Guanyl cyclase activity of human blood lymphocytes

An enzyme, guanyl cyclase, which catalyzes formation of CYCLIC 3',5'-GMP from 5'-GTP, has been identified in human peripheral lymphocytes. The activity in lymphocyte homogenate is 14 pmol (min 10-7 lymphocytes). No activity is detected in red blood cells, and the amount found in platelets is very low. The properties of this enzyme are very close to those reported for other guanyl cyclases studied in other tissues: namely, its intracellular localization, its requirement for cation Mn-2+, its inhibition by Hg-2+, Zn-2+ and by nucleotides especially 5'-ATP. No change in enzyme activity occurs when phytohemagglutinin P is added to disrupted lymphocytes. However, when the mitogen is incubated with intact cells, guanyl cyclase activity increases in a few minutes.