The crystalline yolk-platelet proteins and their soluble plasma precursor in an amphibian, Xenopus laevis.

ABSTRACT

A single lipophosphoprotein complex, vitellogenin, was isolated and purified from the plasma of oestrogen-stimulated female toads by preparative ultracentrifugation and chromatography on TEAE-cellulose (triethylaminoethylcellulose). The protein contains 12% lipid, 1.5% phosphorus, 1.6% calcium and smaller amounts of carbohydrates and biliverdin. In amino acid composition it is identical with total yolk-platelet protein. The platelet protein, however, is fractionated on TEAE-cellulose into two components, a high-molecular-weight lipovitellin and a smaller phosvitin. Analyses of the soluble plasma vitellogenin suggest that it is a complex of two phosvitin molecules covalently bound to one lipovitellin dimer, and that it is the immediate precursor of the yolk proteins, into which it is converted by a molecular rearrangement. Uptake of vitellogenin from the plasma into the growing oocyte, and its subsequent crystallization as a yolk platelet, appear to be enhanced by gonadotrophic hormones.