 kDa protein reacted strongly with concanavalin A, a lectin known to bind to heparitinase, further confirming that the protein was heparitinase. Platelet heparitinase formed dimers and tetramers upon storage in a purified form, possibly accounting for the various molecular weights previously reported for the enzyme. A partial amino acid sequence of the protein revealed that heparitinase has not been previously sequenced.

Partial purification of heparanase activities in Chinese hamster ovary cells: evidence for multiple intracellular heparanases

Heparanases are mammalian endoglycosidases that cleave heparan sulphate glycosaminoglycans from proteoglycan core proteins and degrade them into shorter chains. The enzymes have been proposed to act in a variety of cellular processes, including proteoglycan catabolism, remodelling of basement membranes and release of heparan sulphate-binding ligands from their extracellular storage sites. Additional functions for heparanases may be to generate short heparan sulphate chains that stabilize or activate other proteins. While heparanase activities have been described in a number of tissues and cell lines, it is not known how many different enzymes are responsible for these activities. Our recent studies characterizing the short glycosaminoglycans produced in Chinese hamster ovary (CHO) cells suggested that multiple heparanases are necessary for the formation of the short heparan sulphate chains [Bame and Robson (1997) J. Biol. Chem. 272, 2245-2251]. We examined whether this is the case by purifying heparanase activity from CHO cell homogenates. Based on their ability to bind ion-exchange resins and their elution from gel-filtration columns, four separate heparanase activities were partially purified. All four activities cleave free glycosaminoglycans over a broad pH range of 3.5-6.0 or 6. 5, suggesting that they act in the endosomal/lysosomal pathway. The sizes of the short heparan sulphate chains generated by the partially purified heparanases ranged from 6 to 9 kDa, and for two of the activities the product size is pH-dependent. Three of the four activities degrade proteoglycans as well as the free glycosaminoglycan chain. Interestingly, all four enzymes generate short glycosaminoglycans with a sulphate-rich, modified domain at the non-reducing end of the newly formed chain. Since our previous studies showed that in CHO cells there is also a population of short heparan sulphates with a modified domain at the reducing end of the chain, this suggests that there may be another heparanase in CHO cells that was not purified. Alternatively, our findings suggest that the formation of short heparan sulphate glycosaminoglycans inside CHO cells may be a result of the concerted action of multiple heparanases, and may depend on the proportions of the different enzymes and the environment in which the chains are degraded.

Treatment of central nervous system inflammation with inhibitors of basement membrane degradation

Currently available anti-inflammatory drugs for the treatment of multiple sclerosis (MS) and other inflammatory diseases are generally inadequate, with disease progression not being arrested by the treatments and undesirable side effects posing problems. In response to these deficiencies our laboratories have, over the past 10 years, been developing novel drugs that interfere with the entry of leucocytes into inflammatory sites by inhibiting their passage through the subendothelial basement membrane (BM). This review initially summarizes evidence supporting the hypothesis that the subendothelial BM is a major barrier to the accumulation of leucocytes in inflammatory sites. An important point that has emerged is that breaching of the BM is probably a cooperative process, involving activation- and cytokine-induced degradative enzymes contributed by leucocytes, endothelial cells and platelets. The review then discusses the properties of three separate classes of anti-inflammatory compounds we have developed, namely sulfated polysaccharides/oligosaccharides, phosphosugars, and castanospermine (CS), which inhibit the passage of leukocytes through BM. Each drug type appears to prevent BM degradation by a different mechanism. Sulfated polysaccharides/oligosaccharides mediate their anti-inflammatory effect by inhibiting the endoglycosidase, heparanase, which plays a key role in the solubilization of BM by invading leucocytes. In fact, our studies have highlighted the heparanase enzyme as a major target for future drug development. Phosphosugars probably inhibit inflammation by displacing lysosomal enzymes, which are involved in BM degradation, from cell surface mannose 6-phosphate receptors. This mechanism of expressing degradative enzymes on the cell surface is particularly evident with activated T lymphocytes. On the other hand, CS interferes with appropriate targeting of lysosomal enzymes involved in BM degradation. For reasons which are still unclear, CS specifically inhibits BM degradation by endothelial cells, which results in a characteristic perivascular arrest of leucocytes in inflammatory sites. Overall, our studies have established that inhibitors of subendothelial BM degradation represent viable anti-inflammatory agents. It is hoped that future work will result in the development of a totally new class of highly effective, subtle and non-toxic anti-inflammatory drugs for the treatment of MS and other inflammatory diseases.