Behavior of Activities of Thymidine Metabolizing Enzymes in Human Article

Behavior of Activities of Thymidine Metabolizing Enzymes in Human Leukemia-Lymphoma Cells - Article Example The studies were conducted with cell cultures obtained from 13 human leukemia-lymphoma cell lines consisting of T- and B-cell lines as well as Non-T- and Non-B- cell lines. The various enzymes were assayed in extracts obtained from cells subjected to rapid freezing and thawing in liquid nitrogen. Activities of the catabolic enzymes were higher by several orders of magnitude compared to the synthetic enzymes in normal cells. However, in all leukemia-lymphoma cells examined, the thymidine degrading enzyme activities were decreased for example, by 5-42% in the case of dihydro thymine dehydrogenase (with the complete absence of DHT DH activity noted in chronic myelogenous leukemia K-562 cells) and up to 38% in the case of TP relative to normal cells. In contrast, the activities of the synthetic enzymes namely, thymidylate synthase and TK were increased significantly by up to 407 times and up to 79 times, respectively of the normal human lymphocytes. Thymidine is utilized by cells both for DNA synthesis and energy production through oxidation to CO2 and water. Therefore, the reduction in the activity of the thymidine degrading enzymes is also important since it would lead to the enhanced availability of the compound for DNA synthesis. Furthermore, the enhanced activities of the thymidine synthesizing enzymes would also contribute to DNA synthesis which is  essential for rapid cell growth and proliferation. A comparison of kinetic properties of the catabolic enzymes, DHT DH and TP in the normal lymphocytes showed that the specific activity of DHT DH was considerably less than that of phosphorylase thereby indicating that DHT DH is the rate-limiting enzyme and, therefore, a better enzyme to evaluate the capacity of human leukemia-lymphoma cells to degrade thymidine. Thymidine kinase (TK) converts thymidine, or deoxythymidine (dT) to the respective monophosphate.