Marine corrinoid-binding proteins for the direct determination of vitamin B12 by radioassay

Under conditions of vitamin B-12 deficiency the marine phytoplankton Thalassiosira pseudonana secretes into the growth medium a protein (Gm protein) that binds B-12 specifically with an affinity constant of 2 x 10(11) M-1. When Gm protein was used as a B-12-specific binder in radioassays for the direct determination of the vitamin, the detection limit of the technique approached 5 pg B-12/mL. in its natural state, Gm protein is oligomeric having molecular weight of over 400 kDa. Modification of the solution environment of this protein produces unique changes in its tertiary and quaternary structure. The amino acid and submit composition of the protein is reported. (C) 2001 Academic Press.