Malonic Acid Biosynthesis in Bush Bean Roots. I. Evidence for Oxaloacetate as Immediate Precursor

At least 10 clifferent metabolic pathwvavs leadling to the formation of malonlate have beenl rieported. The enzymes participatiing in these pathways include: AcetylCoA carboxylase (6, 10, 11, 20, 37), transcarboxylase (1, 10, 14, 15, 16, 24), thiokinase and thioesterase (11), barbiturase (12), malonic senmialdehyde dehydrogenase (22, 31, 32), B-alanine transaminase (18), AcylCoA-CO., exchange (33), and metmyglobin (34, 35). Aimiong these enzymes, acetylCoA carboxylase has received the greatest attention. This enzy-me has been demiionstrated in yeast (20), animal tissues (37), andl in plant tissues (11). Hatch and Stumiipf (11) mla(le a survey of the activity of this enzyme in embryos, roots, and leaves of various species. They reporte(d acetvlCoA carboxylase activity wvas highest in the emlbryos, while roots were essentially devoid of activity\. Leaves were about one-fifth as active as the enmbryos. Using enzyme preparations from bush bean roots, the present authors have also been unable to demionstrate acetylCoA carboxylase activity. In view of the fact that malonic aci(d is the mlajor organic acidl in bush bean roots. and in viewv of the apparent absence or lov,activitv of acetxlCoA carboxylase in root tissues (11, 13), experiments were undertaken to investigate further the biosynthesis of malonic acid in bush beani roots.