ii. Langone’s affinity constant calculation
Engvall also relies on the testimony of a Dr. Langone for the calculation of the affinity.
Dr. Langone testified that he calculated the affinity constant from data available in example 1 of the
Engvall specification. ER 3511-3520. Dr. Langone used a different method than Engvall to estimate
55
the affinity. He used a technique based on information in chapter 6 of the Parker text book (E130).
Applying this technique to one of the data points in example 1 (12.5 µg/l), he calculated an affinity
9
value for the labeled antibody of 5.2 x 10 liters per mole. ER 3519.
According to his testimony, Langone used the following relationship from the Parker text to
calculate the affinity constant, (ER 3515):
S=1/K .
a
Langone testified that S in equation is sensitivity of the system and K is the approximate affinity
a
constant. ER 3514-15. Langone identified the Parker text, particularly item 2 on page 137, as the
source for this equation. ER 3515. Item 2 states:
2. Association (K ) or avidity (K ) constants for antibody-a avid
hapten and antibody-protein interactions range between 1x10 and 4
13 -1
1x10 liters/mole . The practical sensitivity of an immunoassay is
approximately equal to 1/K or 1/K . [Emphasis added.]a avid
Parker further explains the equation and gives insight into the meaning of “practical sensitivity” (130,
p. 111):
As noted earlier, antibody affinity can vary over a broad range,
resulting in marked variation in assay sensitivity. The importance of
antibody affinity in hapten binding can be illustrated by a simple[56]
calculation. In the law of mass action
(1) Ka = (Ab·H)
(Ab)(H)
Under conditions in which 50% of the total hapten is antibody bound,
Ab·H=H, and the equation reduces to
55
Charles W. Parker, Radioimmunoassay of Biologically Active Compounds, Chapter 6, “The
Immunoassay, Thermodynamic and Kinetic Considerations,” pp. 111-138, Prentice-Hall, Inc. 1976.
56
A hapten is a small functional group that corresponds to a single antigenic determinant of an antigen.
FUND, p. 235.
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