Aswad D W, Paranandi M V, Schurter B T
Department of Molecular Biology and Biochemistry, University of California, Irvine 92697-3900, USA.
J Pharm Biomed Anal. 2000 Jan;21(6):1129-36. doi: 10.1016/s0731-7085(99)00230-7.
Formation of isoaspartyl peptide bonds (isoAsp) is one of the most common forms of non-enzymatic degradation of peptides and proteins under mild conditions. IsoAsp arises when certain Asn-Xaa and Asp-Xaa sites undergo a spontaneous intramolecular rearrangement to form a succinimide which subsequently hydrolyzes to generate a mixture of isoAsp-Xaa and Asp-Xaa linkages in a ratio of approximately 2:1. This pathway is responsible for the much greater susceptibility of asparagine, compared with glutamine, to deamidation at neutral and alkaline pH. Rearrangement occurs most readily at Asn-Gly, Asn-Ser, and Asp-Gly sequences where the local polypeptide chain flexibility is high. Formation of isoAsp can decrease the biological activity of a protein pharmaceutical, alter its susceptibility to proteolytic degradation, and elicit autoimmunity. The enzyme protein L-isoaspartyl methyltransferase can be used to measure isoAsp sites in the low pmol range with or without the use of radioisotopes.
异天冬氨酰肽键（异天冬氨酸）的形成是在温和条件下肽和蛋白质非酶促降解的最常见形式之一。当某些天冬酰胺-氨基酸和天冬氨酸-氨基酸位点发生自发的分子内重排形成琥珀酰亚胺时，就会产生异天冬氨酸，随后琥珀酰亚胺水解，生成比例约为2:1的异天冬氨酸-氨基酸和天冬氨酸-氨基酸连接键混合物。与谷氨酰胺相比，这条途径导致天冬酰胺在中性和碱性pH条件下更容易脱酰胺。重排最容易发生在局部多肽链柔韧性高的天冬酰胺-甘氨酸、天冬酰胺-丝氨酸和天冬氨酸-甘氨酸序列处。异天冬氨酸的形成会降低蛋白质药物的生物活性，改变其对蛋白水解降解的敏感性，并引发自身免疫。蛋白质L-异天冬氨酰甲基转移酶可用于在低皮摩尔范围内测量异天冬氨酸位点，无论是否使用放射性同位素。
