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Retinol binding Protein (RBP) is small (21kD) transport protein for vitamin A which forms a complex with pre-albumin in blood but loses its affinity for prealbumin once the vitamin has been deliverd to the target cells. The free RBP molecule is rapidly filtered at the glomerulus and catabolized in the renal tubules after resorption by the proximal tubular cells (like other samll molecules e.g. ß-2 Microglobulin). In kidney disease with prevailing tubular changes these proteins are not reabsorbed and appear in the urine.
As actually published by Yang et al. (Nature [2005] 436: 356-362) the plasma form of the RBP, RBP4, seems to play a key role in the development of insulin resistence. The fat cell derived peptide RBP4 which contributes to the transport of vitamin A into the circulatory system for subsequent use also modulates the glucose homeostasis and impairs the insulin sensitivity / insulin resistence. The elevation of serum RBP4 causes systemic insulin resistance, and the reduction of serum RBP4 improves insulin action.
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Retinol binding protein 4, plasma, also known as RBP4, is a human gene. Retinol binding protein 4 (RBP4) belongs to the lipocalin family and is the specific carrier for retinol (vitamin A alcohol) in the blood. It delivers retinol from the liver stores to the peripheral tissues. In plasma, the RBP-retinol complex interacts with transthyretin which prevents its loss by filtration through the kidney glomeruli. A deficiency of vitamin A blocks secretion of the binding protein posttranslationally and results in defective delivery and supply to the epidermal cells.
Retinol binding protein (RBP coded by the gene RBP4) has recently been described as an adipokine that contributes to insulin resistance in the AG4KO mouse model. It is secreted by adipocytes, and can act as a signal to other cells, when there is a decrease in plasma glucose concentration.
RBP ELISA determines the free RBP4 form as well as complex bound RBP4 (RBP4 - prealbumin complex).
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