Soluble syndecan-1 and shed syndecan-1 are identical in size and have similar affinities for type I collagen.

(A) Western blot. Purified syndecan-1 isolated from the media conditioned by cells expressing soluble syndecan-1 (clone sSyn-1.10, lane 1) and from Syn-1 cells that shed syndecan-1 from their cell surface (lane 2) was run on 4% to 12% sodium dodecyl sulfate–polyacrylamide gel electrophoresis, transferred to a cationic nylon filter, and probed with ¹²⁵I-281.2 monoclonal antibody specific for syndecan-1. The 2 samples exhibit broad smears that are identical in size. (B) ACE analysis. Purified³⁵S-labeled syndecan-1 isolated from the media conditioned by cells expressing soluble syndecan-1 (upper panel) and from cells shedding syndecan-1 from their cell surface (lower panel) was electrophoresed through varying concentrations of type I collagen and visualized by phosphorimaging. The K_d for both samples falls within the range of 9 to 36 nM.