N-ACETYLNEURAMINIC ACID, PHOSPHATE AND THIOL-GROUPS OF PYRUVATE-KINASE ISOENZYMES FROM MORRIS-HEPATOMA-7777 AND NORMAL RAT-LIVER

The highest amount of N-acetylneuraminic acid (AcNeu) was found in pyr uvate kinase isoenzyme L from normal rat liver (24 moles/mole of enzym e tetramer), with the highest electrophoretic mobility. On the other h and, isoenzyme M-2 from Morris hepatoma 7777, with the lowest electrop horetic mobility, had the lowest AcNeu content (5 moles/mole of enzyme tetramer). This tumour isoenzyme M-2 of pyruvate kinase was, however, characterised by the highest phosphate content (12 moles/mole protein ), in comparison to isoenzyme L (3 moles/mole protein) or normal liver isoenzyme M-2 (6 moles/mole protein). This could indicate a regulator y change caused by reversible enzyme phosphorylation and dephosphoryla tion or sialization and desialization. Despite these differences, the sum of the two negatively charged residues was lower in tumour pyruvat e kinase isoenzyme M-2, with the slowest migration rate, than in norma l rat liver isoenzyme M-2.Moreover, isoenzyme M-2 from tumour material , in comparison with isoenzyme M-2 from normal rat liver, had a twice as high content of thiol groups (20 moles/mole protein), especially of free and superficially located ones, than the isoenzyme Mt from norma l liver (10 moles/mole protein). This may explain abnormal susceptibil ity of tumour isoenzyme M-2 to stereospecific inhibition by exogenous L-cysteine, and indicate genetically dependent changes in amino-acid c ontent of tumour enzyme which take place during cell tumourigenic tran sformation.