The Journal of General Physiology, Vol 67, 563-578, Copyright © 1976 by The Rockefeller University Press
Sodium permeability of dog red blood cell membranes. I. Identification of regulatory sites
V Castranova and PR Miles
Divalent cations and group-specific chemical modifiers were used to modify
sodium efflux in order to probe the molecular structure of sodium channels
in dog red blood cells. Hg++, Ni++, Co++, and PCMBS
(parachloromercuribenzene sulfonic acid), a sulfhydryl reactive reagent,
induce large increases in Na+ permeability and their effects can be
described by a curve which assumes 2:1 binding with the sodium channel. The
sequence of affinities, as measured by the dissociation constants, reflects
the reactivity of these divalent cations with sulfhydryl groups. In
addition, the effects of Hg++ and PCMBS can be reversed by the addition of
dithiothreitol, an SH-containing compound, to the medium. Much smaller
increases in Na+ permeability are produced by Zn++ and the amino-specific
reagents, TNBS (2,4,6-trinitrobenzene sulfonic acid) and SITS
(4-acetamido-4'-isothiocyano-stilbene-2-2'- disulfonic acid). The Zn++
effect can be described by a curve which assumes bimolecular binding with
the channel, and its effect on Na+ permeability can be reversed by the
addition of glycine to the medium. The effects of Ni++ and SITS can be
completely reversed by washing the cells in 0.16 M NaCl while TNBS binding
is partially irreversible. Measurements of mean cell volumes (MCV) indicate
that the modifier- induced increases in Na+ permeability are not caused by
shrinkage of the cells. It is concluded that the movement of sodium ions
through ionic channels in dog red blood cells can be enhanced by
modification of amino and sulfhydryl groups. Zn++, TNBS, and SITS increase
Na+ permeability by modifying amino groups in the channel while Hg++, Ni++,
Co++, and PCMBS act on sulfhydryl groups.
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