Xenopus fibrinogen. Characterization of subunits and hormonal regulation of biosynthesis

In this paper we describe the purification and characterization of Xenopus plasma fibrinogen and the hormonal factors which regulate synthesis and secretion of fibrinogen in liver parenchymal cells in primary culture. As in other vertebrate species, Xenopus fibrinogen is composed of three nonidentical polypeptide chains, A alpha, B beta, and gamma. In contrast to mammalian fibrinogens, the B beta chain of Xenopus fibrinogen has a higher apparent molecular weight than the A alpha chain. The gamma chain has the lowest molecular weight in the frog protein, as in that of other species. The relatively large size of the frog B beta chain results from the unusually large size of the NH2-terminal B fibrinopeptide, which is released by thrombin cleavage of fibrinogen. Hormonal regulation of fibrinogen biosynthesis was examined using a primary cell culture system. Purified Xenopus liver parenchymal cells, maintained for several weeks in a defined culture medium, gradually decrease the synthesis and secretion of fibrinogen. Sustained production of this protein is dependent upon the addition of a glucocorticoid, dexamethasone, to the culture medium. Fibrinogen production is suppressed if an estrogen, estradiol-17 beta, is added to the culture medium together with dexamethasone and triiodothyronine. The Xenopus system provides new insight into the structure of fibrinogen, the evolution of this protein, and the hormonal factors which regulate its synthesis.