Electron microscope study on human ceruloplasmin.

Abstract

Electron microscopy of human ceruloplasmin (CP) molecules revealed a few distinctive types of particle images. Analysis of these images allows to propose a tentative model for CP: six "subunits" (which we call domains) not much different in size are arranged with 32 point group pseudosymmetry. The determination of the number of polypeptides arising at the spontaneous specific proteolytic fragmentation of CP and their molecular weights conform with this assumption. The electrophoretic studies of the CP samples prepared both with and without potent proteolytic inhibitor, PMSF, revealed that CP is a single-chain protein with molecular weight of 130 000. Isolated and stored without PMSF the polypeptide chain of CP undergoes specific proteolytic cleavage which results in the appearance of polypeptides with molecular weights of 16 000, 48 000, and 64 000. The latter two polypeptides degradate to about two- and three-fold decreased molecular weights fragments, respectively. Therefore, the single polypeptide chain of CP contains at least five peptide bonds which are particularly susceptible to proteolytic attack and which connect six principal segments of the chain. The hydrolysis of these bonds results in liberation of the six fragments which were integrated in the enzymatically active globule of CP.