Subunit and multimer composition of endothelial cell and platelet (Plt) multimerin. The multimerin in the culture media (CM), and cell lysates (Lys) of passage-1 endothelial cells was concentrated by immunoprecipitation and analyzed by immunoblotting with polyclonal antimultimerin or by radioimmunoprecipitation. (A) Reduced (R) 4% to 8% sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) (immunoblot) and nonreduced (NR) multimer gels (origin at top) comparing the multimerin in 12%, 120%, and 24% of the culture media, cell lysate, and Triton-insoluble pellet (Mtx) from a T25 flask (material pooled from 4 flasks, harvested on day 3 at 75% confluence) with platelet multimerin. The radioimmunoprecipitation panel (18-hour labeling; 7% SDS-PAGE) compares equivalent volumes of endothelial cell culture media and cell lysates with ¹²⁵I-labeled multimerin purified from platelets. (B) The mobility of reduced multimerin subunits from platelets and endothelial cell culture media, before (−) and after (+) deglycosylation with N-glycosidase F. Fully glycosylated proM, the predominant 155 kD (p155) subunit of mature platelet multimerin, and cell lysate proM containing endoglycosidase-H–sensitive forms of N-linked carbohydrate (*, panel A), are indicated in the panels showing reduced multimerin.