Transfer of cobalamin from intrinsic factor to transcobalamin II

The process is obscure by which cobalamin (Cbl) in the endocytosed intrinsi c factor (IF)-cobalamin (Cbl) complex is released and transferred to transc obalamin II (TCII) within the enterocyte. Using recombinant IF and TCII, bi nding of Cbl to IF at pH 5.0 was 70% of binding at pH 7.0, whereas for TCII alone, the value was only 12%. TCII binding activity was lost rapidly at l ower pH, but this was not due to protease action. TCII incubated at pH 5.0 with cathepsin L was degraded and could not subsequently bind Chi. Thus, tr ansfer from IF to TCII is unlikely to occur within an acid compartment. Onl y 13-15% of bound Cbl was released at pH 5.0 and pH 6.0 from either rat LF, human IF, or human TCII. The K-a of human or rat IF at pH 7.5 was 2.2 nM; for TCII, the value was 0.34 nM. At pH 7.5, Cbl transfers from IF to TCII, but only to a limited extent (21%), as detected by nondenaturing electropho resis. Transfer of Cbl from IF to TCII could not be demonstrated at pH valu es of 5.0 or 6.0. Thus, luminal transfer of Cbl between IF and TCII is like ly to be limited, but is possible. The most likely mechanism for intracellu lar transfer of Cbl from IF to TCII involves initial lysosomal proteolysis of IF, with subsequent Cbl binding to TCII in a more neutral cellular compa rtment. (C) 2001 Elsevier Science Inc. All rights reserved.