o
Fig. 4-15. Nonlinear regression analysis of the binding of [ H]corticosterone to the
complete ensemble of saturable corticosterone binding sites in perfused mouse
brain cytosol (derived from the total binding data, BT, of figure 4-8 and
displayed in the Scatchard coordinate system).
Experimental conditions are indicated in the legend to figure 4-8, and the use of the
nonlinear regression program is described in Methods. The model to which the data were fit
consists of the sum of one saturable high-affinity (specific) component and one
nonsaturable (nonspecific) component as described by equation (4-6). The actual regression
equation does not contain F; it accepts By and Sy (total [ Hjcorticosterone concentration)
as pairs of input variables and then regresses By on the relatively error-free variable Sy,
fitting the adjustable binding parameters K^, B^ (which replaces Bq in equation 4-6), and
C, (the asymptotic "sink" of nonspecific binding). Each input data point was weighted as
1 9
1/By consistent with the assumption that the coefficient of variation or percentage error
in the measurement of By is constant over the range of By. For display the resulting
complete 3-parameter regression model was plotted in the Scatchard coordinate system
(curved line), and each component was also plotted separately (2 straight lines). (The
x-axis is plotted below y=0 in the figure.) Apparent equilibrium dissociation constant Kd
= 8.8 nM, and total binding site concentration B^ = 4.6 nM (656 fmole/mg cytosol
protein.) Cj = 0.73 x 10^ M~*. Agreement with the binding parameter estimates derived
from the Scatchard plot of specific binding fit by simple linear regression (figure 4-12)
is excellent.