The activity of membrane-bound alkaline phosphatase (ALP) expressed on the external surface of cultured murine P19 teratocarcinoma and human HL-60 myeloblastic leukemia cells was studied at physiological pH using p-nitrophenylphosphate (pNPP) as substrate. The rate of substrate hydrolysis catalyzed by intact viable cells remained constant for eight successive incubations of 30 min and was optimal at micromolar substrate concentrations over the pH range 7.4-8.5. The value of apparent K(m) for pNPP in P19 and HL-60 cells was 120 microM. Hydrolytic activity of the ecto-enzyme at physiological pH decreased by the addition of levamisole, a specific and noncompetitive inhibitor of ALP (K(i) P19 = 57 microM; K(i) HL-60 = 50 microM). Inhibition of hydrolysis was reversed by removal of levamisole within 30 min. Retinoic acid (RA), which promotes the differentiation of P19 and HL-60 cells, induced levamisole-sensitive ecto-phosphohydrolase activity at pH 7.4. After its autophosphorylation by ecto-kinase activity, a 98-kDa membrane protein in P19 cells was found to be sensitive to ecto-ALP, and protein dephosphorylation increased after incubation of cells with RA for 24 h and 48 h. Orthovanadate, an inhibitor of all phosphatase activities, blocked the levamisole-sensitive dephosphorylation of the membrane phosphoproteins, while (R)-(-)-epinephrine reversed the effect by complexation of the inhibitor. The results demonstrate that the levamisole-sensitive phosphohydrolase activity on the cell surface is consistent with ecto-ALP activity degrading both physiological concentrations of exogenously added substrate and endogenous surface phosphoproteins under physiological pH conditions. The dephosphorylating properties of ecto-ALP are induced by RA, suggesting a specific function in differentiating P19 teratocarcinoma and HL-60 myeloblastic leukemia cells.

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Pre-stack wavefield propagators based on generalized phase-shift operators produce accurate depth images of complex geological structures. The standard imaging condition for pre-stack Double-Square-Root (DSR) migration extracts the zero time wavefield at zero offset, resulting in a single depth image. In least-squares (LS) Kirchhoff migration a smoothing constraint along the offset direction in the common reflection point (CRP) domain has been proposed to mitigate artifacts resulting from incompletely and/or coarsely sampled data. In order to make this regularization approach available to least-squares phase-shift migration techniques the common angle imaging condition (CAI) is employed. This imaging condition extracts the zero time wavefield at a set of constant offset ray parameters. This allows for an efficient computation of CAI gathers and the introduction of a ray parameter dependent smoothing in LS migration. The wavefield propagators for modeling and migration used in the LS inversion are based on the split-step approximation of the Square-Root operator. They can be improved by utilizing a multiple reference velocity approach.

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