Direct activation of a protein kinase activity from rat lacrimal gland by PMA in a phospholipid-free system.

The analysis of the cytosolic fraction from rat exorbital lacrimal gland with DEAE-cellulose ion-exchange chromatography showed the presence of a peak of protein kinase activity which was dependent on the presence of phosphatidylserine and diolein as well as calcium. This activity showed the same properties as the previously reported protein kinase C (PKC). Moreover, we have shown for the first time that this kinase or a kinase that coeluted from the column with PKC could be activated by a phorbol ester, PMA, in a phospholipid-free system, i.e. in the absence of any cofactor of PKC. These findings emphasize the need for caution in the interpretation of experimental results obtained when using phorbol esters to probe for a role of PKC in many regulatory processes.