Retinol-binding protein half-life

Transferrin an iron-transport protein in the plasma, has a half-life of about 8 days thyroxine-binding protein, 2 days and retinol-binding protein, 0,4 days-Because of the relatively long half-life of albumin, the serum albumin level is not a particularly sensitive indicator of dietary protein status. Therefore, retinol-binding protein and thyroxine-binding protein have been used to assess malnutrition in poorly nourished populatiorrs. Note that these two proteins occur together as a complex in the bloodstream. 

Vitamin A is readily absorbed from the intestine as retinyl esters. Peak serum levels are reached 4 h after ingestion of a therapeutic dose. The vitamin is distributed to the general circulation via the lymph and thoracic ducts. Ninety percent of vitamin A is stored in the liver, from which it is mobilized as the free alcohol, retinol. Ninety-five percent is carried bound to plasma proteins, the retinol-binding protein. Vitamin A undergoes hepatic metabolism as a first-order process. Vitamin A is excreted via the feces and urine. Beta carotene is converted to retinol in the wall of the small intestine. Retinol can be converted into retinoic acid and excreted into the bile and feces. The elimination half-life is 9 h. 

Transthyretin (also known as prealbumin) Liver plasma circulating form is a tetramer composed of four identical monomers. M.W. 55,000 1-2 days N 20-40 mg/dl Mild 10-15 mg/dl Moderate 5-10 mg/dl Severe <5 mg/dl Circulates in plasma in a 1 1 complex with retinol-binding protein, transports thyroxine, has a small body pool, and has a short half-life. Sensitive indicator of protein deficiency and in the improvement with protein refeeding. 

LBM includes skeletal muscle, somatic protein, and functional proteins such as circulating proteins and the visceral proteins. Biochemically, LBM can be assessed by measuring the serum visceral proteins, albumin (ALB), transferrin (TFN), and prealbumin (thyroxine-binding prealbumin or transthyretin). Other serum proteins, such as retinol-binding protein, flbronectin (an opsonic protein), and somatomedin-C (insulin-like growth factor-1), that have a very short half-life (less than 12 to 24 hours), have been suggested as... 

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. 

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