Studies on the Transport and Cellular Distribution of Vitamin A in Normal and Vitamin A-deficient Rats with Special Reference to the Vitamin A-binding Plasma Protein

Low dietary protein increases vitamin A absorption and deposition into milk in periparturient rats

Our objective was to study the effect of differing dietary crude protein and vitamin A on retinoid metabolism in a periparturient rat model. Sixty female rats, approximately 21 d before parturition, were fed rations containing either low protein (13%; LP) or high protein (22%; HP) crude protein and either low vitamin A (3 IU/g; LA) or high vitamin A (5 IU/g; HA), yielding treatments HPHA, HPLA, LPHA, and LPLA. Samples were collected at d -14, d +3, and +10 relative to parturition and analyzed for all-trans retinoid acid (RA), 13-Cis RA, and retinol. At d -14, serum all-trans RA concentrations decreased compared to baseline. At both d +3 and d +10, serum retinol increased and liver 13-Cis RA decreased. In the small intestine, 13-cis RA was higher in HPHA than HPLA pre-partum (0.93±0.12 vs. 0.40±0.12 ng/ml, P=0.04). Post-partum, 13-cis RA was lower in high vitamin HPHA and LPHA groups (0.35±0.06 and 0.38±0.06 ng/ml) than in low vitamin A HPLA and LPLA treatments (0.50±0.06 and 1.32±0.06 ng/ml, P<0.01). In rats fed LA diets, TNF-alpha expression tended to be lower in HPLA than LPLA groups on day +3 (0.69±0.34 vs 1.00±0.52, P=0.08), but not day +10 (0.56±0.25 vs. 1.00±0.49 Fold Change, P>0.10). Retinoids accumulated during pregnancy and were mobilized during lactation. The sequestration of retinoids was increased when dietary protein content was low. Further studies are needed to investigate how retinoid metabolism could be manipulated to improve vitamin A delivery to milk.

A review of species differences in the control of, and response to, chemical-induced thyroid hormone perturbations leading to thyroid cancer

This review summarises the current state of knowledge regarding the physiology and control of production of thyroid hormones, the effects of chemicals in perturbing their synthesis and release that result in thyroid cancer. It does not consider the potential neurodevelopmental consequences of low thyroid hormones. There are a number of known molecular initiating events (MIEs) that affect thyroid hormone synthesis in mammals and many chemicals are able to activate multiple MIEs simultaneously. AOP analysis of chemical-induced thyroid cancer in rodents has defined the key events that predispose to the development of rodent cancer and many of these will operate in humans under appropriate conditions, if they were exposed to high enough concentrations of the affecting chemicals. There are conditions however that, at the very least, would indicate significant quantitative differences in the sensitivity of humans to these effects, with rodents being considerably more sensitive to thyroid effects by virtue of differences in the biology, transport and control of thyroid hormones in these species as opposed to humans where turnover is appreciably lower and where serum transport of T4/T3 is different to that operating in rodents. There is heated debate around claimed qualitative differences between the rodent and human thyroid physiology, and significant reservations, both scientific and regulatory, still exist in terms of the potential neurodevelopmental consequences of low thyroid hormone levels at critical windows of time. In contrast, the situation for the chemical induction of thyroid cancer, through effects on thyroid hormone production and release, is less ambiguous with both theoretical, and actual data, showing clear dose-related thresholds for the key events predisposing to chemically induced thyroid cancer in rodents. In addition, qualitative differences in transport, and quantitative differences in half life, catabolism and turnover of thyroid hormones, exist that would not operate under normal situations in humans.

Crystal structure of the Pseudomonas aeruginosa PA0423 protein and its functional implication in antibiotic sequestration

Pseudomonas aeruginosa is a widely found opportunistic pathogen. The emergence of multidrug-resistant strains and persistent chronic infections have increased. The protein encoded by the pa0423 gene in P. aeruginosa is proposed to be critical for pathogenesis and could be a virulence-promoting protease or a bacterial lipocalin that binds a lipid-like antibiotic for drug resistance. Although two functions of proteolysis and antibiotic resistance are mutually related to bacterial survival in the host, it is very unusual for a single-domain protein to target unrelated ligand molecules such as protein substrates and lipid-like antibiotics. To clearly address the biological role of the PA0423 protein, we performed structural and biochemical studies. We found that PA0423 adopts a single-domain β-barrel structure and belongs to the lipocalin family. The PA0423 structure houses an internal tubular cavity, which accommodates a ubiquinone-8 molecule. Furthermore, we reveal that PA0423 can directly interact with the polymyxin B antibiotic using the internal cavity, suggesting that PA0423 has a physiological function in the antibiotic resistance of P. aeruginosa.

