Role of sulfhydryl groups in the function of glucosidase I from mammary gland

Response of α-glucosidase in gypsy moth larvae to acute and chronic dietary cadmium

We investigated the effects of acute and chronic treatments with cadmium at 10 μg Cd/g dry food and 30 μg Cd/g dry food on α-glucosidase activity of the 4th instar larvae of Lymantria dispar (Lepidoptera: Lymantriidae) as well as subsequent recovery. Enzyme inhibition was recorded during acute exposure to 30 μg Cd/g dry food and during chronic treatment at the lower metal concentration. After three days recovery from 10 μg Cd/g dry food, the α-glucosidase activity returned to the control level. One-way ANOVA showed that cadmium significantly influenced the activity of α-glucosidase during all treatments. The index of phenotypic plasticity was higher during chronic treatment at 10 μg Cd/g dry food than at 30 μg Cd/g as well as during the recovery. We detected four glucosidase isoforms by NATIVE PAGE. The activities and expressions of the isoforms depended on both larval genotype and cadmium treatment.

Protein glycosylation in Candida

Candidiasis is a significant cause of invasive human mycosis with associated mortality rates that are equivalent to, or worse than, those cited for most cases of bacterial septicemia. As a result, considerable efforts are being made to understand how the fungus invades host cells and to identify new targets for fungal chemotherapy. This has led to an increasing interest in Candida glycobiology, with an emphasis on the identification of enzymes essential for glycoprotein and adhesion metabolism, and the role of N- and O-linked glycans in host recognition and virulence. Here, we refer to studies dealing with the identification and characterization of enzymes such as dolichol phosphate mannose synthase, dolichol phosphate glucose synthase and processing glycosidases and synthesis, structure and recognition of mannans and discuss recent findings in the context of Candida albicans pathogenesis.

The Lec23 Chinese hamster ovary mutant is a sensitive host for detecting mutations in alpha-glucosidase I that give rise to congenital disorder of glycosylation IIb (CDG IIb)

Lec23 Chinese hamster ovary cells are defective in alpha-glucosidase I activity, which removes the distal alpha(1,2)-linked glucose residue from Glc(3)Man(9)GlcNAc(2) moieties attached to glycoproteins in the endoplasmic reticulum. Mutations in the human GCS1 gene give rise to the congenital disorder of glycosylation termed CDG IIb. Lec23 mutant cells have been shown to alter lectin binding and to synthesize predominantly oligomannosyl N-glycans on endogenous glycoproteins. A single point mutation (TCC to TTC; Ser to Phe) was identified in Lec23 Gcs1 cDNA and genomic DNA. Serine at the analogous position is highly conserved in all GCS1 gene homologues. A human GCS1 cDNA reverted the Lec23 phenotype, whereas GCS1 cDNA carrying the lec23 mutation (S440F in human) did not. By contrast, GCS1 cDNA with an R486T or F652L CDG IIb mutation gave substantial rescue of the Lec23 phenotype. Nevertheless, in vitro assays of each enzyme gave no detectable alpha-glucosidase I activity. Clearly the R486T and F652L GCS1 mutations are only mildly debilitating in an intact cell, whereas the S440F mutation largely inactivates alpha-glucosidase I both in vitro and in vivo. However, the S440F alpha-glucosidase I may have a small amount of alpha-glucosidase I activity in vivo based on the low levels of complex N-glycans in Lec23. A sensitive test for complex N-glycans showed the presence of polysialic acid on the neural cell adhesion molecule. The Lec23 Chinese hamster ovary mutant represents a sensitive host for detecting a wide range of mutations in human GCS1 that give rise to CDG IIb.

Genomic organization and promoter activity of glucosidase I gene

Glucosidase I initiates the processing of asparagine (N-) linked glycoproteins by removing the distal alpha1,2-linked glucosyl residue of the tetradecasaccharide Glc(3)Man(9)GlcNAc(2). The gene encoding this enzyme was isolated and its structural organization and promoter activity determined. The major transcript for glucosidase I on northern blot appeared to be 3.1 kb; Southern blotting and DNA sequencing indicated the size of the gene to be 6.8 kb, comprising four exons separated by three introns. The first exon encodes the cytoplasmic tail and transmembrane domain; the fourth encodes the putative catalytic domain of the enzyme. Exon-intron junctions are flanked by consensus splice donor and acceptor sequences. Transcription initiation sites were mapped by primer extension, ribonuclease protection assay and RT-PCR analysis. Primer extension results showed multiple initiation sites at -150, -156, and -272 bp relative to the translation initiation codon ATG. Sequence analysis of 5' flanking region showed no canonical TATA box, a high GC content, Sp1 and ETF binding sites (typical of a housekeeping gene promoter). Also noteworthy, the promoter region contains several generic STAT factor binding sites, one nearly perfect, and two half GR binding elements. Other cis- acting elements recognized by transcription factors such as AP-2, NF-kappaB, estrogen receptor, and progesterone receptor (PR) were also present in the putative promoter region. To determine the promoter activity, a construct encompassing the region between -2114 to -5 bp of the putative promoter was ligated to the chloramphenicol acetyltransferase (CAT) reporter plasmid and transiently transfected into COS 7 cells. CAT assay results clearly show transcriptional activity of the promoter.

Purification and characterization of a glucuronyltransferase involved in the biosynthesis of the HNK-1 epitope on glycoproteins from rat brain

The glucuronyltransferase involved in the biosynthesis of the HNK-1 epitope on glycoproteins was purified to an apparent homogeneity from the Nonidet P-40 extract of 2-week postnatal rat forebrain by sequential chromatographies on CM-Sepharose CL-6B, UDP-GlcA-Sepharose 4B, asialo-orosomucoid-Sepharose 4B, Matrex gel Blue A, Mono Q, HiTrap chelating, and HiTrap heparin columns. The purified enzyme migrated as a 45-kDa protein upon SDS-polyacrylamide gel electrophoresis under reducing conditions, but eluted as a 90-kDa protein upon Superose gel filtration in the presence of Nonidet P-40, suggesting that the enzyme forms homodimers under non-denatured conditions. The enzyme transferred glucuronic acid to various glycoprotein acceptors bearing terminal N-acetyllactosamine structure such as asialo-orosomucoid, asialo-fetuin, and asialo-neural cell adhesion molecule, whereas little activity was detected to paragloboside, a precursor glycolipid of the HNK-1 epitope on glycolipids. These results suggested that the enzyme is specifically associated with the biosynthesis of the HNK-1 epitope on glycoproteins. Sphingomyelin was specifically required for expression of the enzyme activity. Stearoyl-sphingomyelin (18:0) was the most effective, followed by palmitoyl-sphingomyelin (16:0) and lignoceroyl-sphingomyelin (24:0). Interestingly, activity was demonstrated only for sphingomyelin with a saturated fatty acid, i.e. not for that with an unsaturated fatty acid, regardless of the length of the acyl group.