Abstract
Summary
The possibility was investigated that thymidine phosphorylase activity is released from leukocytes during ultrafiltration of crude “plasma,” thus accounting for reports of this enzyme in “cell-free” plasma. Blood samples from nine healthy donors, when centrifuged at only 500g for 12 min, yielded plasma fractions that contained cells identified as leukocytes, erythrocytes and platelets, as well as significant levels of thymidine phosphorylase activity. Centrifugation of the crude plasma at 10,000g for 30 min produced a cell-free plasma that contained negligible levels of thymidine phosphorylase activity. By contrast, the sediment, which contained identifiable granulocytes, possessed very significant levels of enzyme activity. Centrifugation of the crude plasma at 800g for 10 min, when followed by ultra-filtration, yielded a cell-free plasma that contained a twofold higher level of thymidine phosphorylase activity (P < 0.02) than did the truly cell-free plasma produced by high-speed centrifugation.
Our findings substantiate earlier reports, which had indicated that thymidine phosphorylase activity is located primarily in the granulocytes, and suggest that significant levels of the enzyme ordinarily do not occur in true plasma obtained from normal blood, unless the granulocytes are ruptured. Consequently, the proposal that elevated levels of thymidine phosphorylase activity in plasma can be used as diagnostic markers for certain neoplastic states requires reevaluation.
The author wishes to thank Mary Jane Brown and Anne Brooks for their skillful technical assistance and Dr. Arnold D. Welch for his advice during the preparation of this manuscript.
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