Page 193 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

P1: FMX Final Pages
Encyclopedia of Physical Science and Technology EN009J-69 July 19, 2001 22:50

Microanalytical Assays 693

TABLE III Approximate Blood Levels of Plasma Con- in nature or readily genetically modified to have K M
stituents that vary over 5 orders of magnitude. Table II lists
Compound ×10 −3 M Compound ×10 −6 M K M values for some enzymes that have been used in
biosensors.
Sodium 140 Albumin 600 Response time: The response behavior of sensor-based an-
Chloride 105 Urate 250 alytical devices is affected by a number of parameters.
Bicarbonate 30 Phenylalanine 125 With respect to the sensor component, response char-
Glucose 5 Immunoglobulin G 90 acteristics are usually determined by a combination of
Urea 4 Ammonia 30 diffusional and reaction kinetic processes. The usual
Cholesterol 4 Iron 20 configuration for a biosensor includes a membrane that
Calcium 2.5 Bilirubin (Total) 10 protects the sensor element. Thus in the Clark glu-
Triglycerides 1 Immunoglobulin M 1 cose sensor shown in Figure 7, the diffusion processes
(fasting)
include the passage of glucose and oxygen from the
Compound ×10 −9 M Compound ×10 −12 M external fluid through the dialysis membrane into the
enzyme compartment and the subsequent passage of
Oestriol (late 600 Aldosterone 180 oxygen through the silicon rubber membrane into the
pregnancy)
electrode compartment.
Thyroxine binding 500 Insulin 120
globulin
Theapproximatelagtimeintroducedbydiffusionlayers
Cortisol 400 Parathyroid hormone 100
(either membranes or liquid layers) can be expressed as
Placental lactogen 300 Growth hormone 50
2
Thyroxine (total) 125 Luteinising hormone 10 Lag time (sec) = 0.2L /D,
Corticosterone 20 Triiodothyronine (free) 10 2
where D is the diffusivity of solute in the layer (cm /sec)
Triiodothyronine 2 Adrenocorticotrophin 10
and L is the thickness of layer (cm).
Prolactin 1 Thyroid stimulating 5
If the biological element is an enzyme, then the kinetics
hormone
of the reaction of analyte will be governed by an equation
Oestradiol-17 1 Angiotensin II 4
(women) similar to that given above, and the overall rate of reaction
Progesterone 1 Oxytocin 1 will be determined by the enzyme concentration in the
Arginine vasopressin 1 preparation (E) and the inherent substrate turnover of the
enzyme characterized by V m .
If the biological element is an antibody or a biorecep-
biological materials such as enzymes or antibodies, this tor, the kinetics will be related to the magnitude of the
meansthatonehastoselecttheappropriateproteinwith affinity constant K a . The affinity constant is essentially
the correct binding characteristics or reactive charac- the equilibrium constant for the reversible association and
teristics for the molecule that is to be measured. Fortu- dissociation of the ligand for the antibody. Kinetic studies
nately, with the advent of modern molecular biology, of these reactions has shown that increase in binding con-
one can either select or modify the protein to fulfill the stants is directly correlated with a reduction in the inher-
analytical requirements. An equation that depicts the ent dissociation rates of these reactions. The implication
interaction of an antibody with a ligand is of this behavior is that the response rate of sensors based
on antibodies becomes slower with higher affinities.
k f
Antibody + Ligand → Antibody − Ligand. An approximate value for the association rate con-
←
k r 8 −1 −1
stant (k f ) for ligands with antibodies is 10 M sec .
The affinity constant for these antibiodies is the con-
Thus an approximation for the lag time for dissociation of
centration at which the protein is half saturated with
antibiody–ligand complexes is
the ligand.
−8
K a = k f /k r . Lag time (sec) = 10 /K a ,
Enzymes have a kinetic characteristics that show satu- where K a is the ligand–antibody association equilibrium
−1
ration behavior: constant (M ).
v = V m SE/(K M + S).
The parameter for enzymes that characterizes the sen- SEE ALSO THE FOLLOWING ARTICLES
sitivity level for sensors based on the uses of en-
zymes is the kinetic parameter known as the Michaelis– BIOENERGETICS • BIOMATERIALS,SYNTHESIS,FABRICA-
Menton constant (K M ). Again enzymes can be found TION, AND APPLICATIONS • BIOPOLYMERS • ELECTRO-