Thymidine kinase activity and thymidine salvage in adult Brugia pahangi and Dirofilaria immitis

Cytosolic thymidine kinase (EC 2.7.1.75), the initial enzyme in the thymidine salvage pathway, was detected in crude homogenates of adult female Brugia pahangi and Dirofilaria immitis, with respective specific activities of 100 and 460 nmol/h/mg protein. Partially purified filarial thymidine kinases were found to have molecular weights of approximately 180 000, to be most active in the presence of Mg 2+ and ATP, to have a sharp pH optimum (pH 7.0) and to be heat-labile in the absence of added thymidine. For both, the respective K m values for thymidine and ATP were 60 μM and 1.6 mM, and 5-iodo-2′-deoxyuridine was as good a substrate as thymidine. A distinguishing property was the 3-fold higher sensitivity of the B. pahangi enzyme to feedback inhibition by thymidine 5′-triphosphate. Adult female B. pahangi took up and incorporated [ methyl- 3H] thymidine into DNA when they were exposed to this radiolabeled deoxynucleoside in vivo, but the thymidine salvage pathway in these worms was essentially nonfunctional in vitro.