When HeLa cells were grown with 5 µm 5-iodo-2′-deoxyuridine (IUdR) there was an increase in alkaline phosphatase activity after about 24 hr, reaching a maximum usually at about 72 hr. At this concentration, during this time, there was a relatively small inhibition of cell replication. The increment in alkaline phosphatase activity was in large measure prevented by concomitant addition of 10 µm thymidine. Further, IUdR did not increase alkaline phosphatase activity in a mutant line of HeLa cells lacking thymidine kinase. These data indicate that IUdR must be phosphorylated and incorporated into DNA to increase alkaline phosphatase activity. IUdR also usually increased alkaline phosphatase activity of HeLa71 which has a high constitutive level of the enzyme. IUdR did not affect the alkaline phosphatase activity elevated by prior treatment with 1 µm hydrocortisone. On the other hand, 1 µm hydrocortisone further increased the already elevated alkaline phosphatase activity in cells pretreated with 5 µm IUdR, although after hydrocortisone addition the final level of enzyme activity attained in IUdR-treated and untreated cells was the same.

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This work was supported by USPHS Grant CA10748.

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