Elevated glutamate dehydrogenase flux in glucose-deprived hybridoma and myeloma cells: evidence from 1H/15N NMR

The glutamine metabolism was studied in glucose-starved and glucose-sufficient hybridoma and Sp2/0-Ag14 myeloma cells. Glucose starvation was attained by cultivating the hybridoma cells with fructose instead of glucose, and the myeloma cells with a low initial glucose concentration which was rapidly exhausted. Glutamine used in the experiments was labeled with 15N, either in the amine or in the amide position. The fate of the label was monitored by 1H/15N NMR analysis of released 15NH+4 and 15N-alanine. Thus, NH+4 formed via glutaminase (GLNase) could be distinguished from NH+4 formed via glutamate dehydrogenase (GDH). In the glucose-sufficient cells a small but measurable amount of 15NH+4 released by GDH could be detected in both cell lines (0.75 and 0.31 micromole/10(6) cells for hybridoma and myeloma cells, respectively). The uptake of glutamine and the total production of NH+4 was significantly increased in both fructose-grown hybridoma and glucose-starved myeloma cells, as compared to the glucose-sufficient cells. The increased NH+4 production was due to an increased throughput via GLNase (1.6 -1.9-fold in the hybridoma, and 2.7-fold in the myeloma cell line) and an even further increased metabolism via GDH (4.8-7.9-fold in the hybridoma cells, and 3.1-fold in the myeloma cells). The data indicate that both GLNase and GDH are down-regulated when glucose is in excess, but up-regulated in glucose-starved cells. It was calculated that the maximum potential ATP production from glutamine could increase by 35-40 % in the fructose-grown hybridoma cells, mainly due to the increased metabolism via GDH.