Protein kinase cyclic-nucleotide-independent

It has been proposed that hormones and neurotransmitters as well as other types of regulatory agents control physiological processes by means of protein phosphorylation (Greengard, 1975, 1976). Some of the protein phosphorylation occurs as a consequence of a hormone receptor-adenylate cyclase-cAMP-protein kinase sequence of action. In other cases, different cyclic nucleotides may replace cAMP, while with cyclic nucleotide-independent protein kinases, other signal molecules. 

A cyclic-nucleotide-independent protein kinase has been found to transform glycogen synthase I (also termed a4 synthase) into glycogen synthase D (b4 synthase). This cyclic-nucleotide-independent protein kinase is stimulated by CDR (Srivastava et al., 1979). Conceivably, this enzyme mediates the inactivation of glycogen synthase by a-adrenergic agents (Assimacopoulos-Jeannet et al., 1977). 

Inoue, M., Kishimoto, A., Takai, Y., and Nishizuka, Y., 1977, Studies on a cyclic nucleotide-independent protein kinase and its proenzyme in mammalian tissues. II. Proenzyme and its activation by calcium-dependent protease from rat brain, /. Biol. Chem. 252 7610. 

In conclusion, flavonoids exert endothelium-independent vasodilator effects in isolated vascular smooth muscles that are related to the structure of the compound tested. The main vasodilator mechanism of flavonoids seems to be related to the inhibition of PKC, although an inhibitory effect on cyclic nucleotide PDEs and Ca2+ uptake and other protein kinases may also contribute to these actions, Fig. (4). 

E. Butt, M. Bernhardt, A. Smolensk , P. Kotsonis, L. G. Frohlich, A. Sickmann, H. E. Meyer, S. M. Lohmann and H. H. Schmidt, Endothelial nitric-oxide synthase (type III) is activated and becomes calcium independent upon phosphorylation by cyclic nucleotide-dependent protein kinases. J Biol Chem 275, 5179-5187 (2000). 

The term protein kinase was first used to describe the enzymes of yeast, liver, and brain which phosphorylated casein and phosvitin (85-87). More recently it has been proposed that the term protein kinase be restricted to those enzymes which catalyze the transfer of the y-phos-phate of ATP to serine and threonine hydroxyls of proteins and are regulated by cyclic nucleotides (88). The other class of protein kinases, the phosphoprotein kinases, are not affected by cyclic nucleotides, and they can often utilize GTP as a source of phosphate. Wherever appropriate, we shall refer to these two groups of enzymes as cyclic AMP-dependent or cyclic AMP-independent protein kinases, respectively (89). 

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