Two forms of vitellogenin, yielding two distinct lipovitellins, play different roles during oocyte maturation and early development of barfin flounder, Verasper moseri, a marine teleost that spawns pelagic eggs

Two forms of vitellogenin (Vg), Vg A and Vg B, were identified in serum from estrogen-treated barfin flounder (Verasper moseri). Structural changes of lipovitellins (Lvs) derived from the two Vgs were examined during vitellogenesis and oocyte maturation. Two Lvs, vLv A and vLv B, were identified electrophoretically and immunologically in postvitellogenic oocytes. Each appeared to be composed of distinct heavy chains (vLvH A, M-r 107,000, and vLvH B, M-r 94,000) and light chains (vLvL A, M-r 30,000, and vLvL B, M-r 28,000) when analyzed by SDS-PAGE. Results from N-terminal amino acid sequencing and Western blotting using antisera to vLvH A and vLvH B verified that there are two Vg polypeptides in serum from estrogen-treated fish, Vg A (M-r 168,000) and Vg B (M-r 175,000), which give rise to vLvH A-vLvL A and vLvH B-vLvL B, respectively. N-terminal sequencing revealed two sequences for both phosvitin and beta-component, supporting the concept of duality for all three classes of Vg-derived yolk proteins. During oocyte maturation, native dimeric vLv B was dissociated into a native M, 170,000 monomer (oLv B). Meanwhile, vLv A was extensively cleaved including complete degradation of vLvH A into free amino acids. We propose that the quantitative ratio of vLv A to vLv B in postvitellogenic. oocytes regulates the buoyancy of the spawned pelagic eggs by controlling availability of free amino acids which function as osmotic effecters during oocyte hydration. The vLv A/vLv B ratio likely also controls the proportional availability of different types of nutrients, free amino acids versus Lv, for use during embryonic development. (C) 1999 Academic Press.