Cytofluorometric determination of thymidine kinase activity in a mixture of normal and neoplastic cells.

A cytofluorometric assay allowing the measurement of thymidine phosphorylation in single cells had been established (Hengstschläger & Wawra, 1993). This assay enables us to correlate intracellular thymidine kinase (TK) activity with the DNA content of single cells. Enzyme activity levels from neuroblastoma cells and normal fibroblasts derived from the same patient were determined. Using this cytofluorometric assay in a mixture of both cell types the neoplastic cells could be distinguished from the normal fibroblasts because of their higher TK level. A human lymphoblastoid cell line was compared with the cell line KG-1, derived from an acute myelogenous leukaemia, in the same way. The increased enzyme activity enabled us to detect KG-1 cells in a mixture with an 10,000-fold excess of Epstein Barr virus transformed lymphocytes.