Expression of multimolecular forms of pyruvate kinase in normal, benign, and malignant human breast tissue

The levels of multimolecular forms of pyruvate kinase present in four normal human breast specimens, nine benign tissues, and 13 malignant breast carcinomas were determined. The different enzymatic forms were separated by isoelectrofocusing, quantitated photometrically, and characterized further by kinetic studies using phosphoenolpyruvate as the variable substrate in the presence of different effectors. A correlation between specific activity and malignancy was found. The mean specific activities (+/- S.E.) of the normal, benign, and malignant tissues were: 0.078 +/- 0.006, 0.36 +/- 0.072, and 3.50 +/- 0.696 IU/mg protein, respectively. A form of pyruvate kinase with an isoelectric point (pI) of 7.0 predominated in the breast tissues. The properties of this form were consistent with it being the K4 isozyme, known to be widely distributed in mammalian tissues. Higher pI forms were also found. The M4 isozyme, expressed by normal muscle and brain, has a pI value similar to the highest pI form found in the breast tissues. Therefore, the pI data alone suggest that the breast specimens also express some M-type subunits. This conclusion was not supported by the kinetic data. The higher pI forms are thought to be a posttranslationally modified K isozyme. Although this modified form is found in normal specimens, it seems more prevalent in neoplasms.