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On pages 131, 132, 136, and 137, the word conformational should be set off by quotation marks, rather than by parentheses.

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Glycogen phosphorylase in skeletal muscle exists as phosphorylase a, fully active, and phosphorylase b, active only in the presence of 5'-AMP.1 2 Phosphorylase b is phosphorylated at the expense of ATP by phosphorylase b kinase3 and dimerizes to form phosphorylase a. The reverse reaction is catalyzed by phosphorylase phosphatase, formerly called PR enzyme.4 5In 1956, C. F. Cori6 emphasized the speed with which glycogen breakdown occurs in muscle during contraction and suggested that the speed of the b a interconversion was of sufficient magnitude to be the pacemaker of the concomitant changes in the rates of glycolytic reactions. Recently, Rulon et al.7 reported a statistically significant increase in phosphorylase a content of rat anterior tibial muscle during tetanic contraction. The high and variable baseline levels of phosphorylase a in resting muscle obtained with previous methods have precluded a quantitative analysis of changes in the phosphorylase system. Improvements in techniques for the arrest of enzymatic reactions make it now possible to study the time course of the interconversion of phosphorylase a and b in the thin frog sartorius muscle during and following contraction. The present studies show that phosphorylase a rises from less than 5 per cent to nearly 100 per cent of the total phosphorylase (phosphorylase a and b combined) within three seconds after the onset of tetanic contraction at 300. On relaxation, phosphorylase a activity decreases exponentially to baseline values. Kinetic analysis suggests that changes in the phosphorylase b kinase rather than phosphorylase phosphatase activity are responsible for the increase and decrease in phosphorylase a.Variations in muscle phosphorylase a in response to epinephrine, as previously studied by G. T. Cori and B. Illingworth,8 have been reinvestigated with the use of the improved fixation technique. The increase and decrease of phosphorylase a in muscle as a consequence of the addition and removal of epinephrine is slow when compared with the effect of contraction, requiring minutes rather than seconds. Moreover, the response to epinephrine but not that to muscular work can be inhibited by an inactive analogue of epinephrine.Experimental Procedure.-Female Rana pipiens weighing approximately 50 to 70 gm were obtained from Minnesota between November and March. The animals were kept in water at about 30 prior to use. After low spinal transection, the sartorii were carefully dissected and transferred to ice-cold Krebs-Ringer's-bicarbonate solution9 adapted for frog muscle'0 and equilibrated with 5 per cent CO2 in argon (pH -6.8). The muscles were allowed to recover for about 30 min before the experiment was initiated.Muscle stimulation: Muscles to be stimulated were left attached to the pubic bone. The proximal end of the muscle was mounted in a platinum holder, which served as an electrode. The distal tendon was anchored to a small chain. A second platinum electrode surrounded the distal end of the muscle. Once in place, the muscle was immersed in anae...