Reversal of Rous Sarcoma-Specific Immunoglobulin Phosphorylation on Tyrosine (ADP as Phosphate Acceptor) Catalyzed by the src Gene Kinase

To determine the equilibrium constant of the reaction between ATP and protein-bound tyrosine we used as catalyst the highly purified Rous sarcoma src gene transcript. J. M. Sturtevant had earlier found (personal communication) that free tyrosine O-phosphate, upon hydrolysis with alkaline phosphatase in a calorimeter (37 degrees C, pH 9), yielded a Δ H degrees of -2.8 kcal/mol (1 kcal = 4.18 kJ), less than half of that found in ATP hydrolysis. Experience with protein-bound serine phosphate (in phosvitin) had shown it to be energy rich [Rabinowitz, M. & Lipmann, F. (1960) J. Biol. Chem. 235, 1043-1050]. We wondered if the same is true for tyrosine phosphate when it is protein bound. From the equilibrium constant of 2.62 (at pH 6.5 and 5 mM Mg2+), we calculate a Δ G degrees′of -9.48 kcal/mol for hydrolysis of protein-bound tyrosine phosphate, assuming an approximate Δ G degrees′of -10 kcal/mol for hydrolysis of ATP. The experiments show that protein-bound tyrosine phosphate is energy rich, like serine phosphate in phosvitin.