Protein-binding of secretin in human plasma

Protein-binding of endogenous plasma secretin and of 125I-labelled secretin incubated with charcoal-treated plasma examined by gel filtration on a Sephacryl S-200 Superfine column (16 X 980 mm) showed that secretin in plasma appears both to be bound to at least two different plasma proteins where albumin appears to be the major binding protein, and also to occur as a free molecular form. In addition, protein-binding studied by incubating 125I-labelled secretin with charcoal-treated plasma under various conditions followed by charcoal separation of bound from free label indicated the presence of more specific secretin-binding sites on the plasma proteins with an avidity comparable to that otherwise reported for albumin as a binding protein. The protein-binding of 125I-labelled secretin was optimal or reached equilibrium after 2 days incubation at 20 degrees C and first after 8 days incubation at 4 degrees C. Also, the protein-binding of 125I-labelled secretin was higher at an incubation temperature of 20 than of 4 degrees C; was optimal at pH 7.4; increased with increasing amounts of charcoal-treated plasma up to an amount of 800 microliters in our assay system before levelling off; and increased in a constant and predictable manner with increasing amounts of 125I-labelled secretin at least with the amounts of labelled secretin examined here.