GENERAL METABOLISM 171 Vltro 



95 



Table 15. — Changes in the Distribution of Phosphoproteins of 



Guinea Pig Cerebral Slices and Incorporation of Radioactive 



Phosphate during Incubation in Salines Containing Glucose 



Slices of guinea pig cerebral cortex were incubated with radioactive 

 phosphorus for 30 min at 37-5 °C under oxygen in saline containing 

 glucose and buffered with Tham (Heald, 1959). After dispersion in 

 sucrose (0-25 M + 5 X 10~*M ethylenediaminetetra acetate) dispersions 

 were centrifuged at 104,000^ for 30 min. 



included as a contaminant. This has now been partly overcome by 

 the finding that radioactive phosphorylserine is an integral part 

 of the radioactive phosphoprotein molecule (Heald, 1958) thus 

 permitting a more precise identification. In this respect the 

 phosphoprotein fraction of brain is similar to the majority of other 

 phosphoproteins and it seems not unlikely that mechanisms 

 involved in its metabolism w^ill show similarities with those in- 

 volved for example in the metabolism of phosphoproteins of liver 

 (Burnett and Kennedy, 1954). The existence of a phosphoprotein 

 phosphatase active against intrinsic cerebral phosphoprotein has 

 not been clearly demonstrated, principally owing to the absence of 

 a suitable substrate. Nevertheless, brain extracts are capable of 

 splitting casein, with liberation of inorganic phosphate, at rates up 

 to 60 /xmoles P/g hr~i at pH 6-0 (Feinstein and Volk, 1949). 

 Whether such activity represents a true phosphoprotein phos- 

 phatase or is merely the action of a non-specific phosphatase is not 

 clear. 



Mechanisms involved in the synthesis and further metabolism 

 of phosphoprotein are still partly unknown, as is its role in cerebral 

 metabolism. Evidence relating to these points has been provided 