Proteomic analysis of rat plasma by two-dimensional liquid chromatography and matrix-assisted laser desorption ionization time-of-flight mass spectrometry

The proteomic analysis of plasma and serum samples represents a formidable challenge due to the presence of a few highly abundant proteins such as albumin and immunoglobulins. Detection of low abundance protein biomarkers therefore requires either the specific depletion of high abundance proteins using immunoaffinity columns and/or optimized protein fractionation methods based on charge, size or hydrophobicity. Here we describe a two-dimensional (2D) liquid chromatography separation method for the fractionation of rat plasma. In the first dimension proteins were separated by chromatofocusing according to their isoelectric point (pI). In the second dimension, proteins were further fractionated by non-porous, reversed-phase chromatography according to their hydrophobicity. The data from both separations was displayed as a 2D protein expression map of pI versus retention time (relative hydrophobicity). Both separations were carried out on the ProteomeLab PF 2D system (Beckman Coulter), an instrument platform that provides a high degree of automation and real-time monitoring of the separation process. The reproducibility of the first-dimension separation was evaluated in terms of pH gradient formation. The second-dimension separation was evaluated in terms of peak retention times on the reversed-phase column. We found in four consecutive chromatofocusing separations that the pH gradient differed by less than 0.2 pH units at any time during the elution step. Second dimension retention times of peaks from identical pI fractions differed by less than 7 s in six consecutive separations. Each 2D separation generated a total of 540 fractions which were analyzed by matrix assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF MS). We detected approximately 275 peptides and proteins with molecular masses ranging from 3 to 225 kDa. Most fractions were found to contain multiple low and high molecular weight proteins. Differential display of 2D protein expression maps from retinol-sufficient and -deficient rat plasma samples identified a fraction with several proteins that appeared to be down-regulated in the vitamin A-deficient animal. Quantitative proteomic analysis of complex samples such as plasma is still a difficult task. We discuss the potential of this approach for biomarker discovery and address the experimental challenges that remain.

Support for the multigenic hypothesis of amyloidosis: the binding stoichiometry of retinol-binding protein, vitamin A, and thyroid hormone influences transthyretin amyloidogenicity in vitro

The amyloidoses are a large group of protein misfolding diseases. Genetic and biochemical evidence support the hypothesis that amyloid formation from wild-type or 1 of 80 sequence variants of transthyretin causes the human amyloid diseases senile systemic amyloidosis or familial amyloid polyneuropathy, respectively. The late onset and variable penetrance of these diseases has led to their designation as multigenic--implying that the expression levels and alleles of multiple gene products influence the course of pathology. Here we show that the binding stoichiometry of three interacting molecules, retinol-binding protein, vitamin A, and L-thyroxine, notably influenced transthyretin amyloidogenicity in vitro. At least 70 genes control retinol-binding protein, vitamin A, and L-thyroxine levels in plasma and have the potential to modulate the course of senile systemic amyloidosis or familial amyloid polyneuropathy.

Metabolism of a physiological amount of all-trans-retinol in the vitamin A-deficient rat

Because only retinol and not all-trans-retinoic acid (atRA) can satisfy all of the functions of vitamin A, we have investigated the retinol metabolites in tissues of vitamin A-deficient (VAD) rats responding to a radioactive dose of [20-(3)H]all-trans-retinol. As expected, atRA is the major vitamin A metabolite present in the target tissues of VAD rats given a physiological dose (1 microg) of [20-(3)H]all-trans-retinol (atROL). Both atROL and atRA were detected by high-performance liquid chromatographic (HPLC) analysis of the radioactivity extracted from the liver, kidney, small intestine, lung, spleen, bone, skin, or testis of these animals. Novel retinol metabolites were observed in the aqueous extracts from the testis, lung, and skin. However, these metabolites were detected in very small amounts and were not characterized further. Importantly, neither 9-cis-retinoic acid (9cRA), 9-cis-retinol (9cROL), nor 13-cis-retinoic acid (13cRA) was present in detectable amounts. The amounts of atRA varied in each tissue, ranging from 0.29 +/- 0.05 fmol of RA/g of tissue in the femurs to 12.9 +/- 4.3 fmol of RA/g of tissue in the kidneys. The absence of 9cRA in vivo was not due to degradation of this retinoid during the extraction procedure or HPLC analysis of the extracted radioactivity. As atROL completely fulfills all of the physiological roles of vitamin A, and 9cRA is not detected in any of the tissues analyzed, these results suggest that 9cRA may have no physiological relevance in the rat.

Retinol-induced secretion of human retinol-binding protein in yeast

Retinol-binding protein (RBP) functions as a transporter for retinol (vitamin A) in plasma in higher eukaryotes. We have successfully expressed human RBP in Saccharomyces cerevisiae, and its secretion was found to be induced by retinol also in this lower eukaryote. Reduced induction of secretion by retinol in a temperature-sensitive sec18-1 mutant that is blocked in secretion at the restricted temperature suggests that as in mammalian cells, RBP can be released from the endoplasmic reticulum upon addition of retinol. Thus, the molecular mechanism involved in retinol-dependent secretion of RBP appears to be conserved in yeast, and this points to yeast as a putative model system for studying retinol-regulated secretion of RBP. RBP purified from yeast was found to be indistinguishable from RBP purified from human plasma in several functional assays.

Transport and uptake of retinol during chicken oocyte growth

Most, if not all, components found in the yolk of a chicken egg are extracted from the plasma compartment during the rapid growth phase of the oocyte. Uptake of the major yolk constituents, very-low-density lipoprotein and vitellogenin, has been shown to be mediated by a specific receptor in the plasma membrane of the oocyte (Barber, D.L., Aebersold, R., Sanders, E.J. and Schneider, W.J. (1991) J. Biol. Chem. 266, 18761-18770). In the current study, we sought biochemical evidence for the uptake into oocytes of a minor but biologically very important component, the vitamin retinol. For transport in serum, retinol is bound to retinol-binding protein (RBP), which in turn forms a complex with transthyretin (TTR). In order to gain insight into the biochemical details of transport of the vitamin, we have identified, purified and characterized RBP, TTR, and RBP-TTR complexes from chicken serum and yolk. The results demonstrate that both serum and yolk contain the tertiary retinol-RBP-TTR complexes as well as free RBP and TTR. Western blots of yolk collected from oocytes at different stages of growth show that both RBP and TTR, but not albumin, are more abundant at early stages relative to total yolk protein. In addition, we find both RBP and TTR in endocytic clathrin-coated vesicles of the oocyte. Our results support the hypothesis that retinol, which must be imported by the oocyte for proper embryonic development, is internalized by the chicken oocyte bound to its serum protein-transport complex.

