Activation of human creatine kinase isoenzymes by pH and various sulfhydryl and chelating agents.

We report the effect of serum pH and of the presence or absence of mercaptoethanol, N-acetyl-L-cysteine, monothioglycerol, ethylenediaminetetraacetate, and ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetate on the activation of the human creatine kinase isoenzymes. At the serum pH giving maximal enzyme stability and minimal assay lag phase (Nealon et al., Clin. Chem. 26: 1165-1169, 1980) thiol activation of CK-1 and CK-3 is nearly maximal with monothioglycerol in an optimized creatine kinase assay (Szasz et al., Clin. Chem. 22: 650-656, 1976). However, CK-2 is maximally activated at pH 8.5, a pH at which this isoenzyme is least stable on storage and its assay lag phase is prolonged. These findings suggest irreconcilable problems in the storage, activation, and assay of CK-2.