Abstract
Macrophages secrete a large number of proteases, implying in vivo exposure of the cell surface to proteolytic conditions. Mild trypsin treatment of 125I-labeled guinea pig peritoneal macrophages preferentially cleaves one surface component of apparent 160,000 mol wt. Similar trypsin treatment of macrophages with 3H-labeled carbohydrate surface moieties also cleaves a single 3H-labeled 160,000 mol wt glycoprotein, referred to as gp160. Nonreducing sodium dodecyl sulfate (SDS)-electrophoresis established that gp160 of trypsinized cells remains assembled in the membrane as a multichain disulfide-bonded molecule. gp160 was purified by detergent extraction, L. culinaris lectin affinity chromatography and DEAE-cellulose chromatography. The corresponding molecule from trypsinized cells was purified by the same procedure. Reducing SDS-electrophoresis of purified trypsinized 125I-labeled gp160 revealed two proteolytic fragments with apparent molecular weights of 85,000 and 71,000. Thus, mild trypsin treatment of macrophages preferentially cleaves a single surface protein, possibly at a single site. Because the two fragments of gp160 are accessible to lactoperoxidase and trypsin, both must be exposed on the membrane surface. The reactive carbohydrate site was found on the 85,000 mol wt fragment, which alone contains the 3H-label introduced into intact cells by neuraminidase, galactose, oxidase, and [3H]KBH4.