Serum retinol binding protein synthesis

Vitamin A Both vitamin A (= retinol) and A2 (= 3-dehydroreti-nol) occur in nature. Like their derivatives, they are classed under the umbrella term axerophtol. The major provitamin is p-carotin. Vitamin A is stored as a lipoglycoprotein complex in the fat-storing cells of the liver. It is released when necessary by being coupled with a retinol-binding protein (RBP) and is then transported to the cells which require vitamin A. In the case of zinc deficiency the rate of RBP synthesis is markedly increased, and as a result serum retinol concentration is reduced. Retinol deficiency can be compensated by zinc substitution. The daily requirement is approx. 1 mg. (7, 36)... 

Prealbumin is the transport protein for thyroxine and a carrier for retinol-binding protein. The body s content of prealbumin is low (10 mg/kg of body weight), and it has a very short biologic half-fife (I to 2 days). Prealbiunin may be reduced in as few as 3 days after calorie and protein intake is significantly decreased, or when hypercatabolism or severe metabolic stress (tramna or bmns) is present. Because of its short half-life, it is most useful in monitoring the shortterm, acute effects of nutrition support. As with ALB and TFN, sermn prealbumin concentrations are depressed in those with liver disease due to decreased hepatic synthesis. Increased serum prealbumin concentrations have been noted in patients with renal disease due to impaired renal excretion. 

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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