Quantitative detection of tumor M2-PK in serum and plasma

The majority of tumors express an isoform of the glycolytic enzyme pyruvate kinase, the type tumor M2. The isoenzyme exists in an active tetrameric and a less active dimeric form. The dimeric form is strongly overexpressed in tumor cells and this new tumor marker is thus called tumor M2-PK. This isoenzyme is released from tumor cells and is quantitatively detectable in body fluids by a sensitive enzyme-linked immunosorbent assay (ELISA). To establish the ELISA for the routine diagnostic in a clinical laboratory, the sample stability was evaluated. Therefore, blood samples were collected in different ways from healthy donors. Reproducibility of tumor M2-PK detection in EDTA-plasma was not affected by the day to day 'stress' in a clinical routine (e.g. shaking, leaving the samples at room temperature for several hours without prior centrifugation). Similar results were obtained with citrate-plasma, whereas detection in serum and heparin-plasma was only reproducible when the blood samples were centrifuged within 2 hrs after collection. It appears that lymphocytes contain small amounts of the tumor M2-PK isoenzyme. They might release tumor M2-PK in heparin-plasma and serum samples, but not in EDTA-plasma samples. The results indicated that EDTA-plasma appears to be most appropriate for the routine diagnostic of tumor M2-PK as a tumor marker.