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Metabolic pathways of the intermediate metabolism of maize root
tips were identified and quantified after labeling to isotopic and
metabolic steady state using glucose labeled on carbon-1, -2, or -6
with
Volume 270,
Number 22,
Issue of June 2, pp. 13147-13159, 1995
©1995 by The American Society for Biochemistry and Molecular Biology, Inc.
C- or
C-Labeled Glucose
C or C. The specific radioactivity of
amino acids and the
C-specific enrichment of specific
carbons of free glucose, sucrose, alanine and glutamate were measured
and used to calculate metabolic fluxes. The non-triose pathways,
including synthesis of polysaccharides, accumulation of free hexoses,
and to a lesser extent starch synthesis, were found to consume 75% of
the glucose entering the root tips. The cycle of synthesis and
hydrolysis of sucrose was found to consume about 70% of the ATP
produced by respiration. The comparison of the specific radioactivities
of amino acids and phospholipid glycerol phosphate after labeling with
[1-
C] or [6-C]glucose
revealed the operation of the pentose phosphate pathway. The transfer
of label from [2-C]glucose to carbon-1 of starch
glucosyl units confirmed the operation of this pathway and indicated
that it is located in plastids. It was found to consume 32% of the
hexose phosphates entering the triose pathways. The remaining 68% were
consumed by glycolysis. The determination of the specific enrichment of
carbohydrate carbons -1 and -6 after labeling with
[1-C]glucose indicated that both the conversion
of triose phosphates back to hexose phosphates and the transaldolase
exchange contributed to this randomization. Of the triose phosphates
produced by glycolysis and the pentose phosphate pathway, about 60%
were found to be recycled to hexose phosphates, and 28% were directed
to the tricarboxylic acid cycle. Of this 28%, two-thirds were found to
be directed through the pyruvate kinase branch and one-third through
the phosphoenolpyruvate branch. The latter essentially has an
anaplerotic function since little malate was found to be converted to
pyruvate (malic enzyme reaction).
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