Polypeptide Hormones, Extended

Somatomedins are unusual in that unlike most polypeptide hormones they circulate in association with specific binding proteins [51,52]. At least two such proteins are found in blood plasma, with molecular weights of approx. 40-60000 and 150000. The relationship between these has not been determined, though their partial purification has been reported. The function of these binding proteins is not fully understood, though they appear to greatly extend the half-life (tm) of somatomedins - labelled somatomedin injected into animals has a tm of only about 10 min, whereas in association with binding protein tm appears to be about 3 h. 

Avian (aPP), porcine (pPP), bovine (bPP), ovine (oPP), and human (hPP) pancreatic polypeptides have been sequenced (Kimmel et a/., 1975 Floyd et a/., 1977), and the avian hormone has been crystallized (Wood et a/., 1977). X-ray analysis of monoclinic crystals of aPP (Pitts et aL, 1979) initially provided a molecular structure to 3.0 A resolution this has now been extended to 1.4 A resolution (Blundell et a/., 1980 Tickle, 1980). The asymmetric unit comprises one molecule related about a crystallographic twofold axis to an equivalent molecule with which it forms a dimer. The dimers are linked in the crystals through coordination to zinc ions. 

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