Characterization of the alkaline phosphatase expressed on the surface of a Hodgkin's lymphoma cell line

Alkaline phosphatase solubilized from a human Hodgkin's lymphoma cell line (L428) was compared with purified amphiphilic and hydrophilic forms of the enzyme from human liver, and with the enzyme solubilized from a cultured osteosarcoma cell line (Saos-2). Purified hydrophilic alkaline phosphatases from human placenta and intestine were also compared in some experiments. Alkaline phosphatase was released from the plasma membrane of intact lymphocytes by phosphatidylinositol phospholipase C and thus is anchored to the outside of the plasma membrane by covalently attached phosphatidylinositol. Enzyme released in this way was hydrophilic and that solubilized with Triton X-100 was amphiphilic, as assessed by adsorption to octyl-Sepharose. Lymphocyte alkaline phosphatase, when released from the membrane by phosphatidylinositol phospholipase C or solubilized by Triton X-100, had apparent M(r) values on gradient gel electrophoresis of 227 and 494 kDa, respectively. These values were consistently higher than equivalent ones obtained with enzymes purified from human liver, but were similar to those of cultured osteosarcoma cells. Isoenzyme-specific inhibitors of alkaline phosphatase showed similar patterns of inhibition between the enzyme from L428 cells and the tissue-nonspecific (liver/kidney/bone) isoenzyme from human liver. Heat stabilities were similar for the enzymes from L428 and Saos-2 (bone isoform) cell lines, but differed significantly from those of liver, intestine and placenta. We conclude that the alkaline phosphatase expressed in this lymphoma cell line (L428) has properties that most closely resemble those of the tissue-nonspecific isoenzyme found normally in osteoblasts of bone (bone isoform).