The addition of L-Glutamate (L-GLU) and L-Hethionine
~ulfoximine (L-HSO) to mechanically isolated. photosynthetically
competent, Asparagus sprengeri mesophyll cells
~u~pended in 1mM CaS04 cau~ed an immediate transient
alkalinization of the cell su~pension medium in both the light
and dark. The alkalinization response was specific and
stereospecific as none of the L-isomers of the other 19 protein
amino acids tested or D-GLU gave this response. Uptake of
14C-L-GLU was stimulated by the light. The addition of
non-radioactive L-GLU. or L-GLU analogs together with
14C-L-GLU showed that only L-GLU and L-HSO stimulated
alkalinization whilst inhibiting the uptake of 14C-L-GLU.
Both the L-GLU dependent alkalinization and the upt~ke of
14C-L-GLU were stimulated when the external pH was decreased
from 6.5 to 5.5. Increasing external K+ concentrations
inhibited the uptake of 14C-L-GLU. Fusicoccin (FC)
stimulated uptake. The L-GLU dependent alkalinization re~ponse
exhibited monophasic saturation kinetics while the uptake of
14C-L-GLU exhibited biphasic saturation kinetics. In
addition to a saturable component. the uptake kinetics also
showed a linear component of uptake. Addition of L-GLU and L-MSO
caused internal acidification of the cell as measured by a change
in the distribution of 14C-DMO. There was no change in
K+ efflux when L-GLU was added. A H+ to L-GLUinflux
stoichiometry of 3:1 wa~ mea~ured at an external I.-GLU
concentration of O.5mM and increased with increasing external
13
L-QLU concentration. Metabolism of L-GLU was detected
manometrlcally by observing an increase in COa evolution
upon the addition of L-QLU and by detection of i*C02
evolution upon the addition of »*C-L-GLU. »*C02
evolution was higher in the dark than in the light.
The data are consistent with the operation of a
H+/L-QLO cotransport system. The data also show that
attempts to quantify the stoichlometry of the process were
complicated by the metabolism of L-GLU