Purification and Properties of Frog Skeletal Muscle Phosphorylase

The purification of frog skeletal muscle phosphorylase is described. Frog muscle phosphorylase b can be converted into the phosphorylated form (a) by rabbit skeletal muscle phosphorylase b kinase, adenosine triphosphate (ATP), and Mg²⁺. Both types (b and a) of frog muscle phosphorylase appear to be essentially homogenous by a variety of criteria. The molecular weight of frog muscle phosphorylase b was determined by sedimentation equilibrium and found to be 188,000 g. A redetermination of the molecular weight of rabbit muscle phosphorylase b by sedimentation equilibrium and by sedimentation velocity and diffusion gave a value of 185,000. There are 4 moles of pyridoxal 5′-phosphate and 4 moles of orthophosphate bound to each 370,000 g of frog skeletal muscle phosphorylase a. The amino acid compositions of frog, rabbit, and human skeletal muscle phosphorylases are quite similar, but frog and rabbit muscle enzymes differ immunologically. Several kinetic parameters of frog muscle phosphorylase a have been determined. As is the case with rabbit muscle phosphorylase a the dimer a is the active form of frog muscle phosphorylase a with glycogen as the substrate. The enzyme differs from mammalian muscle phosphorylase a in that the equilibrium between phosphorylase a dimer and phosphorylase a tetramer is shifted in the direction of the dimer. Furthermore the rate of dissociation at low temperatures is very much more rapid for the frog enzyme than for the rabbit enzyme. A possible correlation between these observations and the ability of frog muscle to carry out glycogenolysis at temperatures below 37° is discussed.