Sharma Vijay S, Pilz Renate B, Boss Gerry R, Magde Douglas
Department of Medicine, University of California-San Diego, La Jolla, California 92093-0652, USA.
Biochemistry. 2003 Jul 29;42(29):8900-8. doi: 10.1021/bi034469t.
Despite early claims that nitric oxide does not react with cobalamin under any circumstances, it is now accepted that NO has a high affinity for cobalamin in the 2+ oxidation state [Cbl(II)]. However, it is still the consensus that NO does not react with Cbl(III). We confirmed that NO coordinates to Cbl(II) at all pH values and that Cbl(III) does not react with NO at neutral pH. At low pH, however, Cbl(III) does react with NO by way of a two-step process that also reduces Cbl(III) to Cbl(II). To account for the pH dependence, and because of its intrinsic interest, we also studied reactions of NO with cobinamide [Cbi] in the 2+ and 3+ oxidation states. Both Cbi(II) and Cbi(III) react readily with NO at all pH values. Again, Cbi(III) is reduced during the process of coordinating NO. Compared to cobalamin, cobinamide lacks the tethered 5,6-dimethylbenzamidazolyl moiety bound to the cobalt ion. It may, therefore, be considered a "base-off" form of Cbl. To explain the reaction of Cbl(III) at low pH, we infer that the base-off form of Cbl(III) exists in trace amounts that are rapidly reduced to Cbl(II), which then binds NO efficiently. Base dissociation, we postulate, is the rate-limiting step. Interestingly, Cbi(II) has 100 times greater affinity for NO than does Cbl(II), proving that there is a strong trans effect due to the tethered base in nitrosyl derivatives of both Cbl(II) and Cbl(III). The affinity of Cbi(II) for NO is so high that it is a very efficient NO trap and, consequently, may have important biomedical uses.
尽管早期有观点认为一氧化氮在任何情况下都不与钴胺素发生反应，但现在人们已经接受，一氧化氮对处于2 +氧化态的钴胺素[Cbl(II)]具有很高的亲和力。然而，目前仍存在的共识是，一氧化氮不与Cbl(III)发生反应。我们证实，在所有pH值下，一氧化氮都能与Cbl(II)配位，并且在中性pH值下，Cbl(III)不与一氧化氮反应。然而，在低pH值下，Cbl(III)确实会通过一个两步过程与一氧化氮发生反应，该过程也会将Cbl(III)还原为Cbl(II)。为了解释pH依赖性，并且由于其内在的研究价值，我们还研究了一氧化氮与处于2 +和3 +氧化态的钴胺酰胺[Cbi]的反应。在所有pH值下，Cbi(II)和Cbi(III)都能与一氧化氮迅速反应。同样，在与一氧化氮配位的过程中，Cbi(III)会被还原。与钴胺素相比，钴胺酰胺缺少与钴离子相连的5,6 -二甲基苯并咪唑基部分。因此，它可以被认为是Cbl的“碱基脱离”形式。为了解释低pH值下Cbl(III)的反应，我们推断Cbl(III)的碱基脱离形式以痕量存在，并迅速还原为Cbl(II)，然后Cbl(II)有效地结合一氧化氮。我们推测，碱基解离是限速步骤。有趣的是，Cbi(II)对一氧化氮的亲和力比Cbl(II)高100倍，这证明在Cbl(II)和Cbl(III)的亚硝酰基衍生物中，由于相连的碱基存在很强的反位效应。Cbi(II)对一氧化氮的亲和力非常高，以至于它是一种非常有效的一氧化氮捕获剂，因此可能具有重要的生物医学用途。
