Chemical Composition of Membranous Fractions of Rat Liver Microsomes

1. Rat liver microsomes were subfractionated by DOC (deoxycholate) treatment, followed by phenol extraction, gel filtration and finally ammonium sulphate precipitation, paying particular attention to removal of plasma proteins and to the behaviour of the carbohydrate components. A fraction, tentatively designated as structural protein was obtained which was practically free of lipid, RNA and plasma proteins. Structural prottin was heterogeneous and had an extremely high molecular, weight, but after reduction and carboxymethylation it gave a broad, but essentially single peak with a much lower molecular weight in gel filtration and density gradient centrifugation. 2. Immunological studies demonstrated that structural protein contained no plasma protein contaminants, except for a trace of γ-globulin, which was possibly specifically associated with microsomes. 3. structural protein was a glycoprotein with a carbohydrate content of approximately 1.5%, made up of mannose, galactose, glucose, hexosamines and sialic acid. The small, amount of inositol detected was possibly due to phosphoinositides. The molar ratio of mannose to galactose was 2.15: 1, and the hexosamines consisted of both glucosamine and galactosamine in a ratio of 12.1: 1. This composition appears to be membrane specific, being different from that of either plasma proteins or whole microsomes. The amino acid composition was determined and compared with that of other membranous proteins.