Marine corrinoid-binding proteins for the direct determination of vitamin B-12 by radioassay

Under conditions of vitamin B-12 deficiency the marine phytoplankton Thalas siosira pseudonana secretes into the growth medium a protein (Gm protein) t hat binds B-12 specifically with an affinity constant of 2 x 10(11) M-1. Wh en Gm protein was used as a B-12-specific binder in radioassays for the dir ect determination of the vitamin, the detection limit of the technique appr oached 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 stru cture. The amino acid and submit composition of the protein is reported. (C ) 2001 Academic Press.