Non-Transferrin Bound Iron (NTBI) and Labile Plasma Iron (LPI)
Non-transferrin-bound iron (NTBI) is produced when the capacity of the iron-binding protein, transferrin, is exceeded. Thus, it cannot bind incoming iron from the gut or reticuloendothelial system.1 NTBI is a common feature in regularly transfused patients who receive excessive iron quantities, and results in unregulated iron uptake into tissues such as the liver, heart, pancreas, and endocrine system. Within these tissues, labile plasma iron (LPI) pools form, are redox-active, and are the main cause of iron overload toxicity.1
The LPI (or free iron) is a directly chelatable component of NTBI. LPI is highly toxic as it can catalyze the formation of harmful free hydroxyl radicals. LPI is thought to be the iron that is taken up by cells via a mechanism other than the transferrin receptor. Uncontrolled uptake of LPI leads to cell and organ damage.2
The measurement of LPI has been shown to evaluate body iron levels very accurately, although its use outside the clinical research setting is limited.1 A range techniques have been documented for use on processed serum samples.
Nitrilotriacetic Acid and High-Performance Liquid Chromatography
Nitrilotriacetic acid (NTA) is added to serum resulting in the mobilization of iron and the formation of complexes of iron (that is non-specifically bound to serum proteins and low-molecular-weight ligands) and NTA. Consequently, all scavenged NTBI is converted to Fe-NTA complexes. The solution is then ultrafiltrated and the iron in the ultrafiltrate is determined by HPLC or colorimetry.3
An alternative, and newer one-step method, is the fluorescence technique. Serum or plasma is transferred to clear-bottom plates. Next, in order to quantify labile or redox-active iron in the serum of individuals with iron overload, ascorbate is added as a redox-prompting agent that promotes labile iron in redox-cycling. The products of redox-cycling are detected as the oxidation of the fluorogenic probe (dihydrorhodamine 123 [DHR]). The chelator-sensitive component of DHR oxidation that is prompted by ascorbate provides a measure for LPI in a particular biological fluid. This approach is commercially available using feROS™ (Afferix Ltd, Israel).
Implications of LPI and body iron
Labile plasma iron (LPI) is present in patients with iron overload who have transferrin saturation levels > 85%.4 A patient is considered to be iron-overloaded when his / her LPI levels are > 0.4 μm.4
However, current use of LPI in clinical practice is limited, as the baseline levels of NTBI differ based on the underlying disease. Recent data indicate that LPI levels are elevated in patients with beta-thalassemia major and myelodysplastic syndromes (MDS), but not in patients with aplastic anemia.5-7
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- Cabantchik ZI, Breuer W, Zanninelli G, et al. LPI-labile plasma iron in iron overload. Best Pract Res Clin Haematol. 2005;18(2):277-287.
- Porter JB. Concepts and goals in the management of transfusional iron overload. Am J Hematol. 2007;82(12 Suppl):1136-1139.
- Gosriwatana I, Loreal O, Lu S, et al. Quantification of non- transferrin-bound iron in the presence of unsaturated transferrin. Anal Biochem. 1999;273(2):212-220.
- Pootrakul P, Breuer W, Sametband M, et al. Labile plasma iron (LPI) as an indicator of chelatable plasma redox activity in iron- overloaded beta-thalassemia/HbE patients treated with an oral chelator. Blood. 2004;104(5):1504-1510.
- Daar S, Pathare A, Nick H, et al. Reduction in labile plasma iron during treatment with deferasirox, a once-daily oral iron chelator, in heavily iron-overloaded patients with beta- thalassaemia. Eur J Haematol. 2009;82(6):454-457.
- Lee JW, Yoon SS, Shen ZX, et al. Iron chelation therapy with deferasirox in patients with aplastic anemia: a subgroup analysis of 116 patients from the EPIC trial. Blood. 2010;116(14):2448- 2454.
- Gattermann N, Finelli C, Porta MD, et al. Deferasirox in iron- overloaded patients with transfusion-dependent myelodysplastic syndromes: Results from the large 1-year EPIC study. Leuk Res. 2010;34(9):1143-1150.