Glycogen phosphorylase isoenzymes from hepatoma 3924A and from a non-tumorigenic liver cell line. Comparison with the liver and brain enzymes

Abstract

Glycogen phosphorylase isoenzymes were isolated from normal rat liver, rat brain, the glycogen-poor Morris hepatoma (MH) 3924A, and the glycogen-rich non-tumorigenic liver cell line C1I. Electrophoretic and immunological characterization of the enzymes showed that tumour and C1I cells expressed a phosphorylase isoform similar to the brain type; the liver type was not detectable. All enzymes were obtained as dimers; the Mr of the subunits was 96,000 (liver), 93,000 (brain and MH 3924A) and 92,000 (C1I). Isoelectric focusing revealed a main band of pI 6.34 for liver phosphorylase a, pI 5.67 for the enzymes from MH 3924A and brain, and pI 5.68 for C1I phosphorylase. Partial kinetic characterization of the AMP-independent forms of the isoenzymes yielded Km values for glucose 1-phosphate of 3.5 +/- 0.5 mM (liver), 3.9 mM (brain), 1.9 +/- 0.3 mM (MH 3924A) and 2.5 +/- 0.5 mM (C1I); Km values for glycogen were 0.4 mM (liver) and 0.3 mM (MH 3924A and C1I), calculated as glucose equivalents. The AMP-independent phosphorylase was inhibited by glucose 6-phosphate (Glc6P) with Ki values of 0.32 +/- 0.03 mM (C1I), 0.50 +/- 0.04 mM (MH 3924A) and approximately 5 mM (brain). The inhibition could be abolished by 1 mM-AMP, indicating that AMP and Glc6P may partially compete for the same site on the protein. Liver phosphorylase a was not inhibited by up to 25 mM-Glc6P. In contrast with liver and brain isoenzymes, phosphorylase from the cell lines was not affected by NaF and Na2SO4. The data show that both the hepatocellular carcinoma and the non-malignant immortalized liver cells express a phosphorylase isoform different from the liver type. Furthermore, there is some evidence that the enzyme from MH 3924A and C1I cells is distinct from brain phosphorylase a, in spite of electrophoretic and immunological resemblance, and that this isoenzyme is subject to altered metabolic regulation.