IDENTIFICATION OF 2 DIFFERENT GLYCERALDEHYDE-3-PHOSPHATE DEHYDROGENASES (PHOSPHORYLATING) IN THE PHOTOSYNTHETIC PROTIST CYANOPHORA-PARADOXA

Two different glyceraldehyde-3-phosphate (G3P) dehydrogenase (phosphor ylating) activities, namely NAD-and NADP-dependent, have been found in cell extracts of the cyanelle-bearing photosynthetic protist Cyanopho ra paradoxa. Whereas the two G3P dehydrogenase activities were detecte d with similar specific activity levels (0.1 to 0.2 U/mg of protein) i n extracts of the photosynthetic organelles (cyanelles), only the NAD- dependent activity was found in the cytosol. Thus, a differential intr acellular localization occurred. The perfect overlapping of the two G3 P dehydrogenase activity peaks of the cyanelle in both hydrophobic int eraction chromatography and subsequent FPLC (fast protein Liquid chrom atography) gel filtration indicated that the two activities were due i n fact to a single NAD(P)-dependent G3P dehydrogenase (EC 1.2.1.-) wit h a molecular mass of 148,000. SDS-PAGE of active fractions from FPLC gel filtration showed that the intensity of the major protein band (mo lecular mass, 38,000) of the enzyme preparation clearly paralleled the activity elution profile, thus suggesting a tetrameric structure for the cyanelle dehydrogenase. On the other hand, FPLC gel filtration ana lysis of the cytoplasmic fraction revealed a NAD-dependent G3P dehydro genase with a native molecular mass of 142,000, being equivalent to th e classical glycolytic enzyme (EC 1.2.1.12) present in the cytosol of all the organisms so far studied. The significance of these results is discussed taking into account that the cyanobacteria, photosynthetic prokaryotes which share many structural and biochemical features with cyanelles and are considered as their ancestors, have a similar NAD(P) -dependent G3P dehydrogenase.