Deficiency of vitamin B12-binding alpha globulin in two brothers.

E NDOGENOUS VITAMIN B12 circulates bound to an alpha-i glycoprotein.”2 Added vitamin binds primarily to a globulin with beta mobility electrophoretically, which may serve mainly a temporary transport function.3’4 This beta globulin readily gives up its vitamin B12 to cells, while the alpha globulin seems to serve as a circulating storage depot.3’5’6 With the exception of some conditions involving longstanding neutropenia,7’8 deficiency of B12-binding proteins has been reported in only two instances. In one,9 various features suggested poor B12 binding by serum, but subsequently, the serum B12-binding capacity was found to be normal.’#{176} The other report11 was of transient deficiency of B12-binding beta globulin in a subject with per-