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

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Abstract
When thymocytes are treated with iso-osmotic NH4Cl, the sugar incorporation into endogenous acceptors from labeled sugar nucleotides is largely increased compared with that in control thymocytes. This effect was obtained with labeled 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 does not stimulate glycosyltransferase activities themselves, does not lead to the release of soluble glycosyltransferases as the result of an extensive lysis of the thymocytes and does not cause the emergence of glycosyltransferases at the cell surface. In fact, EM 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. This stimulation apparently 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 labeled sugar nucleotides to penetrate the cell and to behave as the cellular pool to be utilized for glycosylation by intracellular vesicles.