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Kinetic comparison of the catalytic domains of SHP‐1 and SHP‐2

The phosphatase activity of SH2‐containing protein tyrosine phosphatase (SHP) is inhibited by its SH2 domains and C‐terminal tail. In order to determine the inhibitory effects of the SH2 domains and C‐terminal tail, we have expressed and purified the catalytic domains of SHP‐1 and SHP‐2, and the SH2 domain truncated SHP‐1 and SHP‐2. We have then measured their kinetic parameters using p‐nitrophenyl phosphate (p‐NPP) and phosphotyrosine (pY) as substrates under the same experimental conditions. The results indicate that the pH‐dependent profiles of SHP‐1 and SHP‐2 are mainly determined by their catalytic domains. Both enzymes have maximum activity at pH 5.0. In addition, the phosphatase activity of different forms of SHP‐1 and SHP‐2 decreases as the salt concentration increases. Without SH2 domains, both SHP‐1 and SHP‐2 are no longer inhibited by their C‐terminal tails. However, the C‐terminal tail of SHP‐1 can further prevent the salt inhibition of the phosphatase activity. Under the same experimental conditions, the catalytic domain of SHP‐1 is two times more active than the catalytic domain of SHP‐2. J. Cell. Biochem. 72:145–150, 1999. © 1999 Wiley‐Liss, Inc.