The effect of anionic and cationic surfactants on acid phosphatase denaturation has been extensively studied. Low molecular mass (LMr) protein tyrosine phosphatase (PTP), a key regulatory enzyme involved in many different processes in the cell, was distinctly affected by anionic (homologous series of n-alkyl sulfates (C8-C14)) and cationic (n-alkyl trimethylammonium bromides (C12-C16)) surfactants. At concentrations 10-fold lower critical micellar concentration (cmc) values, the enzyme was completely inactivated in the presence of anionic surfactants, in a process independent of the pH, and dependent on the chain length of the surfactants. Under the same conditions, the effect of cationic surfactants on the enzyme activity was pH-dependent and only at pH 7.0 full inactivation was observed at concentrations 10-fold higher cmc values. In contrast to cationic surfactants the effect of anionic surfactants on the enzyme activity was irreversible and was not affected by the presence of NaCl. Inorganic phosphate, a known competitive inhibitor of PTP, protected the enzyme against inactivation by the surfactants. Our results suggest that the inactivation of the LMr PTP by anionic and cationic surfactants involved both electrostatic and hydrophobic interactions, and that the interactions enzyme-surfactants probably occurred at or near the active site.265133-40