The concentrations of some constituents of egg yolk in its soluble phase.

Although the particulate fraction of egg yolk (yolk granules) has attracted the attention of investigators since Miescher (1) and Hoppe-Seyler (2)) reliable information concerning the distribution of the yolk constituents between its soluble phase and the granules is lacking. Separation of both phases is difficult, owing to the high viscosity of egg yolk. For this reason, all information concerning the chemical nature of yolk proteins and yolk lipides is based on isolation procedures which involve chemical fractionation of the total yolk, an unusually complex and labile colloidal system. It is possible that proteins isolated in this way from egg yolk are artifacts consisting of mixtures of components derived from both soluble and particulate phases. This possibility should be borne in mind, for example, in evaluating the observation that a large fraction of the yolk phospholipides appears to exist in the form of complexes with phosphoproteins. Information regarding the distribution of yolk constituents may also be of interest in the study of the metabolism of the developing embryo. We have observed that sedimentation of the yolk granules occurs during a 12 hour centrifugation of undiluted egg yolk at approximately 20,000 X g. It was of interest to study the distribution of some yolk constituents between the soluble and insoluble phases with this technique because it offered the only possibility of avoiding artificial alterations of the yolk fractions. The results of these analyses will be reported in the experimental part of this communication.