Taurine Inhibits Protein Kinase C‐Catalyzed Phosphorylation of Specific Proteins in a Rat Cortical P2 Fraction

Abstract

We previously reported that taurine inhibits the phosphorylation of specific proteins in a P₂ synaptosomal fraction prepared from the rat cortex. In the present study, the regulation of the phosphorylation of an ∼20K M_r protein whose phosphorylation is inhibited by taurine was further investigated. The phosphorylation of the ∼20K M_r protein in a hypo‐osmotically shocked P₂ fraction from rat cortex was dependent on the free Ca²⁺ in the reaction medium. Depolarization induced by 30 mM K⁺ stimulated the phosphorylation of the ∼20K M_r protein in an intact synaptosomal P₂ preparation by 30‐fold. This stimulation was inhibited 35% by taurine, whereas guanidinoethanesulfonic acid, a taurine analogue, did not have any effect, thereby indicating the specificity of taurine. Addition of phorbol 12‐myristate 13‐acetate, a phorbol ester, together with phosphatidylserine, stimulated the phosphorylation of the ∼20K M_r protein in the hypo‐osmotically shocked P₂ synaptosomal fraction by fivefold, whereas cyclic AMP, cyclic GMP, and calmodulin did not have any effect on the phosphorylation of this particular protein. Phorbol 12‐myristate 13‐acetate–stimulated phosphorylation of the ∼20K M_r protein is blocked 30% by taurine. Taurine also inhibited phorbol 12‐myristate 13‐acetate‐activated phosphorylation of two other proteins that were similar in molecular weight and isoelectric point to the ∼20K M_r protein on two‐dimensional gels. These results suggest that taurine modulates the phosphorylation of specific proteins regulated by the signal transduction system in the brain. Thus, taurine may modulate neuroactivity by inhibiting the phosphorylation of specific proteins involved in regulatory function.