Bovine probursin tetradecapeptide contains amino acid sequence from somatostatin, tuftsin and bursin

The B cell differentiating tripeptide bursin (lysyl-histidyl-glycyl-amide) is found in avian and mammalian bone marrow and in epithelial cells of the avian bursa of Fabricius and mammalian intrahepatic bile ducts. We now report the structure of probursin (Phe-Phe-Trp-Lys-Thr-Lys-Pro-Arg-Lys-His-Gly-Gly-Arg-Arg) isolated from bovine bone marrow and liver. Amino acids 1-5 correspond to the active site of somatostatin, 5-8 to tuftsin and 9-11 to bursin. Intact probursin has the biological activity of both somatostatin and bursin, and known enzyme cleavages could release free tuftsin, although intact probursin has low tuftsin activity. Probursin and its component peptides could regulate other bone marrow functions in addition to B cell differentiation, and, in mammals, could also regulate the function of hepatocytes and Kupffer cells after transport to the hepatic sinusoids via a local portal system involving the peribiliary capillary plexus.