Pyruvate utilization in castor-bean endosperm and other tissues

Abstract

Micromolar amounts of pyruvate specifically labelled with C14 in the 1, 2, and 3 positions were supplied to 4 different plant tissues (castor-bean petiole, corn mesocotyl, Bryophyllum leaf and castor-bean endosperm) in order to determine the extent of utilization and the fates of individual carbons. The release of C-l of pyruvate in the respired CO2 occurred promptly and regularly from all 4 tissues and from all except Bryophyllum the eventual recovery was virtually quantitative. C atoms 2 and 3 appeared subsequently in sequence in the respired CO2 but in such a way as to indicate that the 2-atom unit was undergoing very different fates in the 4 tissues. In castor-bean petiole, a fully mature tissue, all of C-2 and 70% of C-3 was recovered as CO2. In corn mesocotyl, which grew noticeably during the experiment, the bulk of C-2 and C-3 were retained in the tissue as amino acids and protein. In Bryophyllum leaf, a tissue rich in acids, more than half of C-3 was retained in acids of the tricarboxylic acid cycle. In castor-bean endosperm, where carbo-hydrates were being produced from fat, an active conversion of pyruvate into sucrose occurred. Results are provided to support the conclusion that pyruvate was acting as a sucrose precursor only in so far as it was converted into acetyl units which entered the glyoxylate cycle. Direct reversal of glycolysis from pyruvate plays, at best, a minor role in the production of sucrose from pyruvate; the relatively small incorporation of C-l of pyruvate is ascribed to the (dark) fixation of CO2 into sucrose by endosperm slices.