Cellular retinoid-binding proteins.

A number of specific carrier proteins for members of the vitamin A family have been discovered. Two of these proteins bind all-trans-retinol and are found within cells important in vitamin A metabolism or function. These two proteins have considerable sequence homology and have been named cellular retinol-binding protein (CRBP) and cellular retinol-binding protein, type II (CRBP [II]).

Characterization of rat cellular retinol-binding protein II expressed in Escherichia coli.

Rat cellular retinol-binding protein II (CRBP II) is a small (15.6 kDa) intracellular protein that binds all-trans-retinol. In the adult rat, expression of the CRBP II gene is essentially limited to the small intestinal lining cells (enterocytes), suggesting that CRBP II may be uniquely adapted for intestinal metabolism of newly absorbed retinol.

Binding specificities of cellular retinol-binding protein and cellular retinol-binding protein, type II.

Cellular retinol-binding protein (CRBP) and cellular retinol-binding protein, type ii (CRBP(II] are cytoplasmic proteins that bind trans-retinol as an endogenous ligand. These proteins are structurally similar having greater than 50% sequence homology. Employing fluorescence, absorbance, and competition studies, the ability of pure preparations of CRBP(II) and CRBP to bind various members of the vitamin A family has been examined.

Cellular retinol-binding protein (type two) is abundant in human small intestine.

Human small intestine was found to contain a retinol-binding protein similar to the gut-specific cellular retinol-binding protein, type two [CRBP (II)], described in the rat. This newly detected human protein was immunochemically distinct from human cellular retinol binding protein previously described but immunochemically similar to rat CRBP (II). The partially purified protein bound retinol and exhibited fluorescence excitation and emission spectra distinct from those spectra for retinol bound to pure human CRBP but similar to the spectra for retinol bound to rat CRBP (II).

Comparison of the tissue-specific expression and developmental regulation of two closely linked rodent genes encoding cytosolic retinol-binding proteins.

Cellular retinol-binding protein (CRBP) and cellular retinol-binding protein II (CRBP II) are two highly homologous cytoplasmic proteins that bind all-trans-retinol. We have recently demonstrated that the mouse genes encoding CRBP and CRBP II are closely linked on chromosome 9 and that both human genes are located on chromosome 3 (Demmer, L.A.

Specificity of cellular retinol-binding protein in the transfer of retinol to nuclei and chromatin.

We have reported previously that cellular retinol-binding protein (CRBP) is able to transfer retinol to specific binding sites in nuclei and chromatin. In this report, we have examined the specificity of the interaction of the protein moiety of retinol-CRBP (R-CRBP) with chromatin and nuclei in the transfer process. We first determined the ability of apo-CRBP, apo-serum retinol-binding protein (RBP), and apo beta-lactoglobulin (BLG), all capable of retinol binding, to compete with R-CRBP in the transfer of retinol to chromatin and nuclei.

Liver levels of vitamin A and cellular retinol-binding protein for patients with biliary atresia.

We have examined whether the amount of cellular retinol-binding protein in human liver is related to the amount of vitamin A stored in the liver. Levels of vitamin A, as retinol and retinol esters, and of cellular retinol-binding protein have been determined in liver samples from 6 normal adults and 11 children with biliary atresia, with and without vitamin A treatment. The level of cellular retinol-binding protein in the liver was not related to the liver vitamin A concentration examined over a 300-fold range of vitamin A levels.

Acyl-CoA-independent esterification of retinol bound to cellular retinol-binding protein (type II) by microsomes from rat small intestine.

Cellular retinol-binding protein (type II) (CRBP(II)), a newly described retinol-binding protein, is present in the small intestinal absorptive cell at high levels. Retinol (vitamin A alcohol) presented as a complex with CRBP(II) was found here to be esterified by microsomal preparations from rat small intestinal mucosa. The esterification observed utilized an endogenous acyl donor(s) and produced retinyl esters containing linoleate, oleate, palmitate, and stearate in a proportion quite similar to that previously reported for retinyl esters in lymph and isolated chylomicrons of rat.

Cellular vitamin A-binding proteins in the testis.

Vitamin A plays an important role in the testis, being essential for the maintenance of spermatogenesis. Studies on CRBP and CRABP suggest that both retinol and retinoic acid are involved in maintaining testicular function. The cellular location of the two proteins suggests that retinoic acid may be particularly involved in the later stages of germ cell differentiation, but retinol may be the form of vitamin A that the Sertoli cell receives initially.

Quantitation of cellular retinol-binding protein in human organs.

Levels of cellular retinol-binding protein (CRBP) have been determined for a number of human tissues by a sensitive radioimmunoassay. The protein was detectable in at least one sample of every tissue examined. Samples with the highest levels were obtained from adrenal, liver, ovary, pituitary, and testis.

