Identification of two forms of vitellogenin-derived phosvitin and elucidation of their fate and roles during oocyte maturation in the barfin flounder, Verasper moseri.

A new method for visualizing small and multiple phosvitins (Pvs) in oocytes from a marine teleost was developed by a combination of gel filtration, alkaline phosphatase treatment, and SDS-PAGE followed by silver staining. Three distinct Pv polypeptides having molecular masses of 15 kDa, 8 kDa, and 7 kDa were visualized in vitellogenic follicle extract of barfin flounder, Verasper moseri. N-terminal amino acid sequencing identified two different N-termini that fell into the PvA (7 kDa) and PvB (15 kDa and 8 kDa) groups, which were derived from two forms of vitellogenin (Vg), VgA and VgB, respectively. Analysis of time-course change in phosphorus-rich peaks of gel chromatography fractions of follicle extracts from different maturational stages demonstrated a rapid degradation of Pvs during mid-phase of oocyte maturation. Quantitative analysis of free amino acids in maturing follicles revealed an increment of serine content but not of phosphoserine, indicating the occurrence of dephosphorylation concomitant with Pv degradation. Measurement of phosphatase activity in follicles and eggs at different maturational stages demonstrated a significant activation of phosphatase especially under acidic conditions. This suggested that Pv degradation and dephosphorylation are regulated by changes in ooplasm pH during oocyte maturation. Our results also suggested that the Pvs in barfin flounder vitellogenic oocytes bind to much lower amounts of calcium and magnesium than those of masu salmon, Oncorhynchus masou. This indicates that the Pvs in the barfin flounder, a marine teleost spawning its eggs in seawater, do not play a role in the transport and deposition of calcium and magnesium into oocytes.