Binding Proteins

Several high-affinity (low Kd), soluble binding proteins seem crucial to vitamin A homeostasis and/or function. Two are widely distributed throughout many tissues and cell types, whereas others have limited expression loci (Table 1). These binding proteins occur in all vertebrates, have highly conserved amino acid sequences among orthologs, and show high specificity for distinct retinoids. Where measured, their concentrations exceed the concentrations of their ligands. Indeed, CRBP(II) accounts for of the soluble intestinal enterocyte pro teins. These qualities and experimental data indicate that retinoids exist in vivo bound to specific proteins. For example, purification of CRBP(I) from tissues produces predominantly holoprotein, despite the capacity of membranes to sequester more atROH than occurs physiologically and the time-consuming isolation techniques originally used to isolate these proteins (including tissue homo-genization, centrifugation, and several types of column chromatography). The locus of atROH at equilibrium would depend on both affinity for potential acceptors and acceptor capacity. Nature

Table 1 Examples of retinoid binding proteins

Retinoid binding protein

Ligand(s)

Kd (nM)

Adult distribution

CRBP (cellular retinol binding protein, type I)

atROH

~0.1

Nearly ubiquitous (low in intestine)

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