The binding characteristics of thyroxine (T4), triiodothyronine (T3), and 
reverse T3 (rT3) to rat liver plasma membranes (RLPM) were examined to explore 
the interactions of thyroid hormones with cell surface receptors. Scatchard 
analysis suggested that all three ligands bound to two classes of binding sites. 
The high affinity rT3 binding sites appeared to be distinct from the high 
affinity T4 sites, on the basis of differing optimum physicochemical conditions 
for binding, and analog displacement studies. The higher affinity constant for 
T4 was 1.7 +/- 0.2 X 10(9) M-1 (mean +/- SEM) and binding capacity was 3.1 +/- 
0.3 pmol mg-1 protein whereas for rT3 binding the Ka was 2.5 +/- 0.4 X 10(8) M-1 
and capacity was 6.2 +/- 0.9 pmol mg-1. (125I) T3 bound with lower affinity and 
T3 tracer was readily displaced by T4. Moreover, comparatively higher 
concentrations of T3 were needed to displace either radiolabeled T4 or rT3, 
suggesting that T3 was binding to both the T4 and rT3 sites with lower affinity. 
Marker enzyme studies on RLPM, of varying purity prepared by different methods, 
showed a positive correlation between the activity of the plasma membrane enzyme 
magnesium-stimulated ATPase and high affinity rT3 and T4 binding. Column 
chromatography of the radioligands, after dissociation from membrane binding 
sites, confirmed that the integrity of the hormones was not altered during 
association or dissociation. Our results raise the possibility that the high 
affinity T4 and rT3 binding sites on RLPM may be hormone receptors mediating 
biological actions at the membrane level.