ABSTRACT
Because recent observations indicate that metabolism of cyclic nucleotides may be altered in neoplastic cells, the intracellular levels of cyclic adenosine 3',5'-monophosphate (cAMP) and cyclic guanosine 3',5'-monophosphate (cGMP) were measured in mononuclear leukaemic and normal human leucocytes. The activities of adenylate cyclase, guanylate cyclase and cyclic nucleotide phosphodiesterases were also determined. Under basal conditions, cAMP levels were always higher in the normal leucocytes, whilst cGMP levels were of the same order of magnitude in both normal and leukaemic cells, causing the cAMP/cGMP ratios to be significantly lower in leukaemic leucocytes. Leukaemic cells significantly increased cyclic nucleotide levels in response to theophylline, but did not respond to serotonin, carbamylcholine or D,L-isoproterenol. Preincubation of these leucocytes with theophylline produced a detectable cAMP response to D,L-isoproterenol but no cGMP response to serotonin or carbamylcholine was found. Adenylate cyclase and guanylate cyclase were significantly lower in leukaemic than in normal cells, which could largely explain the abnormal cyclic nucleotide pattern found in human leukaemic leucocytes. In our experiments, cAMP phosphodiesterase activity was comparable in normal and leukaemic cells, whereas cGMP phosphodiesterase activity was undetectable inall mononuclear-leucocyte preparations with the methods used.
