Effect of bis-(p-nitrophenyl) phosphate on the biosynthesis and the utilization of lipid-intermediates

Identification of phosphorylated oligosaccharides in cells of patients with a congenital disorders of glycosylation (CDG-I)

Protein N-glycosylation is initiated by the dolichol cycle in which the oligosaccharide precursor Glc(3)Man(9)GlcNAc(2)-PP-dolichol is assembled in the endoplasmic reticulum (ER). One critical step in the dolichol cycle concerns the availability of Dol-P at the cytosolic face of the ER membrane. In RFT1 cells, the lipid-linked oligosaccharide (LLO) intermediate Man(5)GlcNAc(2)-PP-Dol accumulates at the cytosolic face of the ER membrane. Since Dol-P is a rate-limiting intermediate during protein N-glycosylation, continuous accumulation of Man(5)GlcNAc(2)-PP-Dol would block the dolichol cycle. Hence, we investigated the molecular mechanisms by which accumulating Man(5)GlcNAc(2)-PP-Dol could be catabolized in RFT1 cells. On the basis of metabolic labeling experiments and in comparison to human control cells, we identified phosphorylated oligosaccharides (POS), not found in human control cells and present evidence that they originate from the accumulating LLO intermediates. In addition, POS were also detected in other CDG patients' cells accumulating specific LLO intermediates at different cellular locations. Moreover, the enzymatic activity that hydrolyses oligosaccharide-PP-Dol into POS was identified in human microsomal membranes and required Mn(2+) for optimal activity. In CDG patients' cells, we thus identified and characterized POS that could result from the catabolism of accumulating LLO intermediates.

Intralumenal pool and transport of CMP-N-acetylneuraminic acid, GDP-fucose and UDP-galactose. Study with plasma-membrane-permeabilized mouse thymocytes

Treatment with NH4Cl of mouse thymocytes renders their plasma membrane permeable to sugar nucleotides both inwards and outwards. Using this model, we studied the entry and utilization of CMP-NeuAc, GDP-Fuc and UDP-Gal into intracellular vesicles in situ. It is shown that CMP-NeuAc and GDP-Fuc enter the vesicles in a manner indicating a carrier-mediated transport (substrate saturation curve, inhibition by substrate analogues, temperature dependence) and are entrapped in their uncleaved form. This leads to the formation of an intralumenal pool of these precursors which can be further utilized by the sialyltransferases and fucosyltransferases. The occurrence of an endogenous pool of CMP-NeuAc and GDP-Fuc is demonstrated by the fact that, when the vesicles are disrupted by detergent, the release of the endogenous sugar nucleotides causes an isotopic dilution of the labelled precursors added to measure the glycosyltransferase activities. In contrast, no accumulation of UDP-Gal has been detected, suggesting that transport and transfer reaction are simultaneous events. However, experiments with UDP 2',3'-dialdehyde indicate that UDP-Gal is not transported through the membrane by galactosyltransferase action but by a distinct carrier molecule.

Glycosylation of proteins from sugar nucleotides by whole cells. Effect of ammonium chloride treatment on mouse thymocytes

When thymocytes are treated with iso-osmotic NH4Cl, the sugar incorporation into endogenous acceptors from labelled sugar nucleotides is largely increased compared with that in control thymocytes. This effect was obtained with labelled GDP-mannose, UDP-galactose and CMP-N-acetylneuraminic acid. The stimulation observed with NH4Cl-treated thymocytes does not involve the glycosylation of exogenous acceptors, and it was proved that the NH4Cl treatment (1) does not stimulate glycosyltransferase activities themselves, (2) does not lead to the release of soluble glycosyltransferases as the result of an extensive lysis of the thymocytes and (3) does not cause the emergence of glycosyltransferases at the cell surface. In fact, electron-microscopy observations showed that, although marked changes had occurred in the cytoplasm, the plasma membrane is sufficiently maintained to allow the cell to keep roughly its original shape and to retain the intracellular vesicles. We thus demonstrate that this stimulation is due to an enhancement of the entry of sugar nucleotides into the cell. As demonstrated by the inclusion of Trypan Blue within the cells, and the non-stimulation of glycosylation of exogenous large-molecular-mass acceptors, the effect of NH4Cl seems to be limited to the penetration of small-molecular-sized compounds through the plasma membrane. Thus NH4Cl treatment allows the labelled sugar nucleotides to penetrate the cell and to behave as the cellular pool to be utilized for glycosylation by intracellular vesicles.

Inhibition of glycosyltransferases by bis-(p-nitrophenyl)phosphate: general effect and relation to their membrane integration

The effect of bis-(p-nitrophenyl)phosphate on various glycosyltransferases involved in protein glycosylation (sialyl-, fucosyl-, galactosyl-, mannosyl- and glucosyltransferases) have been studied using crude enzyme preparations solubilized from rat spleen lymphocytes. Bis-(p-nitrophenyl)phosphate appears as a common inhibitor for every glycosyltransferase reaction utilizing sugar nucleotides as direct donors. In most cases 10 mM inhibitor is sufficient to obtain a 90 per cent inhibition. Kinetic studies achieved with a purified galactosyltransferase preparation reveal that bis-(p-nitrophenyl)phosphate exerts a competitive inhibition towards UDP-galactose binding. Concerning membrane-bound enzymes, the interaction of bis-(p-nitrophenyl)phosphate depends on its accessibility to the enzyme active site. This is shown by the different effect obtained with two UDP-Glc utilizing membrane-bound enzymes : UDP-Glc : phospho-dolichyl glucosyltransferase and UDP-Glc : ceramide glucosyltransferase : the first one not being affected but the second one being markedly inhibited under the same condition, although both are inhibited when the membrane environment is disturbed by detergent. Bis-(p-nitrophenyl)phosphate appears to be a tool to study membrane topology of glycosyltransferases.