Galactose, Sensor

The principle of potential-dependent selectivity control of a galactose oxidase membrane (Johnson et al., 1982, 1985) using galactose, raffi-nose, and dihydroxyacetone as substrates has been discussed in Section 2.2.2. The change of the redox potential influences all reactions in the same direction. However, since the activity is different towards different substrates the degree of conversion of a particular substrate can be affected by the applied potential. At first the better substrates, e.g. raffinose or galactose, can be measured and at optimal potential all substrates are detected. 

Lang et al. (1983) devised a microelectrode for galactose determination in tissue. In analogy to the respective glucose sensor, Dicks et al. 

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P(Py) based amperometric galactose sensor data at 0.5 V, in 0.1 M phosphate buffer (pH 6.1) at 25 C. After Reference [821], reproduced with permission. 

Shinkai et al. described the synthesis of dendritic saccharide sensors based on a PAMAM dendrimer labeled with eight boronic acid residues [183]. The dendritic compound showed enhanced binding affinity for D-galactose and d-fructose. The fact that the dendritic boronic acid functions as a saccharide sponge is ascribed primarily to the cooperative action of two boronic acids to form an intramolecular 2 1 complex. When one boronic acid binds a saccharide, its counterpart cannot participate in dimer formation and seeks a guest. 

Biosensors are also available for glucose, lactate, alcohol, sucrose, galactose, uric acid, alpha amylase, choline, and L-lysine. All are amperometric sensors based on O2 consumption or H2O2 production in conjunction with the turnover of an enzyme in the presence of substrate. In the case of glucose oxidase reaction, the normal biological reaction is ... 

Electrochemical transducers work based on either an amperometric, potentio-metric, or conductometric principle. Further, chemically sensitive semiconductors are under development. Commercially available today are sensors for carbohydrates, such as glucose, sucrose, lactose, maltose, galactose, the artificial sweetener NutraSweet, for urea, creatinine, uric acid, lactate, ascorbate, aspirin, alcohol, amino acids and aspartate. The determinations are mainly based on the detection of simple co-substrates and products such as 02, H202, NH3, or C02 [142]. 

Chemically binding enzymes to nylon net is very simple and gives strong mechanically resistant membranes (135). The nylon net is first activated by methylation and then quickly treated with lysine. Finally, the enzyme is chemically bound with GA. The immobilized disks are fixed direcdy to the sensor surface or stored in a phosphate buffer. GOD, ascorbate oxidase, cholesterol oxidase, galactose oxidase, urease, alcohol oxidase (135), and lactate oxidase (142) have been immobilized by this procedure and the respective enzyme electrode performance has been established. 

Other metabolites that have been measured with calorimetric sensors include ascorbic acid, ATP/ADP (adenosine 5 -diphosphate), cephalosporins, galactose, hydrogen peroxide, lactose, malate, phospholipids, uric acid, xanthine, and hypoxanthine. 

Rajendran, V. and Irudayaraj, J. 2002. Detection of glucose galactose and lactose in milk with a microdialysis-coupled flow injection amperometric sensor. J. Dairy Sci. 85 1357-1361. 

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