Protein Kinase C (19-31), a peptide inhibitor of protein kinase C (PKC), derived from the pseudo-substrate regulatory domain of PKCa (residues 19-31) with a serine at position 25 replacing the wild-type alanine, is used as protein kinase C substrate peptide for testing the protein kinase C activity. Protein kinase C (PKC) is involved in controlling the function of other proteins through the phosphorylation of hydroxyl groups of serine and threonine amino acid residues on these proteins.