Some characteristics of fetal and adult isoenzymes of thymidine kinase in human breast cancers.

Two isoenzymes of Thymidine Kinase were isolated from 90 human breast cancers by ion exchange chromatography. One was the enzyme present in adult tissues (TK-A), the second corresponded with the enzyme found in fetal tissues (TK-F). The presence of both isoenzymes was observed in the 90 tumors investigated. The activity of TK-A varied in the same range as in normal tissues, while the activity of TK-F varied to a larger extent and was higher than that of TK-A in a majority of tumors. Moreover, TK-F activity was independent of the presence or of the level of estradiol and progesterone receptors. Both isoenzymes were further characterized and compared.