Retinol-binding protein and transthyretin expressed in HeLa cells form a complex in the endoplasmic reticulum in both the absence and the presence of retinol

To establish a suitable experimental system for studies of the interaction of retinol-binding protein (RBP) with transthyretin (TTR) we have expressed the corresponding cDNAs in HeLa cells. To investigate whether complex formation might occur already in the endoplasmic reticulum (ER), the C-terminal ER retention signal, KDEL, was attached to TTR. The tetrameric TTR-KDEL fusion protein was retained in the ER of HeLa cells. When RBP was co-expressed with TTR-KDEL, RBP was retained intracellularly. A cDNA-encoding purpurin, a protein which is 50% identical to RBP, was then expressed together with TTR-KDEL. Purpurin was not retained intracellularly and did not bind to TTR coupled to Sepharose. The effect of the vitamin A status on the secretion of TTR and RBP was examined. While TTR expressed alone was not retained intracellularly, TTR was retained in vitamin A-deficient cells when co-expressed with RBP. Addition of retinol stimulated rapid secretion of both proteins. These results demonstrate that TTR can form a complex with RBP in the ER. The data suggest that RBP and TTR are secreted as a complex.

The bovine plasma retinol-binding protein. Amino acid sequence, interaction with transthyretin, crystallization and preliminary X-ray data

1. The primary structure of bovine plasma retinol-binding protein (RBP) has been determined and found to be more than 90% identical to human and rabbit RBPs, and more than 80% identical to rat RBP. Main changes in amino acid sequence are observed in two regions on the surface of the protein molecule (residues 138-148 and 169-183). 2. The interactions of bovine RBP with bovine and human transthyretins were investigated using the technique of fluorescence polarization. Bovine RBP was able to form high affinity complexes (K'd = 0.34 +/- 0.02 microM) with both bovine and human transthyretins, displaying a stoichiometry of approximately 2 molecules RBP/molecule transthyretin in both cases. The sites that participate in protein-protein interactions are thus very similar, and this tends to exclude the involvement of the superficial regions more significantly substituted in mammalian RBPs (residues 138-151 and 167-183) in the protein-protein recognition. 3. Bovine RBP has been crystallized (space group P2(1)2(1)2(1), with a = 4.61 nm, b = 4.91 nm, c = 7.61 nm) and the crystals are suitable for high-resolution X-ray diffraction studies.

[The vitamin A requirement and the vitamin A status of growing cattle. 1. Studies of calves]

Five experiments with 18 to 36 male calves each of the black and white dairy cattle breed (age: 14-21 days, initial live weight: approximately 45 kg per animal) were carried out in order to investigate the influence of various vitamin A supply (0-80,000 IU per 100 kg LW and day) on dry matter intake and weight gain as well as the vitamin A status of liver and blood plasma over 84 days. The calves consumed a diet free of carotene and vitamin A consisting of milk replacer, concentrate and chopped wheat straw. The calves were fed in three experiments for a longer time in order to observe the further vitamin A depletion. Nine animals consumed an unsupplemented ration, nine other one got 10,000 IU vitamin A per 100 kg LW and day. Biopsies of liver and plasma samples were taken from 4 animals per group every four weeks. The various vitamin A supplementation did not significantly influence the dry matter intake (Mean: 1.67; 1.48 to 1.80 kg DM per animal and day) and the weight gain of calves (Mean: 702, 599 to 770 g per animal and day). First vitamin A deficiency symptoms (reduced feed intake, decreased weight gain, diarrhoea etc.) were observed in animals of unsupplemented group after 100 days of experiments. After 84 days the vitamin A concentration of liver of animals of unsupplemented groups decreased to 1.3-32.2% compared with the begin of experiments (60.6-155.7 mumol/kg fresh matter). Up to 51% of initial concentration were found when 10,000 IU vitamin A per 100 kg LW and day were fed. About 25,000 IU vitamin A per 100 kg LW and day were required in order to keep the initial level of vitamin A concentration of liver. The plasma vitamin A concentration is unsuitable for estimation of vitamin A status of calves. The concentration of vitamin A of liver and plasma amounted to 114 mumol per kg and 0.25 mumol per litre at the begin of experiments. The vitamin A concentration of liver of unsupplemented group decreased to 20 mumol per kg, that of plasma increased to 0.28 mumol per 1 at the end. A strong vitamin A deficiency (liver concentration: less than 10 mumol/kg) may cause a decrease of vitamin A concentration of blood.

