title: Stabilization of thermally labile alkylcobalamins by a haptocorrin from chicken serum.
abstract: Binding of neopentylcobalamin and benzylcobalamin to the apoprotein of a haptocorrin from chicken serum has been demonstrated spectrophotometrically. The spectra of the protein-bound cobalamins strongly resemble those of base-on alkylcobalamins and show that when unbound these sterically hindered alkylcobalamins are only approximately 75% (benzyl) and 40% (neopentyl) base-on, at neutral pH and at 5 degrees C. The haptocorrin was found to stabilize the spontaneous thermal decomposition of the neutral species of benzylcobalamin and neopentylcobalamin by 470-fold (3.6 kcal) and 166-fold (3.0 kcal), respectively, relative to the protein-free species. After correction of the activation parameters for the thermal decomposition of the protein-free, neutral alkylcobalamins for the relative proportions of base-on and base-off species, the haptocorrin was found to stabilize the base-on species of both alkylcobalamins by 275- to 1400-fold (approximately 3.3 to 4.3 kcal). From the temperature dependence of the decomposition reactions, the enthalpies of activation are found to be essentially identical for the protein-free and protein-bound species of either cobalamin. Thus, stabilization of the thermal decomposition of these sterically hindered alkylcobalamins by haptocorrin is entirely due to entropic factors.
authors.affiliation: Department of Chemistry, Mississippi State University, Mississippi State 39762.
authors.forename: K L
authors.initials: KL
authors.lastname: Brown
authors.forename: H B
authors.initials: HB
authors.lastname: Brooks
authors.forename: D
authors.initials: D
authors.lastname: Behnke
authors.forename: D W
authors.initials: DW
authors.lastname: Jacobsen
mesh_terms: D000818:Animals; D001059:Apoproteins / Q000378:metabolism; D002151:Calorimetry; D002645:Chickens; D004355:Drug Stability; D007700:Kinetics; D011485:Protein Binding; D013053:Spectrophotometry; D013329:Structure-Activity Relationship; D014155:Transcobalamins / Q000378:metabolism*; D014805:Vitamin B 12 / Q000737:chemistry / Q000378:metabolism*
chemical_list: D001059:Apoproteins; D014155:Transcobalamins; D014805:Vitamin B 12