Biomedical Engineering Reference
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simple monovalent cations such as sodium ion. On the other hand,
if the sodium salt of ATP is diluted with an equal volume of a
NaCl solution containing the same concentration of Na + ions, the
resistance decreases by about one order of magnitude, indicating
that SLN allows the translocation of the small chloride anion
across the lipid bilayer. Incorporation of SLN in a tBLM immersed
in a pH 5.3 aqueous solution of 0.05 M NaH 2 PO 4 causes only a
slight decrease in the resistance of the lipid bilayer, as measured
by the diameter of a Z ” vs. Z ' plot (Nyquist plot; cf. Section III);
however, submicromolar additions of ATP decrease the resistance
of the tBLM to an appreciable extent, as shown in Fig. 17 . 195 Plot-
ting the conductance against the ATP concentration yields a curve
that tends asymptotically to a limiting value and can be satisfacto-
rily fitted by the Michaelis-Menten equation, with an association
constant for the SLN-ATP complex of about 0.1 PM (see the inset
of Fig. 17 ).
This behavior was explained by regarding a SLN channel as
consisting of a bundle of four or five SLN D-helixes that turn their
hydrophilic side, containing two hydrophilic threonine residues,
toward the interior of the bundle; this provides a hydrophilic path-
way for small desolvated inorganic anions, such as chloride ion.
Phosphate ion is somewhat bulkier than chloride ion. However,
SLN contains a positively charged arginine residue (Arg6) just
outside the lipid bilayer, on its cytoplasmic side. ATP was as-
sumed to bind electrostatically to this arginine residue. The repul-
sion among the highly negatively charged ATP molecules bound
to Arg6 should widen the lumen of the SLN channel, thus allowing
the passage of phosphate ions. On the basis of this and other fea-
tures of SLN, it was proposed that SLN is just the P i transporter
described by Lee and coworkers 197 on 1991 and whose nature was
not known up to now. The role of SLN is that of translocating
phosphate ions from the cytoplasm to the lumen of the sarcoplas-
mic reticulum, thus causing the precipitation of calcium phosphate
and increasing the level of Ca 2+ accumulation in the lumen.
Phospholamban (PLN) exists in equilibrium between the pen-
tameric and monomeric forms. The pentamer releases to Ca-
APTase a monomer, which forms a 1:1 complex inhibiting the
affinity of Ca-ATPase for Ca 2+ . 198 Phosphorylation of the mono-
mer by ATP removes the inhibition. Both SLN and the PLN mon-
omer have a single D-helix that spans the membrane. As distinct
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