The morphology of release of vitamin A-containing lipid droplets by hepatocytes in rat liver

Vitamin A-containing lipid droplets in the hepatocytes of rat liver were found to be exocytotically released from the cells in the form of a "lipid droplet--retinol-binding protein (RBP)--immunoreactive complex" following intraportal injection of retinol (17, 33, 67, or 100 micrograms). Evidence that the lipid droplets contain vitamin A was obtained by fluorescence microscopy of vitamin A. Intraportal injection of retinol produced varied numbers and sizes of vacuoles in the hepatocytes. The substance within the vacuoles exhibited a meshwork-like configuration in sections from slices incubated in a medium for revealing acid phosphatase activity or the corresponding control medium and was RBP-immunoreactive and proteinaceous in nature. The occurrence and number of the vacuoles depended on the dosage of injected retinol, being greatest at a dosage of 100 micrograms of retinol and becoming progressively less at dosages of 67, 33, and 17 micrograms. The vacuoles were formed by vacuolization of cisternae of the rough endoplasmic reticulum. The formation of vacuoles reached a maximum 30 min after intraportal injection of 100 micrograms retinol, and the vacuoles and lipid droplets had almost disappeared from the hepatocytes after 90 min. Little or no esterase activity was found in lipid droplets in the hepatocytes before intraportal injection of retinol, but after the injection, lipid droplets that had fused with the vacuoles become strongly positive for this enzyme activity. This suggests that hydrolysis of retinyl esters may occur in the process of complex formation in rat hepatocytes.

Retinol-binding protein is synthesized in the mammalian eye

As the chromophoric component of the visual pigment, retinol plays an essential role in vision. In the plasma, retinol is transported by retinol-binding protein (RBP) in complex with transthyretin (TTR, prealbumin). In previous work we demonstrated intraocular synthesis of TTR. To determine whether RBP is also synthesized in the eye, we performed Northern and Western blot analysis of rat eye, and detected both RBP mRNA and immunoreactive RBP. Regional Northern analysis of bovine eye localized RBP mRNA to ciliary body/iris and retina/RPE. Preliminary immunohistochemical studies revealed a widespread but heterogeneous distribution of RBP in rat eye. We postulate that ocular RBP and TTR are involved in the intraocular translocation of retinol.

[Serum vitamin A determinations and their value in determining vitamin A status]

As demonstrated in the literature on vitamin A metabolism and homeostasis of retinol in serum, the concentration of retinol in serum is regulated very exactly if the liver stores are within the physiological range (20-300 micrograms/g liver). Therefore, the serum level indicates the status of vitamin A storage only if there is an extreme depletion or overconsumption of vitamin A. At marginal depletion, however, there is damage to peripheral tissue before changes in the vitamin A level in serum occur. At the beginning of hypervitaminosis A, changes in the level of vitamin A in serum also occur later. Therefore, the determination of vitamin A in serum gives no information on the adequacy of liver reserves for judging the necessity of a substitution.

Characterization of the gene encoding ovine beta-lactoglobulin. Similarity to the genes for retinol binding protein and other secretory proteins

Beta-lactoglobulin is the major whey protein in the milk of ruminants and is expressed in the mammary gland during pregnancy and lactation. Here we describe the isolation and characterization of genomic clones encoding ovine beta-lactoglobulin. Two very similar but non-identical, types of beta-lactoglobulin clone were obtained. DNA sequence analysis of one of these showed that the gene is 4900 bases long and contains seven exons. It codes for a protein of 180 amino acid residues, containing an 18-residue signal peptide, within exons I to VI; exon VII is non-coding. We show that the genes encoding serum retinol binding protein, major urinary protein, alpha-1-acid glycoprotein and apolipoprotein D have a similar organization of exons and introns to beta-lactoglobulin. In particular, a comparison between beta-lactoglobulin and retinol binding protein shows that both genes encode equivalent elements of three-dimensional protein structure within analogous exons. These proteins are all members of a large, diverse family of secretory proteins, many of which function in binding small hydrophobic molecules.

Cellular retinyl esters and retinol among parenchymal and stellate cells in normal rat liver

[11,12-3H] Retinyl acetate (100 micrograms/20 microCi/rat) in corn oil was fed by stomach tube to normal male Wistar-Furth rats (approximately 250 g body weight). After 15 days, the contents of retinyl esters and retinol (total retinol) and their 3H-radioactivity were measured in the whole liver, crude parenchymal cells and the purified parenchymal cells, employing differential centrifugation, centrifugal elutriation and high performance liquid chromatography (HPLC) techniques. Of the total liver retinol (nmol/g liver), the crude parenchymal cells had nearly 90%, whereas the purified parenchymal cells had only 21% based on HPLC analysis. Furthermore, of the total liver retinol radioactivity (dpm/g liver) the crude parenchymal cell fraction had 85%, while the purified parenchymal cell fraction had only 16%. Based on the cell number, the crude parenchymal cell fraction was contaminated by retinoid-rich stellate cells to the extent of 4%. It, therefore, was concluded that the parenchymal cells accounted for 16-21%, whereas the stellate cells contributed 79-84% of total retinol stored in the liver under normal steady-state conditions. It also was calculated that on a per mg basis, stellate cells had 200 times more total retinol than parenchymal cells, whereas on a per cell basis each stellate cell had 74 times more total retinol than a parenchymal cell.

