o
Fig. 4-19. Merged Hill plot of the binding of [ H]corticosterone to the complete ensemble
of corticosterone binding sites in perfused mouse brain cytosol (derived from
28 values of specific binding, B from 7 independent experiments combined
with the individual estimates, fSr each experiment, of the total binding site
concentration, BM.Y, derived from initial Scatchard plots such as that depicted
in figure 4-12).
Only points corresponding to binding site occupancy (e = BSp/B^) between .2 (20%) and .8
(80%) were included in the analysis. The curve (almost linear) was fit by a cubic spline
algorithm that employs polynomials to smooth data in accordance with a statistical
criterion; this method of regression contains no assumptions (other than the smoothness
criterion) concerning the basic form or shape of the curve, and thus is appropriate for
empirically determining the shape of the Hill plot and the abscissa and slope at 50%
occupancy (logit e = 0). Representative experimental conditions are indicated in the
legend to figure 4-8, and the use of the IMSL library cubic spline subroutine is described
in Methods. Equilibrium dissociation constant = 6.2 nM (abscissa at logit e = 0); Hill
coefficient (slope at logit 0 = 0) n = 0.95. Range of total binding site concentration
BMAX: 4,1 nM; range of cytosol protein concentration 4.8 9.3 mg/ml.