Mannosylation of glycoproteins and dolichol derivatives in fibroblasts from patients with cystic fibrosis

Clinical aspects of glycoprotein biosynthesis

Glycoproteins are widely distributed among species in soluble and membrane-bound forms, associated with many different functions. The heterogenous sugar moieties of glycoproteins are assembled in the endoplasmic reticulum and in the Golgi and are implicated in many roles that require further elucidation. Glycoprotein-bound oligosaccharides show significant changes in their structures and relative occurrences during growth, development, and differentiation. Diverse alterations of these carbohydrate chains occur in diseases such as cancer, metastasis, leukemia, inflammatory, and other diseases. Structural alterations may correlate with activities of glycosyltransferases that assemble glycans, but often the biochemical origin of these changes remains unclear. This suggests a multitude of biosynthetic control mechanisms that are functional in vivo but have not yet been unraveled by in vitro studies. The multitude of carbohydrate alterations observed in disease states may not be the primary cause but may reflect the growth and biochemical activity of the affected cell. However, knowledge of the control mechanisms in the biosynthesis of glycoprotein glycans may be helpful in understanding, diagnosing, and treating disease.

Analysis of cell surface glycoconjugates in fibroblasts from patients with cystic fibrosis

Although the basic biochemical defect in cystic fibrosis (CF) is unknown, previous studies have indicated that errors in protein glycosylation may be involved in the pathogenesis of the disease. Utilizing human skin fibroblasts, the present study was designed to quantitatively analyze glycosylation of cell surface glycoconjugates in CF and normal cells. Cell surface glycoconjugates were analyzed using 125I-concanavilin A (Con A), 125I-WGA, and Con A-ferritin conjugates. Under our binding conditions, Con A was used as a probe for mannose residues and WGA was used as a probe for N-acetylglucosamine residues. Saturable binding of both probes was observed and appropriate sugar controls confirmed the specificity of each lectin. When compared on a DNA basis, iodinated lectin binding studies indicated that no consistent differences existed between CF and normal strains of human skin fibroblasts. Ultrastructural quantitative morphometric analysis of Con A-ferritin conjugate binding indicated that neither proteolysis of cell surface glycoconjugates or internalization of lectin probes was occurring at saturable binding concentrations. In summary, our results indicated that no consistent differences in cell surface mannose and N-acetylglucosamine residues could be detected between the normal and CF strains of human skin fibroblasts used in these studies.

Dolichyl phosphate-mannosyltransferase and dolichyl phosphate-N-acetylglucosaminyltransferase activities in liver preparations from normal controls and patients with cystic fibrosis and diabetes mellitus

Optimal assay conditions have been determined in human liver preparations for the catalytic transfer of mannose and N-acetylglucosamine from GDP-mannose and UDP-N-acetylglucosamine, respectively, to dolichyl phosphate. Both enzymatic reactions have an absolute requirement for divalent cation (5 mmol/l Mn2+ optimal), detergent (Triton X-100 or Nonidet P-40) and dolichyl phosphate (as acceptor substrate) and both reactions have optimal activity at a pH value of 7.8. Preliminary characterization of the glycolipid products for both enzymatic reactions indicates that phosphorylated dolichol is the major acceptor substrate for radiolabeled mannose and N-acetylglucosamine. The activity levels and specific activities of dolichyl phosphate-mannosyltransferase are comparable in liver homogenates from normal controls and patients with cystic fibrosis and diabetes mellitus. The activity levels and specific activities of dolichyl phosphate-N-acetylglucosaminyltransferase are comparable in liver homogenates from normal controls and patients with cystic fibrosis and diabetes mellitus but considerably lower than the activity levels of dolichyl phosphate-mannosyltransferase. It appears that two of the initial steps of the lipid-mediated glycosylation pathway are normal in livers from patients with cystic fibrosis and diabetes mellitus.