Study on the mechanism of interference of 3,4,3',4'-tetrachlorobiphenyl with the plasma retinol-binding proteins in rodents

The mechanism of plasma retinol reduction in rodents by 3,4,3',4'-tetrachlorobiphenyl (TCB) was investigated by radioimmunochemical analysis of the amounts of circulating and hepatic retinol-binding protein (RBP) and transthyretin (TTR) in exposed and control animals. Plasma RBP concentrations were markedly reduced in C57BL/Rij mice (50%) at 4 days, in DBA/2 mice (37-41%) at 4 and 8 days, and in Sprague-Dawley rats (58%) at 2 days after exposure to TCB. These reductions paralleled the time course of reduction of plasma retinol after exposure to TCB. Hepatic RBP concentrations were somewhat increased in TCB-treated animals, especially in the C57BL/Rij mouse and Sprague-Dawley rat. However, the release of hepatic RBP into the circulation was not blocked by TCB treatment, as analysed in vitamin A deficient rats. In addition, the amount of plasma TTR was in the normal range in TCB-treated rats. The dissociation constants of the RBP-TTR complex as analysed by polarization of fluorescence appeared to be significantly increased (from 0.5 x 10(-7) M-1 to 2.4 x 10(-7) M-1) in the presence of a TCB metabolite, isolated from plasma of TCB-treated rats. In addition, the estimated number of binding sites for RBP on the TTR molecule was reduced (from 2.8 to 1.7 sites) upon treatment of TTR with the TCB metabolite. These data support the hypothesis that plasma retinol reduction by TCB might result from a weakening of the RBP-TTR complex, in the presence of the TCB metabolite bound to the TTR.

Purification and partial characterization of thyroid hormone binding proteins in canine serum

Thyroxine-binding prealbumin (TBPA) and thyroxine-binding globulin (TBG) were isolated from canine serum and partially characterized. TBPA was isolated by retinol-binding protein (RBP) affinity chromatography and further purified by preparative agarose gel electrophoresis or FPLC ion exchange chromatography. TBG was purified by thyroxine (T4)-Sepharose chromatography followed by gel filtration on Sephacryl S-300 and preparative electrofocusing in a granulated dextran gel. Molecular weights were estimated by SDS-polyacrylamide gradient gel electrophoresis. Canine TBPA had a tetramer molecular weight of 56,000, an extinction coefficient of 12.8 cm2cg.1, an isoelectric point of 5.26-5.70 and a microheterogeneity pattern similar to that of human TBPA. Partial immunochemical identity with human TBPA was also found. Plasma concentrations of TBPA were measured by rocket immunoelectrophoresis in 43 normal and 35 hypothyroid dogs. Reference levels for TBPA ranged between 205 and 474 mg/l. Hypothyroid dogs had a mean TBPA level of 315.0 mg/l (SD: 91.1 mg/l). TBG had a molecular weight of 75,000 and an isoelectric point of 5.0. No immunochemical identity with human TBG was found. Gel filtration of serum on Sephacryl S-200, identification of T4-binding proteins with 125I-T4, and protein- and lipoprotein staining of fractions was performed. Thyroxine-binding was found to TBG in the beta-globulin region, TBPA in the alpha 2-region, albumin, and to the high density lipoprotein (HDL2) in the alpha 1-region and the very low density lipoprotein (VLDL) in the pre-beta region. A corresponding band to the latter protein in serum was masked by TBG and TBPA, and T4-binding in the alpha 1-region was not always seen in serum. Many similarities were found between man and dog regarding TBPA, but not TBG. The differences in structure of TBG may in part be responsible for the low serum T4 levels and rapid T4 metabolism seen in dogs.

A time-course investigation of vitamin A levels and drug metabolizing enzyme activities in rats following a single treatment with prototypic polychlorinated biphenyls and DDT

Xenobiotics previously characterized as selective inducers of drug-metabolizing enzymes were chosen to probe possible relationships between enzyme induction and vitamin A metabolism. Liver, kidney and serum retinol and retinyl palmitate levels were investigated in male Sprague--Dawley rats receiving a single i.p. injection of the polychlorinated biphenyls (PCBs), 2,2',5,5'-tetrachlorobiphenyl, 3,3',4,4'-tetrachlorobiphenyl or 2,2',4,4',5,5'-hexachlorobiphenyl (300 mumol/kg) or 1,1,1-trichloro-2,2-bis-(4-chlorophenyl)-ethane (DDT) (150 mumol/kg). While 2,2',5,5'-tetrachlorobiphenyl, a weak or non-inducer, and 2,2',4,4',5,5'-hexaclorobiphenyl and DDT, phenobarbital-type inducers of cytochrome P-450, led to no reduction in total vitamin A content of liver or kidney during the 7 day time-course, administration of 3,3',4,4'-tetrachlorobiphenyl, a toxic PCB and a potent 3-methylcholanthrene-type inducer of cytochrome P-450, resulted in progressively lowered liver vitamin A levels (to 40% of control values by day 7). During this time, kidney total vitamin A content increased 3-fold. The increase in kidney vitamin A (due primarily to increased retinol content) was only equal to 1/40 of total vitamin A which had disappeared from the liver. Although 3,3',4,4'-tetrachlorobiphenyl specifically induced certain drug-metabolizing enzyme activities, e.g. aryl hydrocarbon hydroxylase and UDP-glucuronosyltransferase (toward 4-nitrophenol), no highly significant correlations were found among the vitamin A levels and drug-metabolizing enzyme activities in the liver (aminopyrine N-demethylase, aryl hydrocarbon hydroxylase, aldrin epoxidase, microsomal epoxide hydrolase, UDP-glucuronosyltransferase toward 4-nitrophenol, glutathione transferase toward 1-chloro-2,4-dinitrobenzene and cytochrome P-450 content) as determined by multiple linear regression analysis.

