ANALYSIS OF NORMAL AND TRUNCATED HOLO-RETINOL-BINDING AND APO-RETINOL-BINDING PROTEIN (RBP) IN HUMAN SERUM - ALTERED RATIOS IN CHRONIC-RENAL-FAILURE

Retinol, the precursor of the retinoic acid hormone, is transported in the serum by a specific carrier, the retinol-binding protein (RBP). C ompared to serum of healthy controls, the serum of patients with chron ic renal failure (CRF) contains markedly increased levels of the RBP f orm truncated at the C-terminal, des((182)Leu-(183)Leu), (RBP(2)), whi ch suggests that RBP(2) is cleared by the kidney in healthy people but accumulates in serum of CRF patients (Jaconi S, et al. J Lipid Res 19 95:36:1247-53). To understand better the mechanism of retinol transpor t, we have developed a new analytical strategy to analyze the various forms of RBP that circulate in the blood: RBP with and without retinol (holo- and apo-RBP, respectively), RBP bound or not to transthyretin (TTR) and to determine in which of these forms RBP(2) circulates. We c onfirm, but now by direct measurement, that holo-RBP and, to a larger extent, apo-RBP are increased in CRF serum compared to normal serum. W e also show that almost all apo-RBP and about 50% of total holo-RBP, c orresponding to RBP excess in CRF serum, circulate free and are not co mplexed to TTR, the remaining 50% being complexed to TTR. This observa tion suggests that the high levels of free holo-RBP, not bound to TTR, which correspond to the increase in total RBPs measured in CRF serum, may alter the tissue uptake of retinol and be responsible for the sig ns of hypervitaminosis A observed in these patients. Secondly, we foun d that the truncation resulting in RBP(2) does not alter its binding p roperties for retinol nor those of holo-RBP(2) for TTR. We observed th at the high amounts of free holo-RBP(2) and holo-RBP in sera of CRF pa tients were low in normal serum, suggesting that these forms are clear ed by the kidney in normal conditions. The possible role of free holo- RBPs is discussed in the context of retinol recycling.