Some of the recent findings which revise our view of the role and regulation
of phosphoenolpyruvate carboxykinase (PEPCK) in
C4 plants are discussed. Evidence is presented that
PEPCK is present at appreciable activities in the bundle-sheath of some
NADP-malic enzyme-type C4 plants, such as maize, but it
was not detectable in NAD-malic enzyme-type C4 plants.
PEPCK is rapidly inactivated in crude extracts of leaves of the
C4 plant, Panicum maximum. This
inactivation could be prevented by high concentrations of dithiothreitol or by
the inclusion of ADP or ATP, suggesting the involvement of thiols at the
active site. PEPCK is also subject to rapid proteolysis in crude extracts of a
range of C4 plants, resulting in cleavage to a smaller
(62 kDa) form. This can be reduced by extraction at high pH and by the
inclusion of SDS, but it means that intact PEPCK has never been purified from
a C4 plant. The molecular mass of PEPCK varies
considerably in C4 plants, unlike
C3 and CAM plants in which it is usually 74 kDa. PEPCK
is phosphorylated during darkness (and reversed by light) in some
C4 plants with PEPCK of a larger molecular mass, such as
Panicum maximum (71 kDa), but it was not phosphorylated
in the PEPCK-type C4 plant,
Sporobolus pyramidalis (69 kDa). The known regulatory
properties of PEPCK are discussed in relation to its role in
C4 photosynthesis, in particular its sensitivity to
regulation by adenylates and by Mn2+.
Isolation of Mitochondria from Leaf Tissue of Panicum miliaceum, a NAD-Malic Enzyme Type C4 Plant