Mode of Activation and Kinetic Properties of Three Distinct Forms of Protein Kinase C from Rat Brain1

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Abstract
Three types of protein kinase C, designated types I, II, and III, were purified from rat brain cytosol, and have been shown to correspond to the cDNA clones γ, β, and α, respectively. Their relative activities in the whole brain tissue were roughly 26, 49, and 25% with H1 histone as a substrate. Type II enzyme was an unequal mixture of two subspecies (roughly 1: 7) encoded by βI and β11 sequences which differ from each other only in a short range of their carboxyl-terminal end regions. Although the three types have closely similar structures, they showed slightly different modes of activation and kinetic properties. Type I enzyme was less sensitive to diacylglycerol but was significantly activated by low concentrations of free arachidonic acid. Type II enzyme exhibited substantial activity without elevated Ca2+ levels, and responded well to diacylglycerol and, to some extent, arachidonic acid. The type III enzyme responded to diacylglycerol as well as to arachidonic acid. The mode of activation of the enzyme by arachidonic acid required elevated levels of Ca2+ but not phospholipid. In the presence of phospholipid, phorbol esters could activate all three types in a manner similar to diacylglycerol. Among various phospholipids tested, phosphatidylserine was the most effective for all three types. Type III enzyme was most sensitive to l-stearoyl-2-arachidonylglycerol for activation. Conversely, type I enzyme was activated most efficiently by synthetic permeable diacylglycerols, such as 1,2-didecanoylglycerol and 1,2-dioctanoylglycerol. Many heavy metal ions exerted variable and distinct effects on the catalytic activities of these three types. Zn2+ inhibited all types when added in the presence of Ca2+, whereas in the absence of added Ca2+ this metal ion could activate type I but not type II or III enzyme. It is concluded that the three types of protein kinase C show subtle individual characteristics, and possibly play distinctly different roles in the regulation of neuronal functions.