Substitution of EMfor (io) and division by Fresults in
2.3
FRT
logaa
joij
zjEMn(EM 60 pH)When we define pjlog
a
a
joij
and substitute 60 mV for 2.3RT/F(30°C), we have ^60 pjzjEMn(EM 60 pH) and Eq. (19) is obtained:
pj
n
6
0z
EMnpH (19)For antiport, the steady-state situation is jnFpmf, and
pj
z
6
0n
EMnpH (20)Equation (19) can be employed to analyze Cl-Hsymport. If electroneutral transport is assumed (n
1 and z-n0)pCl will equal pH, and such transport would not depend on the membrane potential.
When a 100-fold Claccumulation is found (pCl2), a relatively large pH of 2 would be needed
for electroneutral transport. However, if n2 is assumed and EM120 mV, no pH would be needed
for a pCl of 2.
MINERAL CATIONS
Sodium. The first evidence in higher plants for cotransport energized by the proton motive force
was that for Na/Hantiport in barley roots [121]. Much of the information about Na/Hantiport at
the plasma membrane of higher plants emanated from experiments with intact plant tissues (see Ref.
122). The Na/Hantiporter was investigated at the level of plasma membrane vesicles in the halo-
tolerant unicellular alga Dunaliella salina[123,124], in Atriplex nummularia[125,126], and in Gossyp-
ium hirsutum[126]. The Na/Hantiporter in Dunaliellaplasma membrane vesicles had a Kmfor Na
of about 16 mM and was inhibited by amiloride, an inhibitor of Na/Hantiport in animal cells [123].
TheVmaxof the antiport increased when the cells had been adapted to a high NaCl concentration or to
ammonia at a high pH; it decreased in LiCl-adapted cells [127]. The increase of Vmaxwas interpreted
as overproduction of the Na/Hantiporter and was correlated with overproduction of a 20-kDa and a
50-kDa polypeptide. The Na/Hantiport in Dunaliellawas specific for Nain comparison with K,
Cs, and Li[123]. However, in plasma membrane vesicles of Atriplex[125,126] and Gossypium
[126], similar dissipation of pH gradients was found with Naand Kions, and a high concentration
of one of these ions was needed for activity. An additive effect was obtained when Nawas added to
saturating Kconcentrations and vice versa. It was suggested that separate antiporters for Kand Na
may operate.
Evidence for Na/Hantiport at the tonoplast emanated first from experiments of Blumwald and
Poole [128] with tonoplast vesicles. They demonstrated amiloride-sensitive Na/Hamiport in tonoplast
vesicles from Beta vulgarisstorage tissue. At a constant pH the apparent KMfor Naincreased from 7.5
to 26.6 mM when the internal pH decreased from about 7.5 to 6.5. In tonoplast vesicles from suspension
cultured cells of B. vulgaris[129] and from Plantago maritimaroots [130], Na/Hantiport activity in-
creased in response to NaCl in the growth medium. In B. vulgaris[129] such increased activity was as-
sociated with an increase in Vmaxwithout a change of Km. Sodium antiport activity was also increased by
cultivation of the B. vulgariscells in the presence of amiloride [131]. In both cases the increase of Na/H
antiport activity was accompanied by synthesis of a 170-kDa polypeptide. Polyclonal antibodies against
this polypeptide almost completely inhibited the Na/Hantiport activity [131]. Garbarino and DuPont
[132] induced Na/Hexchange in barley roots by pretreatment with sodium salts. The half-time of in-
duction was 15 min and it was attributed to the activation of a preexisting protein. Unlike its effect on the
relatively salt-tolerant crops beet and barley, NaCl treatment did not activate the antiporter in rice roots
[133].
Potassium. Potassium concentrations in soil solutions range from 1 M to 10 mM, with many
soils falling in the range of 0.3 to 5.0 mM, while intracellular Klevels are maintained at 100 to 200 mM
[109]. Hence, assuming an EMof120 mV, plants may obtain Kby uniport from most soils, but a high-
348 JACOBY AND MORAN