[Regulation of cellular functions of protein phosphorylation (author's transl)]

The biochemical mechanisms involved in the regulation of cellular functions by intracellular signals, such as calcium, cyclic AMP and cyclic GMP, are described briefly. Each cyclic nucleotide regulated diverse cellular functions by activating a specific protein kinase. Calcium, on the other hand, has several intracellular receptors, i. e., troponin-C, calmodulin and others, which may be present together in the same cell. In the presence of calcium, calmodulin activates a variety of enzymes including protein kinases, phosphorylase kinase and myosin light chain kinase. The specificity of hormonal effects in different tissues depends not only on the presence of the specific intracellular receptor system for each intracellular signal but also on the presence of specific isoenzymic forms of important regulatory enzymes, whose activity may or may not be regulated by phosphorylation and dephosphorylation.