In vivo phosphorylation following [³²P]orthophosphate injection into neostriatum or hippocampus: Selective and rapid labeling of electrophoretically separated brain proteins

Intracranial injections of [³²P]orthophosphate readily label a number of brain phosphoproteins as resolved by polyacrylamide gel electrophoresis. The majority of these in vivo labeled phosphoproteins co-migrate with phosphoproteins that are labeled in vitro by incubation of brain membranes with [³²P]ATP. Two of the major in vitro labeled phosphoproteins with apparent molecular weights of 47,000 (band F₁) and 41,000 (band F₂) are rapidly labeled in vivo. Since they are rapidly dephosphorylated in vitro, this suggests a high rate of phosphate turnover. The electrophoretic pattern of in vivo labeled phosphoproteins did not appear to be altered by the method of sacrifice (focused microwave irradiation, decapitation or liquid nitrogen immersion) or by the state of the animal at the time of labeling (awake or lightly anesthetized with pentobarbital). The reduction of phosphatase activity during tissue processing at 0 °C may account for the similarities observed with different sacrifice methods. Removal of phospholipids or polynucleotides had little effect on the in vivo labeled ³²P-containing bands. However, alkaline hydrolysis or protease treatment uniformly reduced the radioactivity in the labeled bands. These findings suggest that the ³²P-containing bands consist of phosphoester linkages to serine or threonine residues. The present evidence emphasizes that previously characterized in vitro labeled brain phosphoproteins are, in fact, labeled in the awake, freely-moving animal.