10] Properties of proteins that bind Vitamin B12 in subcellular fractions of rat liver

Abstract

This chapter describes the properties of proteins binding vitamin B 12 in subcellular fractions of rat liver. When a trace amount of radioactive cyanocobalamin ([ 57 Co]CN-Cbl) is administered intravascularly to rats, the vitamin binds immediately to a serum protein and is transported to the tissues in this form. This binding protein has properties analogous to those of human transcobalamin II (TCII). Studies concerning the subcellular distribution of [ 57 Co]Cbl in rat liver at various times after the administration of [ 57 Co]CN-Cbl have demonstrated that: (a) at early times (5 min) after the administration of [ 57 Co]CN-Cbl, the radioactive Cbl in liver homogenates is concentrated in the microsomal fraction in association with membrane vesicles containing 5′-nucleotidase; (b) at intermediate times (30 min to 2 h), lysosomal fractions isolated from liver homogenates contain significant amounts of the newly absorbed [ 57 Co]Cbl; and (c) at later times (24–72 h) the [ 57 Co]Cbl is localized primarily in the mitochondrial and soluble fractions of liver homogenates. The findings of this study demonstrate that [ 57 Co]Cbl associated with the binding proteins in the soluble and mitochondrial fractions is present mainly as methylcobalamin and adenosylcobalamin, the coenzyme forms of vitamin B 12 , while that associated with the binding protein (rat TC II) in the plasma membrane and lysosomal fractions is present mainly as CN-Cbl.