94 



GENERAL METABOLISM in VltVO 



fraction insoluble in trichloroacetic acid. The phosphoprotein 

 fraction has not been isolated and v^ork relating to its metabolism 

 has come exclusively from studies with radioactive phosphate. 



Incubation of cerebral slices with radioactive phosphate in 

 oxygenated salines containing glucose results in a rapid exchange of 

 inorganic phosphorus with the phosphoprotein phosphorus 

 (Table 14), the amount of radioactivity appearing in this fraction 

 accounting for over a third of the total found. At all times during 

 incorporation the specific radioactivity of the phosphoprotein is 

 many times higher than that of any other fraction (Findlay et al.y 

 1954). Although the phosphoprotein fraction incorporating 

 radioactivity in vitro appears to be largely a single entity with respect 

 to radioactive phosphate exchange, brain in vivo contains at least 

 two fractions, one soluble in isotonic sucrose solutions and the 

 other attached to particulate material. However during incubation 

 of slices in saline, half the soluble fraction passes from the slice to 

 the medium, the remainder incorporating radioactive phosphorus 

 to only a small extent compared with that in the particulate material 

 (Table 15). The phosphoprotein attached to particulate matter 

 has been found to occur in the greatest concentration in particles 

 separable from the nuclear fraction. These particles are distinct 

 from microsomes and mitochondria. Analysis of phosphoprotein 

 by means of standard techniques (see appendix) has been open to 

 error since inorganic phosphate adsorbed on tissue residues is 



Table 14. — The Distribution of Phosphorus and Radioactivity in 



Various Fractions of Cerebral Tissues after Metabolizing 



Radioactive Phosphate in vitro 



Data from Findlay et al. (1954) and Heald (1957«). 



