When cells undergo oncogenic transformation, the sialylation of cell surface glycoconjugates is altered, which is thought to be associated with malignant phenotype. To elucidate the significance and the molecular mechanism of the alteration, we have been focusing on sialidase that catalyzes the removal of sialic acid residues from glycoproteins and glycolipids. In mammalian cells, four types of sialidases have been identified to date and were found to behave in different manners during carcinogenesis. A sialidase found in lysosomes is decreased in the activity and mRNA level in cancer cells, while a sialidase in plasma membrane is increased as compared with those in the control cells. The former sialidase affects anchorage-independent growth and metastatic ability and introduction of the sialidase gene leads to reversion of these phenotypes. On the other hand, the latter brings about suppression of apoptosis in cancer cells and knocking down of this gene with short interfering RNA results in acceleration of apoptosis. In this review, we describe and summarize the alteration of sialidases and its possible significance in carcinogenesis.
Two kinds of synthetic methods using transglycosylation or condensation with disaccharide units by endo-glycosidases have been developed. First, in a commercially available cellulase preparation of Trichoderma reesei, we found transglycosylation activity enabling it to transfer the entire lactose (Lac) and N-acetyllactosamine (LacNAc) from p-nitrophenyl β-lactoside (Lacβ-pNP) and p-nitrophenyl β-N-acetyllactosaminide (LacNAcβ-pNP), respectively. Using the enzyme activities, various β-lactosides such as alkyl β-lactosides were directly synthesized. The enzyme catalyzed not only transglycosylation but also condensation reaction between the disaccharides and aglycons. Although 1-alkanols, 2-alkanols, alkan-diols, allyl alcohol, glycerol, Man and Glc can be acceptors for transfer of the disaccharide units, the efficiency of transglycosylation and condensation decreased with the increase in length of the alkyl chain of alkanol acceptors from C2 to C12. The yield of condensation between lactose and glycerol reached up to 40% by elimination of coexistent β-galactosidases. Condensation using the enzyme enabled practical synthesis of various glycosides possessing Lac and LacNAc units and offered a novel method for enzymatic synthesis of neoglycolipid precursors. The results of purification of crude enzyme preparation followed by a substrate competition assay on LacNAcβ-pNP hydrolysis indicated that both transglycosylation and condensation are mediated by a single enzyme, which is thought to be one of the endo-β-(1-4)-glucanases. Next, endo-β-galactosidase from Escherichia freundii, which has been used for structural and functional analyses of glycans involved in glycoconjugates, was shown to catalyze regioselective transfer of the GlcNAcβ1-3Gal unit onto the OH-4 position of non-reducing end GlcNAc of acceptor substrates. As a result, the transglycosylation led to facile preparation of LacNAc-repeating oligosaccharides.
Xyloglucan is a major structural polysaccharide of the primary cell-walls of all higher plants. The xyloglucans from Gramineae and Solanaceae have more unsubstituted glucose residues compared to those from most dicotyledonous plants. A purified Penicillium sp. M451 or Geotrichum sp. M128 xyloglucan specific endo-1, 4-β-D-glucanase, a purified oligoxyloglucan-specific glycosidase, oligoxyloglucan reducing end-specific cellobiohydrolase from Geotrichum sp. M128, and a purified isoprimeverose-producing oligoxyloglucan hydrolase from Eupenicillium sp. M9 are very useful for investigating the arrangement of the glucose residues which are not xylosylated at O-6 in the β-1, 4-D-glucan backbone of Gramineae and Solanaceae xyloglucans. In this review, we summarize the structural study of xyloglucans from Gramineae and Solanaceae in addition to those from most dicotyledonous plants by xyloglucan specific enzymes.
The residents in malaria endemic areas generally acquire protective immunity to malaria by their adulthood. Despite this previously acquired natural immunity, women are highly susceptible to malaria during pregnancy, especially in their first pregnancy. This is due to the sequestration of a phenotypically different Plasmodium falciparum in the placenta by the adherence of the infected red blood cells (IRBCs). Because, prior to first pregnancy, women were not exposed to the placental adherent parasite at significant levels, they lack the phenotype-specific immunity. Therefore, infection during pregnancy leads to placental malaria, which is associated with a number of clinical manifestations. The adherence of P. falciparum IRBCs in the placenta is mediated predominantly by an unusually low sulfated aggrecan family chondroitin sulfate proteoglycan localized in the intervillous space of the placenta. The IRBC binding requires the participation of both 4-sulfated and nonsulfated disaccharide repeats of the chondroitin sulfate chains. The minimal structural motif required for optimal binding is a dodecasaccharide with two 4-sulfated and four non-sulfated disaccharides. P. falciparum erythrocyte membrane protein 1, a product of the var gene family, expressed on the surface of IRBCs has been proposed as the ligand for the IRBC adherence. In this review, we summarize our current knowledge on the structure of the placental chondroitin sulfate proteoglycan receptor, the identity of the parasite ligand, and chondroitin sulfate structural requirements for IRBC adherence.