Wallevik K
Biochim Biophys Acta. 1976 Jan 20;420(1):42-56. doi: 10.1016/0005-2795(76)90343-3.
Column isoelectric focusing separates commercial bovine serum albumin in 5 fractions with isoionic points in the vicinity of that of mercaptalbumin (pI 5.24). About 20% of the bovine albumin have isoionic points higher than mercaptalbumin and are split into two fractions, both recognized as SS-interchanges isomers: (1) pI 5.39 is the "aged" albumin described by Nikkel and Foster (1971, Biochemistry 10, 4479); (2) pI 5.45 represents a further degree of SS-interchange, catalyzed by small amounts of cysteine in the solution ('cysteine-aged' albumin). In 6 M urea the "cysteine-aged" albumin is electrofocused to the same pH value as mercaptalbumin. In 6 M urea 40% of commercial albumin focuses in 3 fractions with isoionic points lower than mercaptalbumin. This percentage will increase during incubation at oxidizing conditions ("oxidized" albumin). Electrofocused in water the oxidized fractions have isoionic points at pI 5.28, 5.18 and 5.12, respectively. The shifts in isoionic point of the "oxidized" albumins are caused by irreversible changes in the primary structure. Although the free SH group of albumin is oxidized during the oxidation reaction, the observed changes in isoionic points are caused by modifications of some other amino acid residues. Both "cysteine-ageing" and "oxidation" are inhibited by alkylation of the SH group. "Cysteine-ageing" is furthermore inhibited when the bovine albumin is "oxidized".
柱等电聚焦可将商业牛血清白蛋白分离为5个组分，其等离子点与巯基白蛋白（pI 5.24）相近。约20%的牛白蛋白等离子点高于巯基白蛋白，并被分为两个组分，二者均被认为是二硫键交换异构体：（1）pI 5.39是Nikkel和Foster（1971年，《生物化学》10卷，4479页）所描述的“老化”白蛋白；（2）pI 5.45代表二硫键交换的进一步程度，由溶液中少量的半胱氨酸催化形成（“半胱氨酸老化”白蛋白）。在6M尿素中，“半胱氨酸老化”白蛋白电聚焦至与巯基白蛋白相同的pH值。在6M尿素中，40%的商业白蛋白聚焦为3个组分，其等离子点低于巯基白蛋白。在氧化条件下孵育时（“氧化”白蛋白），这一比例会增加。在水中进行电聚焦时，氧化组分的等离子点分别为pI 5.28、5.18和5.12。“氧化”白蛋白等离子点的变化是由一级结构的不可逆变化引起的。虽然在氧化反应过程中白蛋白的游离巯基被氧化，但观察到的等离子点变化是由其他一些氨基酸残基的修饰引起的。二硫键的烷基化可抑制“半胱氨酸老化”和“氧化”。此外，当牛白蛋白被“氧化”时，“半胱氨酸老化”也会受到抑制。
