Sucrose synthesis from acetate in the germinating castor bean: kinetics and pathway.

During the germination of fatty seeds such as the castor bean, a rapid conversion of fat to sugar occurs (2). The fatty acids are probably broken down to acetyl units via P-oxidation (3) ; the discovery of the glyoxylate cycle enzymes (4) in the castor bean endosperm (5) provided a mechanism whereby these acetyl units could be quantitatively converted to malate. Since malate could be viewed as a sugar precursor, Kornberg and Beevers (6) suggested that the glyoxylate cycle, coupled with a reversal of glycolysis, afforded a plausible route for the conversion of acetate to sugar, which had been previously shown to occur with high efficiency in this tissue (7). A considerable amount of evidence has now been collected on the existence of the enzymes required for such a conversion (7-11) in the tissues of fatty seedlings, on the rather strict confinement of malate synthetase and isocitritase to such tissues in higher plants, and on the striking changes in activity of these enzymes which occur in phase with the changing pace of fat breakdown in the developing seedling (8, 9). This evidence, all of it encouraging, must nevertheless be complemented by work with the intact tissue to establish the validity of the proposal in hand. Bradbeer and Stumpf (12) have provided valuable data from experiments with peanut and sunflower tissues that show that the products and some of their labeling patterns after several hours in contact with specifically labeled acetate-V were those predicted if the pathway of conversion were that proposed. In this report, more extensive short term experiments are described in which a variety of labeled substrates was provided to slices of castor bean endosperm. Acetate-l-Cl4 and -2-Cl4 were rapidly converted to sucrose and a large portion of the absorbed succinate-1-C14, -2-U4, and malate-Cl4 also gave rise to sucrose. Time studies firmly established the fact that the acetate was first converted to diand tri-carboxylic acids and thence to sucrose. The distributions of Cl4 in malate and in the glucose moiety of sucrose were those expected from the operation of the proposed sequence. The results, therefore, are strong evidence that the glyoxylate cycle and a reversal of glycolysis are functionally important stages in the conversion of fat to carbohydrate in viva.