Use of an Aneuploid Soybean Cell Culture to Examine the Relative Importance of the GS: GOGAT System and GDH in Ammonia Assimilation

Abstract

Summary When the specific activity levels of glutamine synthetase (GS, EC 6.3.1.2), glutamate synthase (GOGAT, EC 2.6.1.53) and glutamate dehydrogenase (GDH, EC 1.4.1.2) were examined in two soybean cell cultures (SB1 and SB3), characteristic differences were seen. The highly aneuploid SB3 cell culture was found to contain much greater amounts of GS and GDH than the SB1 culture. It appeared that the high levels of GDH were specifically induced by placing SB3 cells in media containing high levels of ammonia as the sole nitrogen source (MNHA medium). The addition of α-ketoglutarate or glutamate to MNHA medium inhibited the apparent induction of GDH. The relative importance of the GS: GOGAT system versus GDH in ammonia utilization in the two cell cultures, was examined using methionine sulfoximine (MSX) and azaserine. These experiments showed that in spite of high levels of GDH found in the SB3 cells, the GS: GOGAT system still appeared to be the major system involved in ammonia utilization in both cell lines. The high levels of GDH synthesized in the SB3 line appear to originate either in mitochondria, or in plastids.