The complete amino acid sequence of human serum retinol-binding protein

The complete amino acid sequence of human serum Retinol-binding protein (RBP) including the distribution of its three disulfide bridges, has been determined. The protein consists of 182 amino acid residues, the order of which was determined following the isolation of five CNBr-fragments. Direct amino acid sequence analysis in an automatic liquid phase sequencer provided almost the entire sequences of the five CNBr-fragments. Several sets of enzymatically derived peptides of RBP were also used to elucidate the primary structure. RBP displays significant homology to bovine beta-lactoglobulin, human alpha 1-microglobulin and rat alpha 1-microglobulin. RBP contains an internal homology. Thus, residues 36 to 83 display statistically significant homology with residues 96 to 141.

Decreased hepatic retinyl palmitate hydrolase activity in protein-deficient rats

28-day-old weanling rats were fed a diet containing 3% casein as the only source of protein for eight weeks to induce protein deficiency. When compared to control animals (fed a diet containing 25% casein), these rats had significantly lowered body (5.2-fold reduction) and liver (2.5-fold reduction) weights. The circulatory level of retinol (nmol per ml plasma) as well as retinol (nmol per g tissue) in the liver of these protein-deficient animals were also reduced significantly, although their liver concentration of retinyl palmitate (nmol per g tissue) was comparable to that of the control group. Assay of liver tissue for retinyl palmitate hydrolase activity revealed a 4-fold reduction (compared to that of control animals) of specific enzyme activity (nmol retinol formed per g protein per h). These findings suggest that severe protein deficiency results in a decreased hydrolysis of retinyl esters in the liver, which may be in part responsible for the reduced level of metabolically 'active' retinoids available for normal physiological functions.

Perisinusoidal fat-storing cells are the main vitamin A storage sites in rat liver

Highly purified sinusoidal (fat-storing, Kupffer and endothelial cells) and parenchymal cells were isolated to assess the cellular distribution of vitamin A in liver of adult vitamin A-sufficient rats. A modified simple procedure was developed for the purification of fat-storing cells from rat liver. This was achieved by a single centrifugation step in a two-layer density Nycodenz gradient. Endothelial and Kupffer cells were obtained from the same gradient and further purified by centrifugal elutriation. Reverse-phase HPLC analysis showed that fat-storing cells contained about 300-fold the amount of retinyl esters present in parenchymal cells on a mg cell protein basis. In fat-storing cells, the same retinyl esters, viz. retinyl palmitate, retinyl stearate and retinyl oleate, were present as in whole liver. It was also observed that, within 12 h after intravenous injection of chylomicron [3H]retinyl ester, most of the radioactivity had accumulated in the fat-storing cells. It is concluded that fat-storing cells are the main storage sites for vitamin A in rat liver.

Thyroid hormone binding in serum of 15 vertebrate species: isolation of thyroxine-binding globulin and prealbumin analogs

The binding of [125I]T4 to serum proteins was studied in human, monkey, cattle, sheep, goat, water buffalo, horse, swine, dog, cat, rabbit, rat, chicken, frog, and salmon. Attempts were made to isolate thyroxine-binding globulin (TBG) and thyroxine-binding prealbumin (TBPA) from serum of all species, utilizing purification methods based on the specific properties of these proteins. TBPA was found to exist in all species examined. The protein was found anodal to albumin only in human, monkey, horse, and chicken. In cattle, swine, dog, cat, rabbit, frog, and salmon, TBPA was found cathodal to albumin, while sheep, goat, water buffalo, and rat had identical mobility of albumin and TBPA. The presence of TBG was demonstrated in larger mammals. In cat, rabbit, rat, chicken, frog, and salmon, TBG could not be demonstrated. The thyroxine-binding capacity of TBPA in serum varied from 1000 to greater than 6000 nmol/l and that of TBG between 150 and 600 nmol/l. TBPA from all species except salmon showed affinity to human retinol-binding protein. The presence of TBPA in all vertebrates suggests prealbumin to be a far more important thyroxine carrier than earlier anticipated.

A simplified method for the preparation of rat thyroxine-binding prealbumin. Factors influencing its circulating level

Rat thyroxine-binding prealbumin (TBPA) was isolated in three simple steps by means of a serum precipitation by a 5% phenol solution and two consecutive semi-preparative polyacrylamide gel electrophoreses. The overall yield was 15% and the TBPA preparation contained less than 1% impurities. In addition a monospecific antiserum was raised in the rabbit. In polyacrylamide gel, rat TBPA, as with its human counterpart, migrated anodally to albumin while in agarose gel, its electrophoretic mobility was similar to that of albumin. Serum TBPA measured in adult male Wistar rats did not exhibit a circadian rhythm. However, a significant 13% decrease was observed between 9 and 15 h, followed by the restoration of the initial value by 21 h. TBPA concentration was measured in 1-, 15- and 28-day-old male and female pups as well as in adult rats. The level of this protein increased from 1 to 28 days of age and did not display any sexual difference. Yet, while TBPA concentrations in adult males were similar to those recorded in the 28-day-old pups, for adult females, they returned to the levels measured in the 1-day-old pups.

