118 



FACTORS AFFECTING METABOLISM 171 VltW 



increase in radioactivity represented a true metabolic exchange, 

 and did not involve the synthesis of any new phosphoprotein. 

 Analytical procedures for phosphoproteins involve the measure- 

 ment of inorganic phosphorus liberated from the appropriate 

 fraction during alkaline digestion and since brain contains numerous 

 unknown phosphates analysis of this type is always open to some 



Table 18. — The Effect of Electrical Stimulation upon the 



istribution of radioactivity in the phosphates of guinea 



Pig Cerebral Tissues 



Values for the acid-soluble phosphates are given in i S.E.M. Figures in 

 brackets represent the number of determinations. Data from Heald(1956). 

 Pulses were passed for 10 sec. 



doubt regarding the origin of the phosphorus. Proof of the 

 phosphorus arising from a phosphoprotein has been provided by 

 the isolation of o-phosphorylserine from partial acid hydrolysates 

 of cerebral phosphoprotein fractions. When derived from tissue 

 incubated with radioactive phosphate and subjected to pulses for a 

 few seconds the phosphorylserine increased in specific radio- 

 activity (Heald, 1958) clearly indicating that it was derived from 

 the phosphoprotein originally found to be involved in the response. 

 0-phosphorylserine was the only radioactive phosphorylated 

 amino acid isolated from the acid hydrolysates. Since it is well 

 recognized as a component of many naturally occurring phospho- 



