Effect of glucose on aspartate and glutamate synthesis by human diploid fibroblasts

The quantity of aspartate, but not of glutamate, synthesized by human diploid fibroblasts (HDF) was inversely proportional to the glucose concentration. Forty hours after confluent cells were refed with media containing 1.5 mM or 55 microM glucose, the aspartate concentration was 8 microM in medium high in glucose and 28 microM in medium low in glucose; glutamate was 350 microM at both glucose levels. The label incorporation from [U-14C] glutamine (0.8 C/ml) into aspartate after 40 hours in 1.5 mM glucose medium was less then 1000 DPM/ml and into glutamate 540,000 DPM/ml. The respective label incorporation into aspartate in 55 microM glucose medium was 118,000 DPM/ml and into glutamate 450,000 DPM/ml. In media with 1.5 mM glucose, label incorporation into both aspartate and glutamate was observed from [6-14C] glucose, [3-14C] pyruvate, and [1-14C] pyruvate. Since [1-14C] pyruvate labeled aspartate in 1.5 mM glucose medium, whereas [U-14C] glutamine did not, the observations support a cystolic pathway of aspartate synthesis from glycolytic intermediates in the presence of glucose. However, when the glucose concentration is decreased, glutamine appears to be the primary carbon source for aspartate synthesis. Therefore, both the quantity and the pathway of aspartate synthesis in HDF are functions of the glucose concentration. It is postulated that increased accumulation of aspartate in the media of HDF at confluency may be explained by decreased availability of acetyl CoA or of NADH.