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 N-15, either ther in the amine or in the amide position. The fate of the label was monitored by H-1/N-15 NMR analysis of released (NH4+)-N-15 and N-15-alanine. Thus, NH4+ formed via glutaminase (GLNase) could be distinguished from NH4+ formed via glutamate dehydrogenase (GDH). In the glucose-sufficient cells a small but measurable amount of (NH4+)-N-15 released by GDH could be detected in both cell lines (0.75 and 0.31 mu mole/10(6) cells for hybridoma and myeloma cells, respectively). The uptake of glutamine and the total production of NH4+ was significantly increased in both fructose-grown hybridoma and glucose-starved myeloma cells, as compared to the glucose-sufficient cells. The increased NH4+ 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 upregulated 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. (C) 1998 John Wiley & Sons, Inc.