Biomedical Engineering Reference
In-Depth Information
In this method, glucose can be measured from the oxidative current produced by Med
red
that has
lower oxidative potential. The interruption by the oxidation of other electroactive substances can be
avoided, enhancing the selectivity and accuracy of the measurement.
14.1.1.1.2 Potent iomet ric Glucose Biosensor
Potentiometric glucose biosensor is manufactured with a FET or a pH transducer, which detects the
variation of pH in the solution, while glucose is oxidized to gluconic acid as in Reaction 14.4. The
change of pH induces a shift of current that passes through a FET and a shift of potential that comes
through the sensitive layer of glass bulb of a pH transducer, and then the concentration of glucose is
related to the current or potential difference, respectively.
14.1.1.1.3 Opt ical Glucose Biosensor
When GOD is covalently bonded to a fl uorescein derivative during the enzymatic reaction (14.4),
glucose reacts with the labeled enzyme, and the oxygen in the solution is consumed, as well as an
increase in the fl uorescence intensity of the labeled enzyme is observed. The variation in the fl uo-
rescence intensity is directly related to the glucose concentration [10], and GOD is therefore used as
an optical glucose biosensor for quantitative glucose measurement.
Glucose biosensors are mainly used to detect the concentration of glucose in human blood,
urine, and food and can also be used to detect the concentration of lactose in the presence of
β-galactosidase (E.C. 3.2.1.23) that can catalyze the hydrolysis of lactose to glucose as in Reaction
14.8 [29]:
β-galactosidase
Lactose
+
H
2
O
β-d-glucose
+
β-d-galactose
(14.8)
Examples of the three kinds of glucose biosensors developed in recent years are listed in Table 14.3.
14.1.1.2
Cholesterol Esterase/Cholesterol Oxidase
Cholesterol esterase (CEH) (E.C. 3.1.1.13) is mainly generated from visceras of animals and some
microorganisms. CEH is made up of two identical subunits (2BCE) each of which has a molecular
weight of 63,556 Da. Especially, the molecular weight of CEH prepared from
Pseudomonas
is
about 302 kDa.
CEH can catalyze the hydrolysis of cholesteryl ester to cholesterol:
CEH
Cholesteryl ester
+
H
2
O
cholesterol
+
RCOOH
(14.9)
COD (E.C. 1.1.3.6) is generally a protein with low molecular weight. All kinds of COD
that originated from microorganisms are monomers, while FAD acts as a prosthetic group simi-
lar to GOD. Since Flegg [30] and Richmond [31,32] illustrated the suitability of COD for the
analysis of serum cholesterol, COD has become one of the most widely used enzymes in clinical
laboratories.
The enzyme COD can catalyze the conversion of cholesterol to cholest-4-en-3-one with con-
comitant reduction of O
2
to H
2
O
2
[33]. This reaction is carried out in two steps: cholesterol is fi rst
oxidized to cholest-5-en-3-one by equimolar O
2
in the presence of COD, while O
2
is reduced to
H
2
O
2
(14.10). The intermediate product cholest-5-en-3-one is then isomerized to cholest-4-en-3-one
with the catalysis of COD as in Reaction 14.11.
COD
O
2
Cholesterol
+
cholest-5-en-3-one
+
H
2
O
2
(14.10)
COD
Cholest-5-en-3-one
cholest-4-en-3-one
(14.11)
