The Hormonal Control of Activity of Skeletal Muscle Phosphorylase Kinase

Open Access
Publisher Website
Google Scholar
Abstract
Phosphorylase kinase was activated 5–10‐fold in vivo by an intravenous injection of adrenalin. Sodium fluoride, an inhibitor of phosphorylase kinase phosphatase, was required to prevent the reversal of this process; the activated and non‐activated forms of the enzyme were indistinguishable by dodecylsulphate gel electrophoresis. This suggested that the activation had resulted from a phosphorylation of the enzyme, and that it was not a consequence of the well known activation by proteolytic cleavage that can be demonstrated in vitro. Phosphorylase kinase activated in vivo was purified and digested with trypsin, and the two tryptic peptides which contain the serine residues which are phosphorylated in vitro by the action of cyclic‐AMP (adenosine 3′: 5′‐monophosphate) dependent protein kinase, were isolated. It was found that the same nine‐amino‐acid segment of the β chain and the same seven‐amino‐acid segment of the α chain had become phosphorylated in vivo in response to adrenalin, as were phosphorylated in vitro. The degree of phosphorylation of each of the two sites was at least 50%. The data provide direct proof that the activation of phosphorylase kinase which occurs in vivo in response to adrenalin results from a phosphorylation of the enzyme. They also indicate that the novel form of regulation associated with the phosphorylation of the α subunit, the stimulation of protein dephosphorylation by “second site phosphorylation”, can now be regarded as a new form of enzyme control mechanism which operates in vivo. The regulation of phosphorylase kinase activity was studied in the protein · glycogen complex from skeletal muscle. The enzyme could be rapidly converted to a phosphorylated form in a cyclic‐AMP‐stimulated reaction upon addition of magnesium ions and ATP, but the conversion of phosphorylase b to phosphorylase a in the complex still showed an absolute requirement for calcium ions. The implications of these findings and major problems in the hormonal control of skeletal muscle glycogenolysis which are not yet resolved, are discussed.