The Chemical Structure of RBP

The RBP molecule is a single polypeptide chain of about 180-185 amino acid residues containing three intramolecular disulfide bonds (as cystine residues) (Rask et al., 1981). No fatty acid or fatty acyl chains were detected in purified RBP (Kanai et al., 1968). Analyses for carbohydrate showed that RBP was free of neutral sugar and hexosamine (Peterson, 1971) and had a maximum of 0.9 mol of hexose per mol of protein (Raz et al., 1970). It is likely that RBP contains no bound lipid (other than retinol) and no carbohydrate. 

Confirmation of the proposed carboxy-terminal sequence was very recently obtained from the nucleotide sequence of a complementary DNA (cDNA) segment coding for the carboxy-terminal portion of RBP (Costanzo et al., 1983). A human liver cDNA library was constructed and a large number of individual clones were screened directly by cDNA sequence analysis. One of the 236 sequences screened coded for the region of human RBP from amino acid residue 159 to 182. 

Unlike the case for TTR (see below), detailed information is not available about the three-dimensional structure of RBP. Very recently, the crystallization and preliminary X-ray data of human plasma RBP were reported (Ottonello et al., 1983). The crystals diffracted to a resolution of 2.0 A. 

Information has been sought concerning the possible existence of sequence homologies between the RBP amino acid sequence and the amino acid sequences of other proteins of known primary structure. The initial computer searches 

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