62 



S. KOREY 



VOL. 4 (1950) 



^00 



t 150 



100 



calcium ion at i • io~2 M there was precipita- 

 tion of the nucleotide and therefore the effect 

 cannot be evaluated. On the basis of these 

 observations a solution of ATP in i • lO"^ M 

 MgClg was used as standard to produce con- 

 traction. 



A TPase activity 



The role of ATPase in the interaction 

 between muscle protein and ATP has been 

 repeatedly investigated. It is still a matter 

 of discussion^^' ^^ at which phase of muscle 

 activity the enzyme is required. It appeared 

 therefore of great interest to determine to 

 what extent the ATPase activity is preserved 

 in the preparation used. Table II shows the 

 rate of decrease of the enzyme activity. The 

 determinations revealed a gradual decline of 



activity to about 20% of the initial value, at which level the activity appeared to remain 



stable. 



On the addition of ATP enzymatically inactive fibers when loaded remained at 



resting length and no extension was noted. 



TABLE II 

 ATPase activity of muscle fibers of rabbit 

 preserved in glycerol at io° c, tested at37°c 



Mg(-logM) 



Fig. 4. The effect of magnesium ion in 

 varying concentrations on the extent of 

 isotonic shortening of fibers exposed to 

 0.002 M sodium ATP. The isotonic short- 

 ening caused by sodium ATP alone is 

 arbitrarily assigned as 100%. 



Inhibitors of contraction 



Since it is known that ATPase has -SH groups^^, the effect of -SH inhibitors were 

 studied to find reversible inhibitors of the contractile process. Fiber bundles of a dia- 

 meter of 0.5 mm or less were soaked in solutions of various compounds and then immer- 

 sed in I • io~2 M ATP standard. In suitable cases, the fibers after soaking were set up 

 in the isotonic system and quantitative measurements made. 



It was apparent that compounds which combined with sulfhydryl groups effectively 

 inhibited contraction of the fibers (Table III). Of these compounds sodium o-iodoso- 

 benzoate and mapharsen (we/a-amino-/)flra-hydroxyphenylarsinoxide) proved to be 

 reversible inhibitors. The inhibitory effect of mapharsen was reversed by washing the 

 fibers in saline whereas addition of cysteine to saline was required to remove the inhibi- 

 tion produced by o-iodosobenzoate. HgClg in i • io~* M concentration caused irreversible 

 References p. 6y. 



