Characteristics of the cyclic nucleotide phosphodiesterases of normal and leukemic lymphocytes

The specific activity of cyclic AMP phosphodiesterase and cyclic GMP phosphodiesterase of leukemic lymphocytes was 5-10-fold greater than that of purified normal lymphocytes or of homogenates of spleen, thymus or lymph nodes of normal mice. This rise was demonstrable over a wide range of substrate concentrations. Both normal and leukemic lymphocytes contained a heat-stable, calcium-dependent activator of phosphodiesterase. However, the increased activity of phosphodiesterase in leukemic lymphocytes was not due to this protein activator since (a) phosphodiesterase activity from these cells was not stimulated by this activator and (b) phosphodiesterase activity of leukemic lymphocytes was not inhibited by the calcium chelater, ethylene-glycol-bis,(beta-aminoethylether)-N,N'-tetraacetic acid, suggesting that the enzyme was not already maximally activated. A comparison of several other properties of phosphodiesterase from normal and leukemic lymphocytes showed that the enzymes have similar pH optima, similar stabilities to freezing and thawing and similar sensitivities to inhibition by the phosphodiesterase inhibitors, chlorpromazine, papaverine and isobutylmethylxanthine. However, the subcellular distribution of the phosphodiesterases was different, and the phosphodiesterase of leukemic lymphocytes was significantly more resistant to heat than that of normal lymphocytes. Although no differences were found between the phosphodiesterases of normal and leukemic lymphocytes in their sensitivities to drugs, there were marked differences in drug sensitivity between the phosphodiesterase of lymphocytes and that of other tissue. For example, concentrations of chlorpromazine which inhibited phosphodiesterase of cerebrum by 70% had no effect on phosphodiesterase activity of lymphocytes. On the othere hand, the papaverine-induced inhibition of phosphodiesterase was similar in lymphocytes and cerebrum. Since an optimal concentration of cyclic nucleotides is essential to maintain normal cell growth, these results suggest that the abnormal growth characteristics of leukemic lymphocytes may be explained by their high activity of phosphodiesterase. Furthermore, the qualitative and quantitiative differences between the phosphodiesterases of leukemic lymphocytes and other tissues raise the possibility of selectively inhibiting the phosphodiesterase of the leukemic lymphocytes, thereby reducing their rate of growth, without affecting other tissues.