Biomedical Engineering Reference
In-Depth Information
FIGURE 10.5
Crown ether with potassium. The potassium ion (orange) is squeezed in between the two ring
systems and is coordinated to eight oxygen atoms (red). Hydrogen atoms are not shown.
10.6.1 A
LKALI
M
ETALS
The most important biological role of sodium and potassium is to stabilize cell membranes and
enzymes by electrostatic effects and osmosis. Besides, these ions transmit electrical signals by
diffusion through a certain concentration gradient.
Crown ethers (Figure 10.5) form stable complexes with alkali metals and their biological
importance is binding heteroatoms on the inside of the macromolecule while the surface is more
lipophilic. As a result, pharmacologically active natural products of this kind function as antibiotics
since they may transfer alkali metals in and out of the cells and thereby perturb the natural metal ion
balance. The examples include valinomycin (Figure 10.6) and nonactin. Another efi cient method
for controlled cation transport through lipid double layers involves incorporation of ionic channels
in membranes.
Lithium salts play a particular role in treatment of manic-depressive psychosis. The effective
plasma concentration is 1 mM while 2 mM exhibits toxic side effects, and already 3 mM is a
lethal dose. The Li
+
ion has approximately the same radius as the Mg
2+
ion, and both metal ions
demonstrate high afi nity phosphate binding. Lithium ions inhibit the enzymatic function of inositol
monophosphatase thereby preventing release of phosphate from the active site. Inositol phosphatases
are magnesium-dependent, and structural studies have shown that Li
+
may bind to one of the catalytic
Mg(II) sites. Inositol phosphates are responsible for mobilizing calcium ions, and Li
+
will therefore
inl uence the calcium ion level in cells, which makes it imperative to monitor the calcium concentration
carefully in the patients during lithium treatment.
10.6.2 A
LKALINE
E
ARTH
M
ETALS
The calcium ion is engaged in a series of fundamental physiological processes from skeleton
stabilization, cell division, and blood coagulation to muscle contraction and immune responses.
