Naturally occurring free phosphotyrosine in human liver

We report an endogenous tyrosine-driven phosphorylating activity in human liver extracts. The detection is achieved after selective enrichment of soluble components present in a post-mitochondrial supernatant fraction (PMS) while measuring in vitro kinase activity. A putative functional role is inferred from the competence displayed by exogenous free target amino acids when added to the reaction. We demonstrate that exogenous tyrosine is specifically phosphorylated. In view of the close association between protein phosphorylation and cell function, our observations broader the scope of interpretation for the pivotal role phosphoamino acids might have in cell metabolism.