Calcium ion sensor

K. Suziki, K. Tohda, Y. Tanda, H. Ohzora, S. Nishihama, H. Inoue, and T. Shirai, Fiber-optic magnesium and calcium ion sensor based on a neutral carboxylic polyether antibiotic, Anal. Chem. 61, 382-384 (1989). 

The extent of ion permeation in such tracer studies obviously relates to selectivity, and membranes with calcium ion-sensors and solvent mediators which promote good quality calcium ISEs in turn show high d(C"/C )/dt values. The largest value of 59 x 10" s has been reported for a tripentyl phosphate/calcium bis di[4-(l, 1,3,3-tetramethylbutyl)phenyl] phosphate based sensor [12]. 

Calcium Ion Sensor. Cyclic voltammograms (CV) of ferrocyanide/ferricyanide redox couple with the modified electrode were measured. The peak currents due to the reversible electrode reaction of a Fe(CN) /Fe(CN) system on a bare Pt electrode were almost completely suppressed by the coating witti the polyvinyl-polypeptide block copolymer. This indicates that the electrode was covered with the hydrophobic polymer and was insulated from redox active species. 

Calcium sensors are merely representative of a much wider class of ion sensors, albeit probably the best understood. Fluorescent probes have now been developed for a wide range of metal ions of biological interest, particularly sodium, potassium, magnesium, and zinc. 

System 22 is an earlier example which incorporates Tsien s selective calcium receptor 23. ° System 23 has also been employed for the construction of ratiometric fluorescent sensors involving wavelength shifts. System 22 and other related PET sensors provide some of the most visually dramatic fluorescence off-on switching induced by biologically relevant levels of calcium ions in addition to their consistent predictability of most sensor parameters. 

J. Bobacka. T. Lindfors, A. Lewenstam. and A. Ivaska, All-Solid-State Ion Sensors Using Conducting Polymers as Ion-to-Electron Transducers, Am. Lab., February 2004, 13 A. Konopka, T. Sokalski, A. Michalska, A. Lewenstam, and M. Maj-Zurawska, Factors Affecting the Potentiometric Response of All-Solid-State Solvent Polymeric Membrane Calcium-Selective Electrode for Low-Level Measurement, Anal. Chem. 2004, 76, 6410 M. Fouskaki and... 

Which optical sensor would be more affected by change of the concentration of electrolyte, glucose optical sensor, or calcium ion optical sensor Explain why. 

The development of a sensor for ionized magnesium turned out to be one of the most difficult challenges of recent years. Several carriers have been designed for this purpose but none have been satisfactory. The first report of a successful measurement of ionized magnesium in an automated clinical analyzer (Thermo, prev. KONE) was published only in 1990 [30]. The ionophore ETH 5520 was used as the active compound. Two other carriers have been used since then ETH 7025 (Roche, former AVL), and a derivative of 1,10-phenenthroline (Nova). All of the magnesium sensors are based on a plastic membrane. Numerical compensations of the influence of calcium ion and the ionic strength are used due to insufficient selectivity of the magnesium sensors. 

Q4 The parathyroids produce a peptide hormone, PTH, which controls the level of calcium in the body. A sensor on the surface of the parathyroid cells monitors blood calcium concentration and PTH is secreted in response to a fall in plasma calcium ion concentration. An increase in the level of PTH leads to hypercalcaemia (raised blood calcium) conversely, a reduction in the level of PTH leads to hypocalcaemia. PTH acts on the kidney to reduce reabsorption of phosphate and at the same time to increase reabsorption of calcium. In addition, it promotes the release of calcium and phosphate into the blood by activating osteoclasts, which break down the inorganic matrix of bone. PTH also increases the absorption of calcium by the mucosal cells of the intestine. The latter is a rather slow, indirect action mediated by PTH stimulation of calcitriol secretion by the kidney. 

Figure 4.28. Repeating Motifs in a Protein Chain. Calmodulin, a calcium sensor, contains four similar units in a single polypeptide chain shown in red, yellow, blue, and orange. Each unit binds a calcium ion (shown in green). 

Calcium ion acts by binding to calmodulin and other calcium sensors. Calmodulin contains four calcium-binding modules called EF hands that recur in other proteins. Ca2+-calmodulin activates target proteins by binding to positively charged amphipathic helices. 

CaJciiim sensor protein also occurs in the kidney Specificaliy, the calcium sensor occurs in the thick ascending loop, where the N-terminal half juts Out into the extracellular fluid (not into the lumen of the tubule, but in the space between tubules). When extracellular Ca ions increase in concentration, the thick ascending loop decreases its rale of calcium resorption (Chattopadhyay d (j/., 1996 Pearce and Thakkar, 1997 Pearce ef uf., 1996), The regulatory scenario in the kidney represents a mode of regulating the body s calcium balance that appears completely independent of vitamin D, The overall scenario is that, as extracellular Ca " increases, the parathyroid gland decreases its release of PTH, and the kidney reduces its reabsorption of calcium ions from the developing urine. 

The free calcium ions in blood and extracellular fluid are critical for building and maintaining an adequate bone mass, and also for preventing excessive calcification. The sensor that regulates the free calcium ion concentration of plasma is within the parathyroid glands, where it controls the secretion of parathormone (PTH). This 84 amino acid peptide is split from a large, precursor protein and retained in secretory vesicles. If the concentration of free calcium ions drops below a critical level in blood plasma, the gland is activated to secrete PTH into the bloodstream. 

---