Rat serum albumin is not equal to human serum albumin

9 05 2011

To the Editor:

We read with great interest the recently published article by Kutlu et al. (1). In the article, the authors showed that rats that had been operated to produce testicular torsion had a higher level of ischemia-modified albumin 4 hours after the onset of the disorder. The authors highlighted that although the blood and tissue MDA and MPO levels showed no difference between groups, the ischemia-modified albumin levels showed significant a difference in 4-hour torsion group compared with other groups. The results derived from the animal study showed a high level of ischemia-modified albumin in testicular torsion (TT), indicating a potential value for TT diagnosis, and the value of ischemia-modified albumin level in TT should be investigated with respect to prognosis.

However, we would point out the chemical specificity of ischemia-modified albumin. Ischemia-modified albumin was first reported by Bar-Or et al., who found the new protein, with an altered metal-binding site of human serum albumin, after the onset of cardiac ischemia (2). Human serum albumin is a peptide consisting of 585 amino acids with a unique amino acid sequence at its amino terminus-N-terminus, which serves as the primary binding site for the transitional metals cobalt, copper, and nickel ions. Bar-Or also emphasized that the unique structure of serum albumin was found only in humans, and was not in other species. Rat serum albumin has 608 amino acids, including the signal and pro-peptide sequences at the N-terminus, is of a different size than of its homolog-human. Comparison of the amino acid composition of rat serum albumin with that of human albumin shows rat albumin to contain relatively more tyrosine and relatively less lysine, cystine, and leucine (3).

Based on previous research, Bar-Or et al. postulated that significant molecular changes to or loss of portions of the N-terminus of HSA could reduce the in vivo transitional metal binding capacity of HSA either during myocardial ischemia or during reperfusion immediately after the ischemia event (2). The following research was also about myocardial ischemia, especially in patients following coronary artery bypass operation and percutaneous coronary intervention surgery. However, there was no animal study on ischemia-modified albumin of myocardial ischemia. Specific portions that are highly conserved often correspond to important sites, such as metal ion binding sites, in the final protein or are important to the final configuration of the protein. The human metal ion binding site is not conserved, because it is not found in any other species. So we do not think an animal model is suitable for monitoring the ischemia-modified albumin level changes in any disease before examining the binding capability of rat serum albumin for transitional metal ions, as well as identifying the binding site itself.

Additional research shows that ischemia-modified albumin is affected in skeletal muscle ischemia, mesenteric ischemia, intermittent claudication, stroke, end-stage renal disease, metabolic syndrome, preeclampsia and other vascular ischemia diseases. Summarizing the results, we can conclude that ischemia-modified albumin is sensitive to ischemia, but its lack of specificity for organs or diseases limits its diagnostic value. Therefore we question the clinical applicability of using ischemia-modified albumin to diagnose TT.

Na Zheng, Ph.D.a
Shaohua Zhu, Ph.D.b
Liang Liu, Ph.D.b
Xiaojun Yu, Ph.D.a
aForensic Science Department
Shantou University Medicine College
Shantou, China
bFaculty of Forensic Science
Medicine College of Huazhong Science and Technology University
Wuhan, China

References
1. Kutlu O, Mentese A, Turkmen S, et al. Investigation of the possibility of using ischemia-modified albumin in testicular torsion: an experimental study. Fertil Steril 2011; 95(4):1333-7.

2. Bar-Or D, Lau E, Winkler JV. A novel assay for cobalt-albumin binding and its potential as a marker for myocardial ischemia-a preliminary report. J Emerg Med 2000;19(4):311-5.

3. Peters T Jr. The biosynthesis of rat serum albumin.Ⅲ.amino acid composition of rat albumin. J Biol Chem 1962;237(7):2182-3.

Published online in Fertility and Sterility doi:10.1016/j.fertnstert.2011.05.021

The Authors Respond:

First of all, we would like to thank Zheng et al. for their interest (1) in our study and also to the editor for giving us the chance to share this information.

Ischemia modified albumin (IMA) has recently been investigated in many studies and has largely been validated for the early detection of ischemia. The reduced cobalt binding capacity of albumin (IMA level) was first investigated with a colorimetric assay developed by Bar-Or et al. (2). We used this method described by Bar-Or et al. to determine serum IMA levels in rats and rabbits in our research because of the homology of the cobalt binding site between rat, rabbit and human serum albumin.

As Zheng et al. state in their letter, Peters et al. showed that rat serum albumin differs from human serum albumin in terms of amino acid levels (4). However, Appleton et al. showed that bovine serum albumin, human serum albumin and rat serum albumin exhibit a characteristic preferential binding of one copper (II) ion (5). The amino acid sequence in the N-terminal binding site for rat and rabbit serum albumin is N-Glu-Ala-His-Lys (GenBank accession number: NM_134326.2, NM_001082344.1) and for human serum albumin, N-Asp-Ala-His-Lys (GenBank accession number: NM_000477.5).

The mechanism that causes the modification of albumin during ischemia was explained by Bar-Or et al. (2) They demonstrated that this modification was an alteration in the Asp-Ala-His-Lys sequences in the albumin N terminus (2,6-9). Transient metals including cobalt, nickel and copper binding regions consisting of aspartic acid, alanine, histidine and lysine are the first 4 amino acids of the N terminus indicated by Bar-Or et al. (7). The third residue, histidine, has the major role of binding copper (10,11). Rat and rabbit serum albumin exhibit homology to human serum albumin in the amino acid sequence. Rat and rabbit albumin has 608 and human 609 amino acids, including a signal peptide at the N terminus (12-14). Although Zheng et al. mentioned the difference between the amino acid sequence and size of the peptides, human, rat and rabbit albumin consists of 585, 584, 584 amino acids, respectively, excluding the signal peptides. Metal binding domains differ by one residue, an aspartic acid and glutamic acid. These amino acids are similar structurally and chemically, and both are acidic amino acids. In the binding domain, histidine assumes the major role and this residue is a common one.

