Pitney, Beard, and Van Loon (2) and Ostrowski, Skarzynski, and Zak (3) reported in 1954 that vitamin B12 is bound primarily to a-globulin in normal serum. Pitney and his colleagues (2) also noted that p-globulin can bind B12 added in vitro, but they considered this as "free B12" because, un- like B12 bound to a-globulin, Euglena gracilis is able to utilize it without prior heat treatment. Miller (4) showed that B12 added to serum in vitro binds predominantly to a2-and 8-globulin and that this fraction is nondialyzable. Hall and Finkler (5) confirmed the presence of two main B1,-binding globulins in serum, which Miller and Sullivan (6) and Weinstein, Weissman, and Wat- kin (7) had shown to be constituents of the sero- mucoid fraction.
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Retief et al. (1966) studied this question.