The specificity of the fatty acyl moieties of diacylglycerol for the activation of Ca2+-activated, phospholipid-dependent protein kinase was investigated. Diacylglycerol has been previously shown to activate this enzyme by increasing the affinity for Ca2+ and phospholipid, both of which are indispensable for the enzyme activation. Diacylglycerols containing at least one unsaturated fatty acid at either position 1 or 2 are fully active in this capacity, irrespective of the chain length of the other fatty acyl moiety in the range tested, C2 to C18. Diacylglycerols containing two saturated fatty acids such as dipalmitin and distearin are far less effective. Mono- and triacylglycerols and free fatty acids are totally inactive, indicating that the diacylglycerol structure is essential.

Ca2+-activated, phospholipid-dependent multifunctional protein kinase originally found in rat brain occurs in a variety of mammalian tissues. In most tissues the enzyme activity is comparable to that of cyclic AMP-dependent protein kinase when assayed with calf thymus H1 histone as phosphate acceptor. In some tissues such as platelets, brain, and lymphocytes the enzyme far exceeds the cyclic AMP-dependent enzyme. This species of protein kinase found in various tissues shows very similar physical, kinetic, and catalytic properties, and does not appear to show tissue and species specificities. It is conceivable that this protein kinase plays roles in transmembrane control of protein phosphorylation by a large number of extracellular messengers which induce phosphatidylinositol turnover in their target tissues.

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