The plasma membrane from the human
tumor astrocytoma contains an active
acid phosphatase activity based on hydrolysis of
p-nitrophenyl phosphate. Other
acid phosphatase substrates--
beta-glycerophosphate, O-
phosphorylcholine, and 5'-AMP--are not hydrolyzed significantly. The
phosphatase activity is
tartrate insensitive and is stimulated by
Triton X-100 and
EDTA. Of the three known
phosphoamino acids, only free O-
phosphotyrosine is hydrolyzed by the membrane
phosphatase activity. Other
acid phosphatases tested from potato, wheat germ, milk, and bovine prostate did not show this degree of specificity. The plasma membrane activity also dephosphorylated
phosphotyrosine histone at a much greater rate than did the other
acid phosphatases. pH profiles for free O-
phosphotyrosine and
phosphotyrosine histone showed a shift toward physiological pH, indicating possible physiological significance.
Phosphotyrosine histone dephosphorylation activity was nearly 10 times greater than that seen for
phosphoserine histone dephosphorylation, and Km values were much lower for
phosphotyrosine histone dephosphorylation (0.5 microM vs. 10 microM).
Fluoride and
zinc significantly inhibited
phosphoserine histone dephosphorylation.
Vanadate, on the other hand, was a potent inhibitor of
phosphotyrosine histone dephosphorylation (50% inhibition at 0.5 microM) but not of
phosphoserine histone.
ATP stimulated
phosphotyrosine histone dephosphorylation (160-250%) but inhibited
phosphoserine histone dephosphorylation (95%). These results suggest the existence of a highly specific
phosphotyrosine protein phosphatase activity associated with the plasma membrane of human
astrocytoma.