Recent studies in our laboratory have revealed that Mn2+ is capable of promoting cell spreading and neurite outgrowth in PC12 cells, a process which is dependent on Mn2+ stimulation of the interaction between extracellular matrix (ECM) components and their corresponding integrin receptors. Since the major ECM proteins implicated in the Mn(2+)-induced morphogenesis, fibronectin and vitronectin, are both tyrosine sulfated, it was of interest to determine whether Mn2+ can regulate the activity of the enzyme responsible for tyrosine sulfation, tyrosylprotein sulfotransferase (TPST). Results of the present studies demonstrated that Mn2+ can suppress TPST activity in PC12 cells in both a time and concentration-dependent manner at concentrations of Mn2+ (0.1 to 1 mM) that promote morphological changes in PC12 cells. Since uptake of Mn2+ may occur via the Ca2+ channel, LaCl3, an inhibitor of Ca2+ transport, was examined and found not to prevent the suppression of TPST activity induced by Mn2+, suggesting that Mn2+ may function at an extracellular site. Suppression of TPST activity occurred with cells plated in serum-free medium on a substrata consisting of either serum or polylysine, suggesting that attachment to extracellular matrix was not an absolute requirement for regulation of activity. Consistent with this is the fact that an RGD-containing pentapeptide did not prevent suppression of TPST activity. Results from the present study demonstrated that Mn2+ is capable of promoting the suppression of TPST activity in PC12 cells at concentrations that have been shown to induce cell spreading and neurite outgrowth. However, unlike the Mn(2+)-induced morphological changes, the presence of ECM proteins is not an absolute requirement for suppression of TPST activity.