BIOSYNTHESIS AND ISOLATION OF CARBON-14 LABELED DIGITOXIN

¹ Department of Pharmacology, The University of Chicago, Chicago 37, Illinois
² Department of Physiology and Pharmacology, The University of South Dakota, Vermillion, S. D.

A procedure has been described for the biosynthesis of radioactive digitoxin from Digitalis purpurea L. plants grown in an atmosphere of radioactive CO₂. Employing the growing chamber described, it was possible to administer as much as 4.2 mc. of C¹⁴ to each plant without causing any noticeable inhibition in growth rate when compared to control plants grown simultaneously in a second chamber under similar conditions, but without the labeled CO₂. However, histological studies of radioactive plant leaves (Beal, 1949) indicate that C¹⁴ does cause injury to the plant leaves. It is believed that larger amounts of C¹⁴ can be administered to the plants although this has not yet been attempted.

An extraction procedure for the labeled drug utilizing chemical and chromatographic methods has also been described. The yield of non-crystalline digitoxin was approximately 0.06 per cent: after crystallization the yield was reduced to approximately 0.04 per cent. Depending upon the amount of C¹⁴ administered, the specific activity of the labeled glycoside ranged from 0.26 to 0.54 µc. per milligram. This range of specific activity was found to be sufficient for biological tracer studies.

Using the radioactive method of assay and adding non-labeled carrier digitoxin to the biological sample, it is possible to isolate and detect as little as 0.02 microgram of radioactive digitoxin (original sp. activity—0.537 µc./mgm.) per 100 ml. of human urine.

Identification tests performed on the radioactive drug indicated that the isolated compound was digitoxin of high purity.