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SCIENCE AND TECHNOLOGY

Sensing Biological Signal

Biotechnological Approach

by Md Iqbal Hassan Khan
HE momentum of biosensor development stems from the exciting advances of the life science. The preparation of pure enzymes in large quantities. evolving potential of protein engineering techniques, propagation of monoclonal antibodies by animal tissue culture on an industrial scale, progress in our understanding of function of receptor molecules, and availability of an ever Increasing number of microorganisms and mutants covering a host of metabolic pathways and ambient conditions are extremely useful for analytical exploitation and stimulating the field considerably.
In a simple sense, the general scheme of a biosensor consists of bio-catalytic component and the transducer which are closely connected. thus
combining specificity and sensitivity. In view of the large number of enzymes available and the wide range of reactions catalyzed, it is not normally difficult to find an enzyme that will accept a given analyte as a substrate and provide the basis for an analytical procedure. The enzymes are not used in soluble form, rather it is used in un-mobilized state.
Immobilized enzymes tend to have greater resistance to loss of activity and have featured in the development of sev-
eral analytical biosensor devices. namely the enzyme electrode the enzyme analytical reactor. and the diagnostic reagent strip.
The enzyme electrode system is a combination of thin layer of enzyme with a suitable electrochemical electrode sensor. Practically, the analyte diffuses into the enzyme layer and the catalytic reaction occurs and the products are formed. The function of the sensor is to monitor and signal the biochemical change; i.e. the biological signal is converted into electrochemical change. For example, it is the appearance of product which gives rise to an electrochemical signal that can be registered by an appropriate meter. The signal produced by. the transducer could follow either amperometric or potentiometric principle. An amperometric sensor gives rise to a flow of current (micro-amps) between the sensor and a reference electrode and the current is a linear function of the concentration of analyte. A potentiometric transducer gives rise to a potential difference (milli volts) between the sensor and a reference electrode.
A number of enzymes are used in enzyme electrodes. These enzymes have got potential application In the areas of medicine, bio-process control, food process control arid analysis and environmental analysis.
Clinical biochemistry has become an Important branch of modern medicine, struggling with Increasing pressure from the clinician who is looking for quick answers and with economic pressure from the public health systems, sophisticated automated analytical devices have been developed which allow measurement of hundreds of samples within an hour. An apparent situation can be resolved if it is highlighted with the inside fact of estimation of blood cholesterol. About half of all deaths in the developed countries are caused by arthen>s< lerosis (hardening of the itu -iesl In this diseas.
fatty deposits containing mostly cholesterol, accumulate on the walls of arteries. The deposits harden and enlarge to form plaques, gradually narrowing the bore of the artery and restricting the flow of blood. The irregular and rough textured plaques represent a form of tissue damaged and as such cause formation of blood clots (thrombosis). Eventually a blood clot forms in one of the arteries serving the heart, thus starving the heart of oxygen and causing a heart attack. If a person's blood cholesterol level is normally below 150mg per 100cmJ, that person should be fairly safe from the possibility of heart attack. If blood cholesterol level is regujarly above 250mg per 100cm2, then the risk of serious heart disease is substantially increased.
Another serious disease is familial hypercholesterolemia in which blood cholesterol attains level as high as 650 to lOOOmg per 100 cmJ
Qi^n^atiye^rftqrmjnatiOB,. of serum cholesterol can be tnade by a calorimetric test. The procedure is based on the reaction of acetic anhydride with cholesterol in a chloroform solution to produce a characteristic blue green colour. But in this system the specificity of reaction is decreased due to the interference imposed by proteins such as hemoglobin. So the test procedure is developed where extraction of cholesterol by other means takes place. An enzymes based biosensor system can easily replace this tedious and lengthy procedure. Cholesterol occurs in blood serum as free cholesterol and cholesteryl esters. The biosensor determines the total cholesterol by employing two enzymes. Cholesterol esterase hydrolyses the esters to free cholesterol, and cholesterol oxidase detects total free cholesterol. Cholesterol oxidase converts cholesterol to cholesterone and hydrogen peroxide. This system comprises two enzymes Immobilized in an enzyme reactor, and the electrodes are positioned close to the reactor. With a response time of two minutes this procedure provides a substantial saving of time compared with the chemical procedure. So. this bio-sensing device becomes a magic bullet in the arena of diagnostic test proce-