Cultured Novikoff rat hepatoma, human HeLa, and mouse L cells contain 2 cyclic 3′,5′-nucleotide phosphodiesterases, 1 with a low apparent Km of 1 to 2 εm for cyclic 3′,5′-monophosphate and another with an apparent Km of 100 to 400 εm. The high-Km phosphodiesterases are inhibited by theophylline in a simple, competitive manner, whereas the low-Km enzymes are inhibited in a noncompetitive manner, although the Ki's of inhibition are similar for both activities (1 to 7 mm). For cell extracts from all 3 cell lines, specific activities for the low-Km phosphodiesterase are similar, although there is slightly higher activity in extracts of the fastest-growing Novikoff cells than in extracts from the other cell lines. In contrast, the specific activity for the high-Km phosphodiesterase is 30 to 40 times higher in L than in Novikoff cell extracts and intermediate in HeLa cell extracts, and in Novikoff cells there is only 5 to 10% as much activity as in rat liver. The specific activities for the 2 phosphodiesterases do not change significantly during the growth cycle in suspension culture.

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Supported by USPHS Research Grant AI 07250 from the National Institute of Allergy and Infectious Diseases.

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