A study of the problems of inactivation of creatine kinase in serum.

Addition of sulfhydryl groups to the reaction mixture, to stabilize serum creatine kinase (creatine phosphokinase, CPK), results in apparent increases in the activity of this enzyme in many sera. In addition, in sera from patients just after myocard ial infarction, assays for sulfhyd ryl-activated CPK have a different clinical pattern than do those for CPK assayed without sulfhydryl activators: activities are greater and remain abnormally high longer in assays in which the enzyme is sulfhydryl activated. If the assay is done without sulfhydryl activators present, technical difficulties appear, because CPK in serum is rapidly inactivated at room temperature. CPK is apparently inactivated in at least two ways. One, which is irreversible, is inhibited by albumin; the other, which is reversible by sulfhydryl groups, seems to result from the presence of substances in the pooled serum used that are both protein bound and free.