REGULATION OF PEANUT GLUTAMATE-DEHYDROGENASE BY METHIONINE SULFOXIMINE

Peanut glutamate dehydrogenase (GDH) was electrophoretically purified to homogeneity. Rotofor IEF fractionated the peanut GDH to 7 isoelectr ic (charge) isomers, which focused in the pH 5-8 range. Western blot a nalysis of the charge isomers using anti-GDH serum showed that methion ine sulphoximine (MSX) treatment suppressed the b-subunit (69 KDa), bu t enhanced the a-subunit (45 KDa), and alpha-subunit (46 KDa) of the e nzyme. The MSX-mediated suppression of the b-subunit increased the NH4 + K-m value of the acidic charge isomers from 7.7 mM in the control to 50 mM in the MSX-treated peanut, and also increased the 2-ketoglutara te K-m value of the basic charge isomers from 0.4 mM in the control to 7.0 mM in the MSX treatment. Therefore, the control peanut could salv age NH4+ with its GDH activity. But by increasing the NH4+ K-m value, the MSX rendered the enzyme ineffective in NH4+ salvage. In the deamin ation direction, despite the enhancement of the a-, and alpha-subunits by MSX, the basic charge isomers of GDK had very high K-m value for L -glu (50 mM), similar to that (25 mM) of the control. Thus, the GDHs o f the control, and MSX treatment could not function in the deamination reaction in vivo. These results show that the treatment of peanut wit h MSX impaired the amination function of GDH. (C) 1997 Elsevier Scienc e Ltd.