摘要：

strain and the GST-MKP-1 was produced and purified to apparent homogeneity as described previously. 6 Next puri- fied phosphatase was incubated with the 40 mM p-nitro- phenyl phosphate in 20 mM Tris (pH 8.0), 1 mM EDTA, and 0.2 mM DTT at 37 C. Figure 1 depicts the time course of the reaction in the absence or presence of increasing concen- tration of menadione. In the absence of menadione, a straight line was obtained from the plot of absorbance (yield of p-nitrophenolate) at 410 nm versus time. In the presence of menadione, p-nitrophenylate formation decreased with in- creasing concentration of menadione. This result indicated the menadione inactivates the GST-MKP-1. Like cdc25 phosphatase inactivation by menadione, the activity of inac- tivated enzyme with menadione did not return after dialysis, suggesting an irreversible inactivation process. 6 Moreover protection from inhibition in the presence of the competitive reversible inhibitor, arsenate, showed that menadione reacts with the active site of the enzyme ( data not shown). This experiment is consistent with the observations of Carr and co-workers, who found that vitamin K-related quinoid com- pound stimulates MAP kinase phosphorylation, which may be caused by MKP inactivation.11 Previously, in the experiment reported by others super- oxide dismutase did not antagonize the growth inhibitory effects by menadione.12 This result can be understood as a consequence of the one-electron reduction potential of menadione. Quinone redox cycling implies autoxidation of quinone reduction products. During autoxidation, two one- electron transfer steps with formation of semiquinone inter-

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