Circular dichroic (CD) spectra were obtained on solutions of human plasma retinol-binding protein and prealbumin, the two proteins involved in the transport of vitamin A in plasma. CD spectral studies were also carried out on the following solutions: (1) apo-retinol-binding protein (retinol-binding protein not containing bound retinol); (2) a 1:1 molar mixture of prealbumin and l-thyroxine, and (3) two mixtures of prealbumin and retinol-binding protein, one of which consisted almost entirely of the prealbumin-retinol-binding protein complex. The spectra were subjected to computer analysis and yielded information about the secondary structures of both proteins. Retinol-binding protein appears to have a relatively high content of unordered structure, a significant but small complement of β-structure, and little or no α-helix. Prealbumin appears to have a higher content of β-conformation (one-third to one-half the overall structure) and a lower content of unordered structure than does retinol-binding protein; prealbumin also appears to have little or no α-helical structure. It is likely that the structures of both proteins are highly organized and specific, but mainly lack regular repeating characteristics. The CD spectrum of apo-retinol-binding protein was very similar to that of holo-retinol-binding protein from 208 to 240 nm, suggesting that disruption of the ligand-protein complex did not lead to extensive alteration of the secondary structure. Small differences in fine structure of the spectra retinol-binding protein and apo-retinol-binding protein between 240 and 300 nm cannot at present be interpreted in terms of specific chromophores or conformational significance. There was no evidence from the CD spectra to indicate that the interaction of prealbumin with l-thyroxine resulted in a significant change in the secondary structure of the protein. The spectra of mixtures of prealbumin and retinol-binding protein were additive, indicating that formation of the retinol-binding protein prealbumin complex resulted in very little if any alteration in the secondary structure of the two proteins. It is, of course, possible that small conformational changes, not detected by the methods used, occurred as a result of the interactions of the molecules studied.
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