Which substance is used to activate CK in the assay?

In the context of activating creatine kinase (CK) in assay procedures, N-acetylcysteine is a substance that can enhance enzyme activity by maintaining a suitable environment for the enzyme's stability and function. N-acetylcysteine is known for its role as an antioxidant and a reducing agent, which can help protect the enzyme from oxidative degradation. This preservation of CK's structural integrity allows it to remain active and functional in the assay, thus ensuring accurate measurement of creatine kinase levels.

The other substances listed have different roles within biochemical assays but do not specifically activate creatine kinase. Flavin adenine dinucleotide (FAD) is involved as a coenzyme in various reactions but is not necessary for CK activation. Imidazole serves as a buffer or stabilizing agent in some enzyme reactions, but it does not activate CK. Pyridoxyl-5′-phosphate is a cofactor for certain enzymes such as amino acid decarboxylases, but it is not relevant to the activation of creatine kinase. Therefore, N-acetylcysteine is the substance that effectively supports the activation of CK in the assay context.