Benesch R E, Benesch R, Kwong S, Baugh C M
Proc Natl Acad Sci U S A. 1983 Oct;80(20):6202-5. doi: 10.1073/pnas.80.20.6202.
The binding of a physiological concentration of pteroylhepta(glutamate) to oxy- and deoxyhemoglobin in large excess was measured by ultrafiltration. The variation of free to bound folate with hemoglobin concentration showed that a single molecule of the pteroylpolyglutamate is bound by deoxyhemoglobin tetramers and by alpha beta dimers in oxyhemoglobin. Although the binding sites are different, the affinity constants are the same and very similar to the 2,3-bisphosphoglycerate binding energy. Nevertheless, in view of the small proportion of dimers in oxyhemoglobin much more pteroylhepta(glutamate) is bound by deoxyhemoglobin over a wide range of hemoglobin concentrations. Because even 2% deoxyhemoglobin is enough to bind all of the erythrocyte folate as polyglutamate, the bulk of it will be bound at physiological oxygen pressures. Free folate could only be expected in fully oxygenated erythrocytes. Therefore, the reaction of pteroylpolyglutamates with hemoglobin represents an oxygenation-dependent storage mechanism that can account for the 40-fold excess of the vitamin in the erythrocyte over the amounts in the serum. Because methotrexate is also converted to polyglutamate derivatives in the erythrocyte, this drug is likely to be concentrated and stored there by the same mechanism.
通过超滤法测定了生理浓度的蝶酰庚（谷氨酸）与大量过量的氧合血红蛋白和脱氧血红蛋白的结合情况。游离叶酸与结合叶酸随血红蛋白浓度的变化表明，一个蝶酰多聚谷氨酸分子可被脱氧血红蛋白四聚体和氧合血红蛋白中的αβ二聚体结合。尽管结合位点不同，但亲和常数相同，且与2,3-二磷酸甘油酸的结合能非常相似。然而，鉴于氧合血红蛋白中二聚体的比例较小，在较宽的血红蛋白浓度范围内，脱氧血红蛋白结合的蝶酰庚（谷氨酸）要多得多。由于即使2%的脱氧血红蛋白就足以将所有红细胞叶酸以多聚谷氨酸的形式结合，所以在生理氧分压下，大部分叶酸会被结合。只有在完全氧合的红细胞中才会有游离叶酸。因此，蝶酰多聚谷氨酸与血红蛋白的反应代表了一种依赖氧合的储存机制，这可以解释红细胞中维生素的含量比血清中高出40倍的现象。由于甲氨蝶呤在红细胞中也会转化为多聚谷氨酸衍生物，这种药物很可能通过相同的机制在那里被浓缩和储存。
