Rate of protein glycosylation in rat cerebral cortex

Quantitative aspects of the pathway leading to the formation of asparagine-linked oligosaccharides were investigated in rat cerebral cortex. Steady-state labeling conditions were achieved with [2-3H]mannose by developing a micromethod of incubation of cerebral cortex particles in the presence of physiological concentrations of glucose (1 g/L). The rate of [2-3H]mannose uptake and incorporation into protein was markedly affected when the concentration of glucose was lowered to 0.05 g/L. It was found that in the presence of glucose (1 g/L), a minor fraction of the utilized [2-3H]mannose is used in glycoprotein formation and the remaining labeled sugar enters the other major metabolic pathways, generating tritiated water which is rapidly exchanged with that of the medium. Under these conditions, the intracellular isotopic dilution of [2-3H]mannose-labeled precursors was calculated to be about 11.5-fold. These data allow determination of the rate of the net transfer of mannose into proteins. Comparison of the rate of glycosylation between 5- and 30-day-old cerebral cortex revealed a striking difference: 2.1 and 0.3 ng of mannose/mg protein/h, respectively.