Retinol-binding protein (RBP4) TechNotes

Retinol-binding protein 4 (RBP4) belongs to a lipocalin protein family and functions as a carrier protein for vitamin A in serum. Human retinol-binding protein circulating in blood consists of 183 amino acid residues. Several truncated isoforms of RBP4 lacking 1, 2, 4 or 6 of the very C-terminal residues were also described in literature (7). In blood RBP4 carries retinol (vitamin A) which is bound to RBP4 in equimolar ratio. Besides, a major part of circulating RBP4 forms complex with prealbumin (transthyretin) and according to Jaconi et al. only a small fraction of free RBP4 can be found in serum. (7)

RBP4 has been studied since the 1960s, mainly as a transporter of retinol. However, recent data suggests that RBP4 may contribute to pathogenesis of type 2 diabetes. Yang et al. demonstrated that serum RBP4 levels are elevated in patients with obesity and type 2 diabetes. Studies in mice showed that serum RBP4 may cause insulin resistance (1). Therefore, while on the one hand there is a growing body of evidence demonstrating that RBP4 is a promising marker of the risk of type 2 diabetes, on the other hand there is a conflicting situation in the literature regarding RBP4 clinical utility in terms of predicting insulin resistance and type 2 diabetes (3). Some authors show a strict correlation between circulating RBP4 and magnitude of insulin resistance in subjects with obesity and type 2 diabetes and non-obese subjects with a family history of type 2 diabetes (2). On the contrary, others (4, 5) had not found any correlation between those variables. This confusing situation could at least partially be explained by the heterogeneity of the RBP4 in serum and by methodological shortcomings in determining level of circulating RBP4 (6). If epitope of diagnostic antibody is influenced by RBP4 truncation or by complex formation with retinol or prealbumin, then the level of RBP4 determined by the assay, utilizing such an antibody, would be different from the results of measurements by the assays with antibodies that are not susceptible to such modifications.

Hytest offers a set of mouse monoclonal antihuman RBP4 antibodies that are suitable for the development of sandwich immunoassays for the quantitative detection of circulating RBP4 in human plasma as well as for the immunodetection of RBP4 in direct ELISA, Western blotting or that can be used for the immunoprecipitation of the antigen.


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