The activity of acyl CoA: retinol acyltransferase in the rat: variation with vitamin A status

Retinol esterification in the small intestine, liver and kidney of rats given a normal diet or a vitamin-A-free diet and of rats given large doses of vitamin A was studied. The active enzyme is a microsomal acyl CoA:retinol acyl transferase (ARAT). In the small intestine ARAT activity was 0.37 nmol ester/mg microsomal protein per min. Large doses of vitamin A increased the activity significantly, while the enzyme activity in the vitamin-A-deficient rats was in the range of that of the controls. Retinoic acid in physiological doses (0.064 mg three times per week) had no influence on ARAT activity. In the liver, ARAT activity of the controls was 0.58 nmol ester/mg microsomal protein per min. The activity was increased after large doses of vitamin A. It was not significantly reduced in vitamin-A-deficient animals. The kidney had a low, but significant ARAT activity, both in normal and vitamin-A-deficient animals and after large doses of vitamin A (range 0.08-0.14 nmol ester/mg microsomal protein per min). The vitamin-A-esterifying enzyme in the small intestine and liver of the rat seems to be influenced by the amount of retinol in the diet.

Newly administered [3H]retinol is transferred from hepatocytes to stellate cells in liver for storage

We have recently shown that newly administered vitamin A (retinol) is initially taken up by the parenchymal cells of the liver, and subsequently (within 1-2 h) transferred to non-parenchymal liver cells (NPC) (Blomhoff et al., ref. [10]). In the present study we have separated the NPC by different methods to determine the cell type responsible for this uptake of [3H]retinol. When liver cells were prepared between 5 and 18 h after intraduodenal administration of [3H]retinol, the radioactive retinol was recovered mainly in the stellate cells. Other liver cells (i.e., hepatocytes, endothelial cells and Kupffer cells) contained only small amounts of [3H]retinol. Further, fluorescence microscopy studies indicated that stellate cells contain large quantities of retinol. Our results show that newly administered [3H]retinol, which is initially located in the hepatocytes, is transferred to the stellate cells and stored there.

A histomorphometric study of the effect of chronic vitamin A deficiency on the circumvallate papilla of the rat

Although a deterioration of taste sensitivity is known to occur during severe vitamin A depletion, the structural changes in the taste buds themselves have not been described. Circumvallate papillae from rats deprived of vitamin A were examined histomorphometrically at intervals, and changes in the gustatory apparatus were quantitated. The cross-sectional area of the circumvallate trenches increases 30-fold because of keratin accumulation when vitamin A deficiency is extreme. After 190 days on a vitamin A-free diet, 38% of rats had no detectable taste buds and the remaining taste buds appeared atrophied. The involution of the taste buds may be the result of keratin buildup in the adjacent trench or a direct consequence of vitamin A deficiency. The atrophy of the gustatory apparatus during vitamin A deficiency and the consequent presumed loss of taste sensation may contribute to a diminished interest in food and the subsequent protein-calorie malnutrition seen in vitamin A-deficient populations.

Retinol homeostasis in lambs given low and high intakes of vitamin A

Four groups of lambs were fed on a low-carotene basal diet. One group received no supplemental vitamin A (mildly deficient). Remaining groups were supplemented daily with vitamin A acetate equivalent to 100 (control) 9000 (mildly intoxicated) and 18000 (severely intoxicated) microgram retinol/kg body-weight. After 16 weeks lambs received a bolus of [15-3H]retinol intravenously; blood, urine and faeces were sampled for 48 h. Plasma retinol was complexed to a protein of 20000 molecular weight (MW), which in turn was complexed to a protein of 65000 MW; these proteins correspond respectively to retinol-binding protein and prealbumin. Plasma retinol concentration reached plateau values in intoxicated lambs, but plasma retinyl ester concentrations increased rapidly when liver contents of both retinol and retinyl esters exceeded approximately 10 and 100 mg respectively and kidney contents of both retinol and retinyl esters exceeded 30 micrograms. Labelled compounds, more polar than retinol, were found in plasma; their concentration increased tenfold in intoxicated lambs within 48 h. Plasma retinol transport rates were 0.1, 10.5 and 11.8 times control values, and clearance rates were 0.3, 14.1 and 14.3 times control values in mildly-deficient, and mildly- or severely-intoxicated lambs respectively. Turnover of retinol increased rapidly when liver contents of retinol and retinyl esters exceeded approximately 10 and 100 mg respectively and kidney contents of both retinol and retinyl esters exceeded approximately 30 micrograms. Plasma clearance of retinyl esters was unchanged with intake. Faecal excretion of tracer increased linearly with plasma retinol clearance. Our findings identify several variables that appear to be involved in retinol homeostasis, including plasma retinol clearance and excretion.

Structure and tissue distribution of some retinoid-binding proteins

Vitamin A has, apart from its function in the visual pigments, general effects on several organs. Early signs of vitamin A deficiency include keratinization of epithelia and hyperkeratosis of the skin. To elucidate a generalized function for vitamin A, we have taken the approach of tracing the vitamin from its storage site in the liver via its blood transport by the retinol-binding protein (RBP) to its uptake by susceptible cells. We have also examined the intracellular occurrence of vitamin A as regards its binding to specific receptor proteins. Here we summarize data on the amino acid sequences of several vitamin A-binding proteins. The finding that CRBP and CRABP, the two intracellular proteins, are homologous to each other, to a myelin protein, and to a fatty acid-binding protein may shed light on the functions of these proteins. Retinoic acid, which binds to CRABP but not CRBP, induces differentiation of teratocarcinoma cells. This is accompanied by a lowering of the CRABP concentration, an increase of the CRBP level, and an increase in the uptake of retinol from RBP. The epidermis contains both CRBP and CRABP, and their distributions are rather similar. However, in contrast to CRBP, CRABP is most abundant in cells lining the hair follicles. CRBP occurs in greatest relative amounts in the outer layers of the epidermis. Since techniques have been developed to measure CRBP and CRABP, normal and disease-affected skin may now be explored as to quantity and cellular distribution of the retinoid-binding proteins.

