Regulation of muscle phosphorylase b kinase activity by inorganic phosphate and calcium ions

Publisher Website
Google Scholar
Abstract
The regulation of the activity of blowfly flight-muscle phosphorylase b kinase by Pi and Ca2+ was studied, and the actions of these effectors on the kinases from insect flight and rabbit leg muscles were compared. Preincubation of blowfly kinase with Pi increased activity severalfold. The effect was concentration-dependent, with an apparent Km of about 20mm, and time-dependent, requiring at least 10min for maximal activation. Neither ATP nor cyclic AMP was needed, suggesting that a protein kinase may not be involved. Maximal activation of the insect kinase required Mg2+ in addition to Pi. The apparent Km for Mg2+ was 3mm. Rabbit leg-muscle phosphorylase b kinase was slightly inhibited, rather than stimulated, by Pi, and was strongly inhibited by K+, Na+ and Li+. At physiological concentrations, Ca2+ activated the phosphorylase b kinases from both blowfly flight and rabbit leg muscles. However, the responses to Ca2+ of the enzymes from the two tissues were different. The mammalian kinase had virtually no activity in the absence of Ca2+, and showed a large increase in activity over a narrow range of Ca2+ concentrations. Flight-muscle kinase had appreciable activity in the absence of Ca2+, and had a smaller increase over a wide range of Ca2+ concentration. The concentrations of Ca2+ required for half-activation were 0.1 and 1μm for the blowfly and rabbit enzymes respectively. The pH–activity profiles of the non-activated, phosphate- and Ca2+-activated kinase revealed considerable enhancement of activity with little, if any, increase in the ratio of activities at pH6.8 to those at 8.2. These results are discussed in relation to the mechanism coupling contraction to glycogenolysis and to the biochemical distinction between asynchronous and synchronous types of muscle.