THE METABOLISM AND FATE OF TRITIATED THYMIDINE IN MAN *†

Abstract

Tritiated thymidine (I) of high specific activity was rapidly administered by intravenous injection into 2 male patients with normal hematopoiesis. Plasma clearance of I began in the first circulation time and was exponential. Components with half-times of 0.2 minute and 1.0 minute were identified. This rapid clearance was associated with the incorporation of the compound into new desoxyribonucleic acid of proliferating cells as early as 1 minute after injection. Labeling of marrow cells was essentially complete in 10 minutes, and thereafter the label appeared to remain in those cells or their progeny for their life span. The short availability time of I permits studies of the time parameters of labeled cell populations. About 1/3 of I was catabolized to tritiated water a few hours after injection. Small amounts of non-volatile H3 compounds appeared in the urine during the first day. Tritiated Beta-aminoisobutyric acid was identified in these urines as a catabolic product of I. Severe malnutrition in one of the patients subsequently resulted in increased degradation of repeated I injection to tritiated water and other tritiated compounds which were excreted in the urine.