Decreased activity of cytidine 3':5'-monophosphate (cyclic CMP) phosphodiesterase in the fast-growing Morris hepatoma 3924A, but not in the slow-growing Morris hepatoma 9618A.

The activity level of the newly-identified cyclic CMP phosphodiesterase in the fast-growing Morris hepatoma 3924A was found to be much lower than the control (normal or host) liver. Its level in the slow-growing Morris hepatoma 9618A (a minimal deviation tumor), on the other hand, was the same as the host liver. The level of cyclic AMP phosphodiesterase was higher, whereas that of cyclic GMP phosphodiesterase was lower, in hepatoma 3924A than the control liver. In comparison, the levels of the two enzymes were both depressed in hepatoma 9618A. These findings suggest that depression of cyclic CMP phosphodiesterase may be related to the process and the rate of malignant growth, and that metabolism of cyclic CMP may be more crucial than that of cyclic AMP or cyclic GMP in the neoplastic cell proliferation.