Rabbit serum is shown to contain a cAMPdependent protein kinase (biochemically characterized as type H) that specifically phosphorylates a 135-kDa endogenous protein. This endogenous phosphorylation can be reproduced with platelet-rich plasma, after stimulation with thrombin, but not with plasma devoid of platelets. Stimulation of isolated platelets ("washed" by gel rftration) with either thrombin or ADP brings about a release of this kinase. The supernatant of these stimulated platelets, which contains the kinase, does not undergo a cAMP-dependent endogenous phosphorylation because it does not contain the 135-kDa protein substrate. On the other hand, plasma devoid of platelets does not contain cAMP-dependent protein kinase. By combining the supernatant of the physiologically stimulated platelets with the plasma devoid of platelets, it is possible to reconstitute the system and to reproduce the specific endogenous phosphorylation of the 135-kDa target substrate. On the basis of the above evidence it is proposed that upon physiological stimulation of platelets, they release into the blood a cAMP-dependent protein kinase in addition to the well-known release of MgATP. This kinase specifically phosphorylates the 135-kDa plasma protein.Following the discovery of cAMP (1) and of cAMPdependent protein kinase (2), it became apparent that cAMPmediated protein phosphorylation plays a key role in the regulation of intracellular metabolism (3). cAMP-dependent protein kinase was shown to act as a major sensor of changes in cAMP levels within the cell, changes that take place in response to extracellular hormonal stimuli. This kinase is thus involved in the intracellular implementation of some extracellular (hormonal) messages (4), making cAMP a "second messenger" (1).cAMP-dependent protein kinases have been isolated from quite a few mammalian tissues (5). They were shown to be composed of catalytic (C) and regulatory (R) subunits assembled, in the inactive form, into an R2C2 complex (6-9). When there is a rise in the intracellular level of cAMP, this complex is activated by binding of the cyclic nucleotide to its regulatory subunits and subsequent dissociation to release free, catalytically active C subunits (6-10).The occurrence of a cAMP-triggered proteolysis of cAMPdependent protein kinase in brush-border membranes (11) led us to the discovery ofa membranal proteinase that selectively clips the C subunits of cAMP-dependent protein kinase. Further characterization of this proteinase showed (12-15) that it possesses a rather unusual biochemical specificity for C, which would suggest that C itself could be a physiological substrate ofthe kinase-splitting membranal proteinase. However, a recent study on the orientation of the proteinase showed that in cell membranes its active site faces predominantly the cell exterior (16), whereas cAMP-dependent protein kinase is so far known to act within the cell. This prompted us to consider a possible extracellular function(s) for this kinase and thus its possible occurrence in...
