High Affinity Binders for Cyclic Adenosine 3′, 5′-monophosphate on Plasma Membranes Isolated from Rat Liver and Ascites Hepatomas1

Abstract

Plasma membranes from rat liver were found to contain at least two types of specific binding sites for cyclic [3H] adenosine 3', 5'-monophosphate (c[3H]AMP) with apparent dissociation constants of 0.51 +/- 0.14 and 2.9 +/- 0.6 nM (O degrees), respectively. The levels of these binding sites in liver plasma membranes were about 0.60 +/- 0.20 and 1.3 +/- 0.5 pmole/mg protein. The highest affinity binders for c[3H]AMP were found to be reduced in amount in plasma membranes of ascites hepatomas to 1/3 to 1/4 as compared with liver membranes in the cases of AH-130 and AH-7974 and to an almost undetectable level in the case of AH-130F(N). No difference in the endogenous phosphorylation of plasma membranes by (gamma-32P])ATP was, however, detected among liver and hepatoma plasma membranes. Addition of cAMP or cGMP at various concentrations did not affect the endogenous phosphorylation of plasma membranes of these cells.