Our presented results showed that IMA levels can be an important marker in the diagnosis of testicular torsion (TT) (15). The literature contains many studies, similar to our own, investigating IMA levels in rats and rabbits (16-18). These studies, which enjoy wide acceptance in the literature and have attracted wide interest are in fact of the nature of preliminary studies.

In the light of all this information, we think that our research method is valid and widely accepted in the literature and that it can serve as a guide on such an important subject as TT.

Ahmet Mentese, Ph.D.a
Suleyman Turedi, M.D.b
Suha Turkmen, M.D.b
Abdulkadir Gunduz, M.D.b
Aysegul Uzun, Ph.D.a
Omer Kutlu, M.D.c
aDepartment of Biochemistry
Karadeniz Technical University Faculty of Medicine,
Trabzon, Turkey
bDepartment of Emergency Medicine
Karadeniz Technical University Faculty of Medicine
Trabzon, Turkey
cDepartment of Urology
Karadeniz Technical University Faculty of Medicine
Trabzon, Turkey

References
1. Zheng N, Zhu S, Liu L, Yu X. Rat serum albumin is not equal to human serum albumin. Fertil Steril (in press).

2. Bar-Or D, Lau E, Winkler JV. A novel assay for cobalt-albumin binding and its potential as a marker for myocardial ischemia – a preliminary report. J Emerg Med 2000; 19: 311–5.

3. Aparci M, Kardesoglu E, Ozmen N, Ozcan O, Cebeci BS, Cingozbay BY, et al. Prognostic significance of ischemia-modified albumin in patients with acute coronary syndrome. Coron Artery Dis 2007; 18: 367–73.

4. Peters T Jr. The biosynthesis of rat serum albumin.III.amino acid composition of rat albumin. J Biol Chem 1962;237:2182-3.

5. Appleton DW, Sarkar B. The absence of specific copper (II)-binding site in dog albumin. A comparative study of human and dog albumins. J Biol Chem 1971; 246: 5040-6.

6. Christenson RH, Duh SH, Sanhai WR, Wu AH, Holtman V, Painter P, et al. Characteristics of an albumin cobalt binding test for assessment of acute coronary syndrome patients: a multicenter study. Clin Chem 2001; 47: 464–70.

7. Bar-Or D, Curtis G, Rao N, Bampos N, Lau E. Characterization of the Co(2+) and Ni(2+) binding amino-acid residues of the N-terminus of human albumin. An insight into the mechanism of a new assay for myocardial ischemia. Eur J Biochem 2001; 268: 42–7.

8. Bhagavan NV, Lai EM, Rios PA, Yang J, Ortega-Lopez AM, Shinoda H, et al. Evaluation of human serum albumin cobalt binding assay for the assessment of myocardial ischemia and myocardial infarction. Clin Chem 2003; 49: 581–5.

9. Fagan GJ, Wayment H, Morris DL, Crosby PA. The albumin cobalt binding test: analytical performance of a new automated chemistry assay for the detection of ischemia modified albumin (IMA TM ). J Clin Ligand Assay 2002; 25: 178–87.

10. Laussac JP, Sarkar B. Characterization of the copper(II)- and nickel(II)-transport site of human serum albumin. Studies of copper(II) and nickel(II) binding to peptide 1–24 of human serum albumin by 13C and 1H NMR spectroscopy. Biochemistry 1984; 23: 2832–8.

11. Mohanakrishnan P, Chignell CF, Cox RH. Chloride ion nuclear magnetic resonance spectroscopy probe studies of copper and nickel binding to serum albumins. J Pharm Sci 1985; 74:61–2.

12. Available at: http://www.ncbi.nlm.nih.gov/nuccore/NM_134326.2 . Accessed May 6, 2011.

13. Available at: http://www.ncbi.nlm.nih.gov/nuccore/NM_001082344.1 . Accessed May 6, 2011.

14. Available at: http://www.ncbi.nlm.nih.gov/nuccore/NM_000477.5 . Accessed May 6, 2011.

15. Kutlu O, Mentese A, Turkmen S, Turedi S, Gunduz A, Yulug E, et al. Investigation of the possibility of using ischemia-modified albumin in testicular torsion: an experimental study. Fertil Steril 2011; 95:1333-7.

16. Gunduz A, Turkmen S, Turedi S, Mentese A, Yulug E, Ulusoy H, et al. Time-dependent variations in ischemia-modified albumin levels in mesenteric ischemia. Acad Emerg Med 2009;16: 539-43.

17. Livaoglu M, Kerimoglu S, Karahan SC, Mentese A, Sozen E, Karacal N, et al. Ischemia-Modified Albumin and Flap Viability. Eur Surg Res 2009;42:87–90.

18. Turedi S, Patan T, Gunduz A, Mentese A, Tekinbas C, Topbas M, et al. The Value of Ischemia-Modified Albumin in the Diagnosis of Pulmonary Embolism: an experimentaly study. American Journal of Emergency Medicine 2009; 27:635-40.

Published online in Fertility and Sterility doi:10.1016/j.fertnstert.2011.05.022

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