Ovalbumin synthesis in a homologous cell-free system prepared from hen's oviduct

Abstract

Hen's oviduct polysomes are present in the precipitates prepared from the oviduct homogenates by low-speed centrifugation, in constrast other eukaryotic polysomes. The polysomes possessed synthesizing activity for ovalbumin. The amount of the released form of ovalbumin (soluble ovalbumin) synthesized in cell-free system A or B, consisting of the cell sap and the total ribosomal fraction or the polysomes, respectively, was about a half of that bound to the polysomes (nascent ovalbumin). The amount of soluble ovalbumin synthesized in cell-free system C, consisting of the pH 5 fraction and the polysomes, was only about 5% of that of nascnet ovalbumin. These results indicate that factors required to release nascent ovalbumin from polysomes are present in the pH 5 supernatant fraction. The soluble and nascent ovalbumins, which were purified by chromatography on a CM-cellulose column and by the use of antiovalbumin antiserum, respectively, seemed to be elongation products the initiations of which were supposed to occur in the oviducts before preparation of the cell-free system. The initiated chains in vitro were found to exist as nascent peptides bound to polysomes. Thus, the cell-free systems prepared in the present study lacked the ability to complete initiated peptide chains. The soluble ovalbumin synthesized in the cell-free systems was indentical with ovalbumin A1 containing two residues of phosphates, which was crystallized from hen's egg-white and was different soluble ovalbumin devoid of the prosthetic group (ovalbumin A3) prepared in the oviduct minces. This result suggests that an enzyme necessary for incorporation of the phosphate is present in the cell sap.