Identification and Comparative Analysis of Thymidine Phosphorylase in the Plasma of Healthy Subjects and Cancer Patients: Brief Communication

Abstract

Thymidine (TdR) phosphorylase (EC 2.4.2.4) was identified in human plasma with the use of a simple microassay that measured the catabolism of [³H]TdR to [³H]thymine ([³H]Thy). TdR phosphorylase also degraded [³H]deoxyuridine ([³H]UdR) but did not cleave [³H]uridine; thus the enzyme exhibited specificity for the “deoxy” moiety of these pyrimidine nucleosides. In a comparative analysis of TdR phosphorylase in the plasma of 146 patients with uncontrolled neoplastic diseases and 59 healthy subjects, 53 patients showed levels of enzyme activity that were greater than the highest value recorded for healthy subjects. In a similar study of 60 patients and 24 healthy subjects, appreciable amounts of [³H]UdR were degraded by the plasma enzyme; nevertheless, mean values recorded for the two groups were not significantly different. [³H]Thy and [³H]uracil, products arising from the degradation of [³H]TdR and [³H]UdR, are usually not incorporated to any appreciable extent by DNA- and RNA-synthesizing cells. Accordingly, failure to account for the presence of TdR phosphorylase in the plasma used for supplementing culture medium may lead to the erroneous interpretation of results of experiments that utilize incorporation of radiolabeled TdR or UdR as a means of defining the proliferation of cells in vitro.