A cell-type specific glycoprotein antigen (SFA) from fibroblast surface appears in human plasma and serum. The amount of SFA in serum was reduced if the blood coagulation clot was removed at a low temperature. SFA could be bound to Sepharose-conjugated fibrinogen and to fibrin powder at 0 degrees C and was subsequently released when the temperature was elevated to plus 37 degrees C. This procedure resulted in a 10-fold enrichment of SFA relative to other serum proteins. SFA was found to be concentrated in the cryoprecipitate fraction of human plasma and was copurified with the cold insoluble globulin (CIG) with procedures published for the purification of the latter component. SFA/CIG is not soluble at low temperatures as such and its appearance in the cryoprecipitate fraction of plasma is likely to be due to its affinity to cryofibrinogen evident from these experiments. The biological significance of the interaction of fibroblast surface SFA moleculres with fibrin(ogen) is not known.

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