DOLICHOL-MEDIATED ENHANCED PROTEIN N-GLYCOSYLATION IN EXPERIMENTAL DIABETES - A POSSIBLE ADDITIONAL DELETERIOUS EFFECT OF HYPERGLYCEMIA

In liver cells from diabetic rats, an increased incorporation of label led glucosamine into cellular and secretory proteins was found, when r elated to the incorporation of labelled leucine. This increased N-glyc osylation was present in the face of decreased synthesis of hepatic ce llular and secretory proteins evident from reduced leucine incorporati on and diminished glycosyltransferase activity. To elucidate the mecha nisms involved we incubated isolated hepatocytes with two N-glycosylat ion inhibitors: tunicamycin and 2-deoxyglucose. Tunicamycin exerted a marked inhibitory effect on the incorporation rate of labelled glucosa mine into proteins in liver cells from diabetic rats, while 2-deoxyglu cose had a negligible effect on this process in these cells. These div erse effects might be explained by the fact that tunicamycin acts thro ugh strong association with the enzyme catalysing the first step in gl ycoprotein synthesis, namely, the transfer of UDP-GlcNAc to dolichol-P (indicating noncompetitive inhibition). This enzyme is reduced in liv er cells from diabetic animals. On the other hand, 2-deoxyglucose exer ts its effect by being attached to dolichol-P, preventing further elon gation of oligosaccharide chain on the protein backbone. This latter e ffect might be eliminated by excess dolichol-P (indicating competitive inhibition). The dolichol content in liver extract from diabetic rats was about 2.5-fold higher compared with nondiabetic rats (51.6 mu g/g versus 20.6 mu g/g wet liver weight). These two lines of evidence con firm the notion that the enhanced enzymatic glycosylation in liver fro m diabetic animals is maintained by an increased hepatic dolichol conc entration, which is most probably related to the hyperglycemia. Thus, the dolishol-N-glycosylation pathway may represent another detrimental aspect of hyperglycemia and may operate by dolichol mass action rathe r than through glycosylating enzyme activity. (C) Elsevier Science Inc ., 1997.