Metabolism of prealbumin in rats and changes induced by acute inflammation

Prealbumin was purified from rat plasma by chromatography on Blue Sepharose CL-6B, followed by chromatography on concanavalin-A--Sepharose and preparative electrophoresis in polyacrylamide gel. The overall recovery was 20%. The purified prealbumin was homogeneous upon electrophoresis in detergent containing polyacrylamide gel and was used to raise a monospecific anti-serum in rabbits. During perfusion of rat liver, [14C]-leucine was incorporated into prealbumin secreted into the perfusion medium, suggesting that the liver was synthesizing prealbumin. The ratio of the rate of incorporation of leucine into prealbumin to that into total protein was 1.5%. 125I-prealbumin had a half-life of 29 h in the bloodstream of healthy Buffalo rats on a diet containing 20% protein. Whole body homogenates from healthy rats contained 6.2 mg prealbumin/100 g body weight. The rate of synthesis of prealbumin, calculated from half-life and total body pool, was 3.6 mg prealbumin X (100 g body weight)-1 X day-1. Two days after induction of inflammation by a subcutaneous injection of turpentine, both the concentration of prealbumin in the plasma and the amount of prealbumin in the whole body homogenates decreased to a minimum of about one third of normal. The relative rate of degradation of 125I-prealbumin did not change during inflammation. The rate of synthesis of prealbumin decreased considerably, or even ceased, during acute inflammation.

Retinol binding protein in normal human skin and in cutaneous disorders

Circulating vitamin A is transported by a carrier termed retinol binding protein (RBP). Autoradiography and immunofluorescence studies showed that, in normal human skin, RBP is confirmed exclusively to the dermis. Following oral vitamin A loading, a large increase in dermal RBP but only a very small rise of serum RBP was found. Serum RBP was slightly decreased in eczema patients and markedly decreased in patients with porphyria cutanea tarda. Values in other dermatoses, including psoriasis, were normal.

Retinol esterification in lamb kidney

Retinol esterification was demonstrated in vivo and in vitro in lamb kidney. Retinol labeled with hydrogen-3 injected into the renal artery of lambs appeared in the labeled retinyl ester fraction in the renal vein. Degree of tracer incorporation into retinyl esters increased with vitamin A intake. In homogenates, activity was concentrated in microsomes and enhanced fourfold by addition of adenosine 5'-triphosphate and coenzyme A to the incubation mixture. These findings suggest that the esterification is a function of fatty acyl coenzyme A:retinol acyl transferase. Renal esterification may play a role in retinol homeostasis.

Vitamin status in treated patients with cystic fibrosis

The water-soluble (B1, B2, B6, C, folic acid) and fat-soluble vitamin (A, carotene, E, and D) status of 36 patients with cystic fibrosis was assessed and compared with a control group of 21 age-matched normal children. Twenty-seven of the patients were receiving vitamin supplements (except folic acid and vitamin E) at the time of investigation. Vitamin B1, B2, and B6 status was adequate in all patients, and there was little evidence of folic acid deficiency. Vitamin C stores might not have been adequate in some of these patients, despite daily supplements with 50 mg of the vitamin. Steatorrhoea, often severe, was present in most of them. Serum carotene and vitamin E concentrations were low in over 90% of patients and were related to the severity of steatorrhoea. Vitamin A was low in over 40% of the patients despite daily vitamin supplements of 4000 IU and correlated with the serum retinol-binding protein level. Serum 25-OH cholecalciferol was low in some patients whether or not they were receiving a daily supplement of 400 IU vitamin D. In a short-term supplementation trial with water-miscible preparations of vitamin A and E in 14 patients, the serum levels of both vitamins responded well to 2 weeks of treatment with 50 mg vitamin E and 4000 IU vitamin A. Except for serum vitamin A, which was lowest in patients with the poorest clinical grading, the other vitamins were not influenced by the clinical grade of the patients.

Retinoid-binding proteins in plasma and in cells

Much has been learned during the past decade about the specific retinoid-binding proteins that exist in plasma, and in the intracellular compartment in a number of tissues. Vitamin A is mobilized from liver stores and transported in plasma in the form of the lipid alcohol retinol, bound to a specific transport protein, retinol-binding protein (RBP). A great deal is now known about the chemical structure, metabolism, and biological roles of RBP. Vitamin A mobilization from the liver is highly regulated by factors that control the rates of RBP production and secretion. Retinol deficiency specifically blocks the secretion of RBP, which can then be rapidly stimulated by intravenous retinol repletion. The cellular and molecular mechanisms that mediate these phenomena are under investigation. Delivery of retinol to peripheral tissues appears to involve specific cell surface receptors for RBP. The retinol so delivered enters the target cell, where it may become associated with the intracellular binding protein for retinol (CRBP). A number of tissues of rats, humans, and other species contain soluble proteins with binding specificity for retinol (CRBP) or for retinoic acid (CRABP). These proteins have been purified from several tissues and partly characterized. They differ in a number of ways from plasma RBP, and differ from each other in regard to binding specificity and immunoreactivity. It has been suggested that these intracellular proteins may play a direct role in the biological expression of vitamin A activity in the cell. Studies are in progress to explore this and other possibilities.