Department of Animal Medicine and Surgery, University of Murcia, 30100 Espinardo, Murcia, Spain.
J Theor Biol. 2013 Jan 21;317:119-25. doi: 10.1016/j.jtbi.2012.10.011. Epub 2012 Oct 13.
Serum amyloid A (SAA) is a major acute phase protein in most species, and is widely employed as a health marker. Systemic SAA isoforms (SAA1, and SAA2) are apolipoproteins synthesized by the liver which associate with high density lipoproteins (HDL). Local SAA (SAA3) isoforms are synthesized in other tissues and are present in colostrums, mastitic milk and mammary dry secretions. Of systemic SAA the bulk is monomeric and bound to HDL, and a small proportion is found in serum in a multimeric form with a buried HDL binding site. In most species, systemic SAA could easily be studied by purifying it from serum of diseased individuals by hydrophobic interaction chromatography methods. For years, we were not able to isolate systemic pig SAA using the latter methods, and found that the bulk of pig SAA did not reside in the HDL-rich serum fractions but in the soluble protein fraction mainly as a multimeric protein. Based on these surprising results, we analysed in silico the theoretical properties and predicted the secondary structure of pig SAA by using the published pig primary SAA amino acid sequence. Results of the analysis confirmed that systemic pig SAA had the highest homology with local SAA3 which in other species is the isoform associated with non-hepatic production in tissues such as mammary gland and intestinal epithelium. Furthermore, the primary sequence of the pig SAA N-terminal HDL binding site did differ considerably from SAA1/2. Secondary structure analysis of the predicted alpha-helical structure of this HDL binding site showed a considerable reduction in hydrophobicity compared to SAA1/2. Based on these results, it is argued that systemic acute phase SAA in the pig has the structural properties of locally produced SAA (SAA3). It is proposed that in pig SAA multimers the charged N-terminal sequence is buried, which would explain their different properties. It is concluded that pig systemic SAA is unique compared to other species, which raises questions about the proposed importance of acute phase SAA in HDL metabolism during inflammation in this species.
血清淀粉样蛋白 A (SAA) 是大多数物种的主要急性期蛋白，被广泛用作健康标志物。系统性 SAA 同工型（SAA1 和 SAA2）是由肝脏合成的载脂蛋白，与高密度脂蛋白（HDL）结合。局部 SAA（SAA3）同工型在其他组织中合成，存在于初乳、乳腺炎奶和乳腺干燥分泌物中。在系统性 SAA 中，大部分是单体并与 HDL 结合，一小部分以多聚体形式存在于血清中，具有埋藏的 HDL 结合位点。在大多数物种中，可以通过疏水性相互作用色谱方法从患病个体的血清中纯化系统性 SAA 来轻松研究它。多年来，我们无法使用后一种方法从猪血清中分离出系统性猪 SAA，并且发现猪 SAA 的大部分不存在于富含 HDL 的血清部分，而是存在于可溶性蛋白质部分，主要作为多聚体蛋白。基于这些令人惊讶的结果，我们通过使用已发表的猪初级 SAA 氨基酸序列，在计算机上分析了猪 SAA 的理论特性并预测了其二级结构。分析结果证实，系统性猪 SAA 与局部 SAA3 的同源性最高，在其他物种中，SAA3 与乳腺和肠道上皮等组织中的非肝脏产生的同工型相关。此外，猪 SAA N 端 HDL 结合位点的原始序列与 SAA1/2 有很大不同。该 HDL 结合位点预测的α螺旋结构的二级结构分析显示，与 SAA1/2 相比，疏水性明显降低。基于这些结果，可以认为猪的系统性急性期 SAA 具有局部产生的 SAA（SAA3）的结构特性。有人提出，在猪 SAA 多聚体中，带电荷的 N 端序列被埋藏，这可以解释它们的不同性质。结论是，与其他物种相比，猪的系统性 SAA 是独特的，这引发了关于在该物种炎症期间急性期 SAA 在 HDL 代谢中的重要性的问题。
