Characteristics of Phosphoproteins (Phosvitins) from a Variety of Fish Roes

Phosvitins were isolated from ling, flounder, cod, shad, and salmon roes.A general isolation procedure is described which was used, with some modifications, for all the roes analyzed.Prepurified preparations were subjected in the final stage to fractionation on diethylaminoethyl cellulose with the use of a sodium chloride gradient for elution.This yielded distinct fractions in the case of ling, cod, and flounder with maxima between 0.08 and 0.25 M NaCl and increasing phosphate contents.The ling roe fractions were the most thoroughly analyzed; symmetrical peak fractions of ling containing 3.2, 5.7, 7.2, and 9.5% alkali-labile phosphorus eluted at 0.08, 0.125, 0.185, and 0.225 M sodium chloride.Amino acid analyses of these proteins showed essentially the same serine content and general composition.Cod and flounder phosvitins yielded similar discrete peaks.Salmon phosvitin, however, in analogous chromatography, yielded only one peak, containing approximately 10% phosphorus.In general, amino acid analysis showed a composition similar to hen phosvitin, i.e. deficient in sulfurcontaining and low in aromatic amino acids, and histidine.Analysis of partial acid hydrolysates indicated a general similarity among phosvitins from various sources; on hydrolysis of ling phosvitin, strongly negatively charged fractions consisting of blocks of serine phosphate were obtained by high voltage electrophoresis.These results confirm the view that phosvitins are a class of analogously constituted phosphoproteins.The discrete phosphorylation levels of the same backbone structure found in several roe preparations seem to deserve attention.In the early thirties, one of us (F.L.) became interested in the nature of the phosphate bond in phosphoproteins (1, 2).At that time, only a few representatives of this group were known: those found in egg yolks and in milk (3, 4), earlier identified by alkali lability of the phosphate (5, 6).