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Cytidine deaminase levels in cultured mammalian cell lines measured by the growth tests and enzyme assays.

We developed a test medium for cytidine deaminase in order to examine the distribution of this enzyme in cultured cell lines. The growth of various mammalian cell lines was tested in culture medium containing 2 microM pyrazofurin and 100 microM cytidine. Enzymological assays for the enzyme also were made spectrophotometrically with cell extracts. A good correlation was found between results of cell growth tests and the levels of enzyme activity. Twelve of twenty cell lines were killed in the test medium, but the remaining lines showed good growth. The levels of enzyme activities were lower in the former lines than in the latter. The critical level of enzyme activity required to support cell growth was approximately 30 units per mg protein. These findings indicate that culture medium containing 2 microM pyrazofurin and 100 microM cytidine serves as a test medium for cytidine deaminase. The possibility that the cytidine deaminase may be useful in determining the embryonic origin of cultured cell lines is discussed, based on the growth properties of various cultured cell lines in the test medium.