Abstract
Rat parotid glands were shown to possess protein phosphatase activity capable of catalyzing the dephosphorylation of several model phosphotase substrates, including p-nitrophenyl phosphate, tyrosine phosphorylated myelin basic protein and serine phosphorylated casein. A portion of this activity closely resembled dephosphorylation patterns of known protein tyrosine phosphatases. The reaction showed sensitivity to sodium orthovanadate, proceeded efficiently in the presence of metal chelators and favored acidic pH for optimum activity. Cell lysates from EGF-or isoproterenol-stimulated parotid glands, when immunoprecipitated with anti-Syp antibody, showed the induction of protein tyrosine phosphatase activity significantly higher than the unstimulated controls. The protein of M r = 65kDa also had elevated levels of tyrosine phosphorylation following isolation from cells treated to undergo proliferation. Thus parotid gland acinar cells possess protein tyrosine phosphatase activity of the PTPase 1D class associated with inducible cell growth, in addition to other phosphatases.
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