Reminder of an earlier report.

/ 12 Fig. 1.Assay ofserum creatine kinase A denotesthe IncreasedCX activity (predominantly MM isoenzyme)of serumfrom a patient with myo-cardlal infarction; B, elevated CX activity (predom-inantlyMM isoenzyme)of serumfrom a patientwith Rey&s syn&ome, following a greatly extended Incubation period(a 10-fold Increase in the concen-trationofs*ifrsy&)l activating agentsdoesnotaffect the dizatlon of the lag for either A or B); C, assay of a mixtwe of sera A and B. Reye's syndrome apparently represents another form of pathology in which a similar inactivation occurs. The kinetic properties of the reactivation of CK from a Reye's syndrome patient are analyzed to provide an explanation for the origin of the inhibition and a possible biochemical link between the two pathologies. Creatine kinase normally has an activation lag phase of about 3 mm in the presence of the reagents used in its analytical determination. Pre-incubation for the duration of this lag phase is followed by the appearance of a linear initial velocity, allowing assay of the enzyme activity (Figure 1). The essential activating substance in the reagent mixture can be one of severalsulfhydryl agents Figure 1 shows assay of increased CK for serum from a patient with myocar-dial infarction (control, A), from an acutely ill Reye's syndrome patient (B), and an equal mixtureof the two sera (C). Several conclusions can be derived directly by inspection of Figure 1: (a) The identical reactivity of the mixed serum assay, C, with the control assay, A, indicates that no component of the Reye's syndrome patient's serum is interfering in the assay system. This experiment also precludes the presence of a free, circulating inhibitor for CK in Reye's syndrome. (b) Regardless of changes in concentration (over more than a 10-fold range) of the sulfhydryl activation reagent (RSH) addedto the Reye's syndrome patient's serum, we saw no change in the lag period of about 12 mm (Figure 1, B). For both sera, an activation reaction occurring directly, as shown in equation 1 CKinactive + RSH CKactive (1) would have a lag period varying inversely with theconcentration ofRSH. The total absence of any such dependence , over a wide range of concentrations , requires the postulation ot a pre-equilibrium binding step as in enzyme kinetics, with inactive enzymesaturated by sulfhydryl reagent: CKinactive + RSH zi CKinactive. RSH k CKactjve (2) The actual activation step, designated by k, is rate limiting. The occurrence of unique lag periods that CKinactive in the usual clinical condition (myocardial infarction) …