Studies on the Apoproteins of the Major Lipoprotein of the Yolk of Hen's Eggs II. The Dimer-Tetramer Transition of Apovitellenin I

Abstract

Physical properties of hen''s apovitellenin I, the principal low-molecular-weight protein from the high-lipid low-density lipoprotein of the yolk of hen''s eggs were studied. The methods used included chromatography, sedimentation, viscosity, optical rotation and spin labeling; the solvents used were aqueous urea, and, for some experiments, aqueous formamide. At neutral pH the protein is apparently present in these solvents as an aggregate of MW 36,000 corresponding to a tetramer. Below .apprx. pH 4.5, solutions of the tetramer increased greatly in viscosity; a covalently bound spin label increased in mobility. These changes were reversible and were apparently the result of dissociation of the tetramer to a dimer. This dissociation did not involve a change in the proportion of .alpha.-helix. The apovitellenin I dimer is probably stabilized by an interchain disulfide bond.