The vitamin D3 binding properties of the common and rare Gc variants were examined. Vitamin D3 labeled with 14C was added to serum. Gc phenotypes were demonstrated autoradiographically following separation by immunofixation electrophoresis on agarose. This qualitative analysis did not reveal differences in vitamin D3 binding by the group-specific components of the common types Gc 1-1, Gc 2-1, and Gc 2-2. The double-band variants Gc Darmstadt, Gc Y/Ab, Gc Toulouse, Gc Norway, and Gc Caucasian were examined; the phenotypes Gc Ab-Ab, Gc Ab-1, Gc Ab-2, Gc T-l, Gc T-2, Gc Norw-2, and Gc 1-Cau showed normal D3 binding. The double bands of Gc Darmstadt in the phenotype D-2 appeared somewhat weak. The singleband mutants Gc Wien, Gc Chippewa, Gc Opava, and Gc Z were analyzed; the phenotypes Gc W-l, Gc W-2, Gc Chip-l, Gc Chip-2, Gc 1-Op, Gc Op-2, Gc l-Z, and Gc 2-Z showed normal D3 binding. A mutant in the Gc system with clearly defective vitamin D3 binding properties remains to be delineated.
Recently, Daiger et al. (1975) demonstrated binding of vitamin D3 and of 25hydroxyvitamin D by the group-specific component (Gc) proteins of human serum. The binding was shown in vitro after addition of 14C-vitamin D3 to serum and subsequent electrophoresis on polyacrylamide gels or immunoelectrophoresis on agar followed by autoradiography. It was shown, furthermore, that vitamin D and 25-hydroxyvitamin D were bound by the gene products of both common alleles in the Gc system, Gc 1 and Gc 2 (Daiger et al., 1975), as well as by both electrophoretic subcomponents of the Gc 1 allele product (Daiger, 1976). Daiger et al. (1975) assumed that vitamin D binding to Gc also occurs in vivo and proposed that vitamin D transport in blood is the biological function of the group-specific component. Bouillon et al. (1976) confirmed Daiger's original observation and established the identity of the group-specific component with the transport protein of human serum for vitamin D3 and 25-hydroxyvitamin D3