Rat cellular retinol-binding protein II: use of a cloned cDNA to define its primary structure, tissue-specific expression, and developmental regulation.

The primary structure of rat cellular retinol-binding protein (CRBP) II has been determined from a cloned cDNA. Alignment of this 134-amino acid, 15,580-Da polypeptide with rat CRBP revealed that 75 of 133 comparable residues are identical. Both proteins contain four tryptophan residues, which occupy identical relative positions in the two primary structures, providing a structural explanation for their similar fluorescence spectra when complexed to retinol.

Transfer of retinoic acid from its complex with cellular retinoic acid-binding protein to the nucleus.

Cellular retinoic acid-binding protein (CRABP), a potential mediator of retinoic acid action, enables retinoic acid to bind in a specific manner to nuclei and chromatin isolated from testes of control and vitamin A-deficient rats. The binding of retinoic acid was followed after complexing [3H]retinoic acid with CRABP purified from rat testes. The binding was specific, saturable, and temperature dependent.

Vitamin A status affects chromatin structure.

We have examined RNA synthesis by nuclei isolated from testes of rats of varying vitamin A status. Nuclei from retinol-deficient animals showed substantially decreased RNA synthesis by polymerase II when compared to nuclei from normal animals. Within 4 hours after oral administration of retinyl acetate (as the source of retinol) to deficient animals, RNA synthesis by polymerase II had significantly increased.

Cell-specific immunohistochemical localization of a cellular retinol-binding protein (type two) in the small intestine of rat.

One of us recently has reported the purification of a new retinol-binding protein that is distinctly different from the well-known cellular retinol-binding protein, CRBP. This protein, which we propose to name cellular retinol-binding protein type II [CRBP(II)], was found almost exclusively in the small intestine of the adult rat at levels 1000 times greater than that of CRBP. Here we have determined the cellular location of these two proteins in the small intestine of the rat.

Localization of cellular retinol-binding protein and cellular retinoic acid-binding protein in the rat testis and epididymis.

The distribution of cellular retinol-binding protein (CRBP) and cellular retinoic acid-binding protein (CRABP) in rat testis and epididymis was examined by the peroxidase-antiperoxidase immunolocalization technique. In the testis, cellular retinol-binding protein was localized exclusively in the Sertoli cells. Staining varied with the stages of the seminiferous epithelium cycle and was maximal prior to the maturation divisions.

Partial characterization of nuclear binding sites for retinol delivered by cellular retinol binding protein.

Retinol (vitamin A alcohol), which plays an important role in the differentiation of epithelia, can be transferred to chromatin in vitro. Rat liver chromatin can accept retinol in a specific and saturable manner only when the retinol is presented as a complex with cellular retinol-binding protein (CRBP). A partial characterization of the nuclear components responsible for accepting retinol is reported here.

Immunocytochemical localization of cellular retinol binding protein in the rat retina.

Cellular retinol binding protein (CRBP) was localized in the rat retina by means of light and electron microscopic immunocytochemistry. The peroxidase-antiperoxidase (PAP) method employed at the light microscopic level showed that CRBP is sharply localized to the retinal pigment epithelium (RPE). None was detectable in the epithelium of the pars plana or pars plicata of the ciliary body.

A miniature molecular-sieving column assay for cytoplasmic vitamin A-binding proteins.

Cytoplasmic vitamin A-binding proteins were measured by a method using centrifugation of gel-exclusion columns and compared to the sucrose gradient method. The gel-exclusion method analyzed 18 samples simultaneously on one table-top centrifuge, while the sucrose gradient method required use of three ultracentrifuges to process 18 samples simultaneously. Multiple 2-min low-speed centrifugations of test cytosol applied to miniature molecular-sieving columns was a faster (1/2 the working time for 18 samples), more convenient, and more accurate method for measuring cytoplasmic vitamin A-binding proteins than was the sucrose gradient method.

A novel retinol-binding protein from rat. Purification and partial characterization.

A novel retinol-binding protein, resolved during purification into two essentially identical forms, has been discovered in the rat. It was purified to apparent homogeneity, using whole neonatal rat pups as source. The protein is distinct from other known retinol-binding proteins by behavior during purification, spectra of bound retinol, and immunochemical reactivity.

Localization of cellular retinol-binding protein in several rat tissues.

The distribution of cellular retinol-binding protein (CRBP) in rat liver, ileum, and epididymis was examined by the peroxidase-antiperoxidase immunolocalization technique. Positive cytoplasmic staining was seen in the liver when antiserum prepared against purified CRBP was used but not when antiserum absorbed with purified CRBP was used. Ileal mucosa, a tissue that contains no detectable CRBP, showed no positive staining.