On the possible role of phosphoproteins as iron carriers

The reaction of αs1- and β-casein with ferrous ions in the presence of oxygen

“…The fate of this O 2 is unknown, but the possibility cannot be ruled out that it was occluded within the metal-protein complex possibly in co-ordination with the iron. The binding of iron by a si -and /ff-casein was extremely firm and was similar to that reported to occur with rat cytosol phosphoproteins and with phosvitin (Donella et al 1972) where it was attributed to covalent rather than electrostatic forces.…”

“…The results of the present study are consistent with the hypothesis that phosphoproteins may be involved in intermediary metabolism by acting as carriers of iron and more specifically in the translocation of iron across the mitochondrial membrane (Donella et al 1972). They do not support the role proposed by Grant & Taborsky (1966) which involved the generation of energy-rich phosphate from phosphoseryl residues by reaction with Fe 2+ and O 2 .…”

“…Relatively little consideration has been given to possible biochemical functions which may be performed by the phosphorus-containing portions of these phosphoproteins although it has been suggested that the biochemical value of phosphoproteins in general may be based on their ability to act as complexing agents for metals of physiological importance such as iron (Osterberg, 1959). This contention is supported by the findings of Mellander & Vahlqvist (1957), who reported that phosphopeptides released from casein on digestion with pancreatic enzymes were effective in promoting the intestinal absorption of Ca and iron and more recently by Donella, Pinna & Moret (1972) who have suggested that cellular phosphoproteins may transport iron across mitochondrial membranes in the form of stable complexes between iron and phosphorylated sites in the proteins. The iron complexing ability of phosvitin, the phosphoprotein of hen egg yolk, has been studied by Taborsky and his co-workers (Taborsky, 1963;Grant & Taborsky, 1966;Rosenstein & Taborsky, 1970) who have advanced the hypothesis that phosvitin may be involved directly in phosphorylating systems in the chick embryo through an initial reaction with iron.…”

Characterization of cytosol phosphopeptides

“…Although this problem is still open, at present our knowledge about the presence of iron in cytosol phosphopeptides, the small size of their molecules and the intracellular localization of the enzymes involved in their phosphorylation and dephosphorylation [13] suggest that cytosol phosphopeptides might be involved in iron translocation across the metochondrial membrane. Experiments supporting such an hypothesis have been already reported [14].…”

A comparative analysis of the evolution of the egg envelopes and the origin of the yolk

“…The most striking feature that emerges is the presence of uninterrupted P-Ser sequences of 6 residues or more terminating usually with a basic amino acid (Arg or Lys). This unusual and "specific" structure may have evolved to provide, in addition to the obvious function of amino acid and phosphorus storage, an efficient role of metal binding carrier: iron is strongly bound in the ratio of 2-3 atoms per molecule of phosvitin (Donella et al, 1972). Its presence seems to be limited to vertebrates, in that it is absent in other animal groups such as insects, crustaceans (\Vallace et al, 1967) and echinoderms ( Ozaki, 1980).…”