Two ammonium-inducible, chloroplast-localized NADP-specific glutamate dehydrogenase isoenzymes were purified to homogeneity from Chlorella sorokiniana. These isoenzymes were homopolymers of either a-or a-subunits with molecular weights of 55,500 or 53,000, respectively.The a-isoenzyme was preferentially induced at low ammonium concentrations (2 millimolar or lower), whereas only the f,-isoenzyme accumulated after cells were fully induced (120 minutes) at high ammonium concentrations (29 millimolar 53,000 (4, 8, 10, 11, 17-19, 22 In a recent study, Prunkard et al. (18) compared the kinetics of induction of NADP-GDH catalytic activity with the accumulation ofNADP-GDH antigen induced in cells under different cultural conditions. The apparent mol wt of the catalytically active NADP-GDH holoenzyme forms were estimated by use of an enzyme activity stain after native polyacrylamide slab-gel electrophoresis. The NADP-GDH antigens were examined after native and SDS slab-gel electrophoresis by use ofa Western blot/ immunodetection procedure. Native gel electrophoresis of extracts, from cells harvested during a 3 h induction time-course in continuous light in 29 mM ammonium medium, revealed that the number of electrophoretic forms of NADP-GDH holoenzyme(s) that were catalytically-active (and antigenic) increased from several forms, at the earliest induction times, to seven forms (Mr = 290,000-360,000) by the end of the 3 h induction period. When these same cell extracts were examined by SDS gel electrophoresis, two NADP-GDH antigens were detected with mol wt of 55,500 and 53,000. The 53,000 D NADP-GDH antigen increased continuously throughout the 3 h induction period. The 55,500 D antigen increased in concentration only during the first 1.5 h of the induction period and decreased in concentration thereafter. In fact, this larger antigenic species was barely detectable by the end of the 3 h induction period and essentially was absent from cells induced for 6 h. The 55,500 D antigen was shown to be associated only with NADP-GDH holoenzyme-size proteins. Therefore, it was assumed to be a subunit of NADP-GDH holoenzyme(s). The 55,500 D and 53,000 D antigens were designated as being a-and ,B-subunits of NADP-GDH, respectively. Depending upon the cultural conditions (i.e. autotrophic versus heterotrophic; light versus dark) and the length of the induction periods, a wide variation was observed in the a:,3 subunit ratio and in the number and sizes of the NADP-GDH holoenzymes.Tischner (23)