Drabik G, Naskalski J W
Department of Diagnostics, Collegium Medicum, Jagiellonian University, Kraków, Poland.
Acta Biochim Pol. 2001;48(1):271-5.
N-acetyl-L-tyrosine (N-acTyr), with the alpha amine residue blocked by acetylation, can mimic the reactivity of exposed tyrosyl residues incorporated into polypeptides. In this study chlorination of N-acTyr residue at positions 3 and 5 in reactions with NaOCl, chloramines and the myeloperoxidase (MPO)-H2O2-Cl- chlorinating system were invesigated. The reaction of N-acTyr with HOCl/OCl- depends on the reactant concentration ratio employed. At the OCl-/N-acTyr (molar) ratio 1:4 and pH 5.0 the chlorination reaction yield is about 96% and 3-chlorotyrosine is the predominant reaction product. At the OCl-/N-acTyr molar ratio 1:1.1 both 3-chlorotyrosine and 3,5-dichlorotyrosine are formed. The yield of tyrosine chlorination depends also on pH, amounting to 100% at pH 5.5, 91% at pH 4.5 and 66% at pH 3.0. Replacing HOCl/OCl- by leucine/chloramine or alanine/chloramine in the reaction system, at pH 4.5 and 7.4, produces trace amount of 3-chlorotyrosine with the reaction yield of about 2% only. Employing the MPO-H2O2-Cl- chlorinating system at pH 5.4, production of a small amount of N-acTyr 3-chloroderivative was observed, but the reaction yield was low due to the rapid inactivation of MPO in the reaction system. The study results indicate that direct chlorination of tyrosyl residues which are not incorporated into the polypeptide structure occurs with excess HOCl/OCl- in acidic media. Due to the inability of the myeloperoxidase-H2O2-Cl- system to produce high enough HOCl concentrations, the MPO-mediated tyrosyl residue chlorination is not effective. Semistable amino-acid chloramines also appeared not effective as chlorine donors in direct tyrosyl chlorination.
N-乙酰-L-酪氨酸（N-acTyr）的α-胺基残基被乙酰化阻断，它可以模拟掺入多肽中的暴露酪氨酸残基的反应活性。在本研究中，研究了N-acTyr残基在3位和5位与次氯酸钠、氯胺以及髓过氧化物酶（MPO）-H₂O₂-Cl⁻氯化系统反应时的氯化情况。N-acTyr与HOCl/OCl⁻的反应取决于所采用的反应物浓度比。在OCl⁻/N-acTyr（摩尔）比为1:4且pH为5.0时，氯化反应产率约为96%，3-氯酪氨酸是主要反应产物。在OCl⁻/N-acTyr摩尔比为1:1.1时，3-氯酪氨酸和3,5-二氯酪氨酸都会形成。酪氨酸氯化的产率也取决于pH，在pH 5.5时为100%，在pH 4.5时为91%，在pH 3.0时为66%。在反应体系中，用亮氨酸/氯胺或丙氨酸/氯胺替代HOCl/OCl⁻，在pH 4.5和7.4时，仅产生痕量的3-氯酪氨酸，反应产率仅约为2%。在pH 5.4时使用MPO-H₂O₂-Cl⁻氯化系统，观察到产生了少量的N-acTyr 3-氯衍生物，但由于反应体系中MPO的快速失活，反应产率较低。研究结果表明，在酸性介质中，未掺入多肽结构的酪氨酸残基会在过量的HOCl/OCl⁻作用下直接发生氯化。由于髓过氧化物酶-H₂O₂-Cl⁻系统无法产生足够高的HOCl浓度，MPO介导的酪氨酸残基氯化并不有效。半稳定的氨基酸氯胺在直接酪氨酸氯化中作为氯供体似乎也无效。
