Differences between Liver and Hepatoma Cells in Their Complements of Adenosine 3’ : SMonophosphate- binding Proteins and Protein Kinases*

The soluble fractions from both rat liver and an established hepatoma cell line (HTC cells) contain protein kinases and proteins which bind adenosine 3’: 5’-monophosphate (cyclic AMP). Liver cytosol exhibits somewhat more binding activity and less protein kinase activity than HTC cells. Protein kinase activity from both sources is stimulated by cyclic AMP. Scatchard plots for the binding of cyclic AMP have been compared at pH 4 and 6.5. At both pH values, liver shows nonlinear plots which are increased in curvature and apparent aflinities by prior dialysis of the samples, mostly because of the removal of endogenous cyclic AMP. For HTC cells the Scatchard plot at pH 4 is also nonlinear and exhibits high binding affinity (KD < 0.5 naa), but prior dialysis is unnecessary and the shape of the curve differs from that of liver. At pH 6.5, the binding affinity observed in HTC extracts (KD > 10 IBM) is lower than seen with liver and is insensitive to dialysis for a day or 2. However, this binding can be partially converted to higher affinity by aging or by overnight treatment at pH 4. The binding activity present in liver extracts is considerably less stable than that from HTC cells at pH 4 in the absence of cyclic AMP. Increasing the pH and adding cyclic AMP stabilize the binding activities from both sources. Much of the nonlinearity observed in the Scatchard binding plots is in the opposite direction from that caused by the instability of binding components at low concentrations of cyclic AMP. Chromatography on DEAE-cellulose shows that HTC extracts lack one of two major cyclic AMP-stimulated protein kinase fractions which are present in liver. The deficiency seems to be due primarily to the absence of a cyclic AMPbinding fraction. Chromatographically analogous protein kinase peaks are present in both HTC and liver extracts, although their proportions differ. The cyclic AMP-binding fraction which is present only in liver is very labile at pH 4 and accounts for the greater instability of binding activity in