Cation binding to yolk platelet phosvitin
In addition to non-heme iron [23], yolk phosvitin also contains Ca2+, Mg2+, Na+ and K+ [11]. Partially relaxed 23Na Fourier transform NMR spectra revealed the existence of at least two major intracellular compartments of NMR-visible Na+ [35]. A large fraction of the Rana oocyte Na+ was NMR-invisible and could be recovered in the yolk platelets [35]. During the first meiotic division there is a net increase in NMR-visible Na+; by completion of the second meiotic division (following fertilization), about 70% of the total Na+ becomes NMR-visible. Thus, phosvitin not only serves as a site for energy storage, but also as a storage site for iron and other ions essential for embryonic development in ponds and streams that contain little dissolved salts and minerals.