summarized in fig2 vanadate inhibited the dephosphorylation of the 32 p labeled insulin receptor in a dose dependent manner in contrast the inhibition by pervanadate was much less than that by vanadate alone also 10 3 m h 2 o 2 alone was without effect fig1 lanes 8 and 9 since vanadate and pervanadate may have different specificities for various phosphoprotein phosphatases their inhibitory effects were separated tested on insulin receptor tyrosine dephosphorylation catalyzed by a crude preparation of phosphotyrosine phosphatase activity extracted from rat liver microsomes the samples were processed as in example 1 except that after being washed the immunoprecipitate was incubated with or without 10 4 m vanadate 10 4 m h 2 o 2 or 10 4 m pervanadate in the presence or absence of an equal volume 50 u03bcl of rat hepatocyte phosphotyrosine phosphatase as described above for 20 min at 30 u00b0 c with vigorous shaking in contrast to the above results catalase treated pervanadate powerfully inhibited tyrosine dephosphorylation fig3 lanes 5 and 6 while h 2 o 2 fig3 lanes 1 4 was without effect in this case vanadate also inhibited dephosphorylation but with less efficacy fig4 lanes 5 to 8 then pervanadate not treated with catalase fig4 lanes.