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

Abstract

Further studies have been made of the 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. The methods used included chromatography, sedimentation, viscosity, optical rotation, and spin labelling; the solvents used were aqueous urea, and, for some experiments, aqueous formamide. It is concluded that at neutral pH the protein is present in these solvents as an aggregate of molecular weight 36000 corresponding to a tetramer. Below about pH 4·5 solutions of the tetramer increased greatly in viscosity; furthermore, 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 a-helix. In contrast to the results of previous experiments, it now seems probable that the apovitellenin I dimer is stabilized by an interchain disulphide bond.