Retinol-binding protein properties

The overall metabolism of vitamin A in the body is regulated by esterases. Dietary retinyl esters are hydrolyzed enzymatically in the intestinal lumen, and free retinol enters the enterocyte, where it is re-esterified. The resulting esters are then packed into chylomicrons delivered via the lymphatic system to the liver, where they are again hydrolyzed and re-esterified for storage. Prior to mobilization from the liver, the retinyl esters are hydrolyzed, and free retinol is complexed with the retinol-binding protein for secretion from the liver [101]. Different esterases are involved in this sequence. Hydrolysis of dietary retinyl esters in the lumen is catalyzed by pancreatic sterol esterase (steryl-ester acylhydrolase, cholesterol esterase, EC 3.1.1.13) [102], A bile salt independent retinyl-palmitate esterase (EC 3.1.1.21) located in the liver cell plasma hydrolyzes retinyl esters delivered to the liver by chylomicrons. Another neutral retinyl ester hydrolase has been found in the nuclear and cytosolic fractions of liver homogenates. This enzyme is stimulated by bile salts and has properties nearly identical to those observed for... 

Sammar, M., P.J. Babin, M. Durliat, I. Meiri, I. Zchori, A. Elizur and E. Lubzens. Retinol binding protein in rainbow trout molecular properties and mRNA expression in tissues. Gen. Comp. Endocrinol. 123 51-61, 2001. 

Measurements of the levels of semm proteins such as albumin, transthyretin (also known as prealbumin), transferrin and retinol-binding protein are used as biochemical parameters in the assessment of protein energy malnutrition (Table 17-1). An ideal protein marker should have rapid turnover and present in sufficiently high concentrations in semm to be measured accurately. Transthyretin has these properties it is a sensitive indicator of protein deficiency and is effective in assessing improvement with refeeding. 

Horwitz, J. Heller, J. Properties of the ehromophoie binding site of retinol-binding protein from human plasma. J. Biol. Chem. 1974, 249 (15), 4712-4719. 

Blaner, W. S. and Goodman, D. S. 1990. Purification and properties of plasma retinol-binding protein. Methods Enzymol 189 193-206. 

The peculiar spectral properties of CRBP-bound retinol and RME are indicative of specific ligand-protein interactions (see Fig. 2) Instead, other holo-retinoid-binding proteins exhibit less characteristic spectra. For example the absorption spectrum of the complex of retinol with plasma retinol-binding protein (RBP) is characterized by a single, well-shaped peak centered at approx 328 nm On the other hand, the absorption spectra of some CRBP-bound retinoids, like CRBP-bound a l-trans retinal (12,13), do not resemble those of CRBP-bound retinol and RME. 

Vitamin A is transported m the plasma as retinol bound to a carrier protein, called retinol-binding protein (RBP), which itself forms a complex with the thyroxine-binding protein, known as transthyretin (TTR). This complex exists in equilibrium with free holo-RBP, which can then interact with a specific cell-surface receptor, thereby inducing the release of its retinol to the target cell. Thus, RBP possesses at least three molecular-recognition properties it binds retinol and it interacts with both TTR and the cell-surface receptor (for reviews, see refs. I and 2). 

The cellular metabolism of retinoids involves the interaction of several enzymes together with a number of retinoid-binding proteins44 64 (Fig. 3). While these binding proteins have been well described in mammals, little is known about their properties in fish, although orthologues of the mammalian proteins do exist29,55. Retinol can be metabolized to retinal and, in turn, be irreversibly converted to RA in tissues that... 

Two independent families have been described in the literature that have abnormally low blood RBP-ROH levels [114-117]. In one family from Japan, several members reportedly have serum RBP-ROH levels that are approximately 50% that of normal [114-116]. These diminished RBP-ROH levels did not respond to oral administration of retinol or to a protein-rich diet [114-116]. RBP isolated from one affected member of this family demonstrated no differences in its molecular weight, isoelectric point, binding to TTR or immunological properties as compared to RBP isolated from unaffected family members [114-116]. At present it is not clear whether the diminished RBP-ROH levels measured in these individuals arises from some defect in the gene for RBP or if the defect exists in another gene that has an important influence on the normal physiology of RBP-ROH (i.e. on RBP synthesis, secretion, turnover or catabolism). 

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