PLANT PHYSIOLOGY , Vol 109, Issue 4 1301-1307, Copyright © 1995 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
Decarboxylation of Malate in the Crassulacean Acid Metabolism Plant Bryophyllum (Kalanchoe) fedtschenkoi (Role of NAD-Malic Enzyme)
R. M. Cook, J. G. Lindsay, M. B. Wilkins and H. G. Nimmo
Plant Molecular Science Group, Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland, United Kingdom
The role of NAD-malic enzyme (NAD-ME) in the Crassulacean acid metabolism
plant Bryophyllum (Kalanchoe) fedtschenkoi was investigated using
preparations of intact and solubilized mitochondria from fully expanded
leaves. Intact, coupled mitochondria isolated during the day or night did
not differ in their ability to take up [14C]malic acid from the surrounding
medium or to respire using malate or succinate as substrate. However,
intact mitochondria isolated from plants during the day decarboxylated
added malate to pyruvate significantly faster than mitochondria isolated
from plants at night. NAD-ME activity in solubilized mitochondrial extracts
showed hysteretic kinetics and was stimulated by a number of activators,
including acetyl-coenzyme A, fructose-1,6-bisphosphate, and sulfate ions.
In the absence of these effectors, reaction progress curves were nonlinear,
with a pronounced acceleration phase. The lag period before a steady-state
rate was reached in assays of mitochondrial extracts decreased during the
photoperiod and increased slowly during the period of darkness. However,
these changes in the kinetic properties of the enzyme could not account for
the changes in the rate of decarboxylation of malate by intact
mitochondria. Gel-filtration experiments showed that mitochondrial extracts
contained three forms of NAD-ME with different molecular weights. The
relative proportions of the three forms varied somewhat throughout the
light/dark cycle, but this did not account for the changes in the kinetics
behavior of the enzyme during the diurnal cycle.
