Brault D, Vever-Bizet C, Le Doan T
Biochim Biophys Acta. 1986 May 28;857(2):238-50. doi: 10.1016/0005-2736(86)90352-4.
The effects of hydrophobicity and charges of dicarboxylic porphyrins upon their interactions with membrane model systems are investigated. Four protonation steps are evidenced from fluorescence emission studies of hematoporphyrin IX and its more hydrophobic parent compound lacking of alcoholic chain, deuteroporphyrin IX. They are attributed to the successive protonations of the inner nitrogens of the porphyrin cycle (pK = 4.7 and 2.9 for hematoporphyrin and 4.4 and 2.7 for deuteroporphyrin) and successive deprotonations of propionic groups (pK approximately equal to 5.0 and 5.5 for hematoporphyrin and 5.4 and 6.0 for deuteroporphyrin). These porphyrins, as well as their dimethyl ester forms, are shown to incorporate as monomers into the hydrophobic bilayer of egg phosphatidylcholine small unilamellar vesicles, although the esterified forms are highly aggregated in aqueous solutions. In the case of the non-esterified forms, the incorporation of the porphyrins into the lipidic bilayer is reversible and strongly pH-dependent. A theoretical model is presented which takes into account the various protonation steps and the partition equilibria of the porphyrin between the vesicle lipidic phase and the water medium. The neutral form of the porphyrin (i.e., carboxylic groups protonated) presents the higher affinity, with constants of K approximately equal to 2 X 10(5) and K approximately equal to 6 X 10(6) M-1 (relative to lipid concentration) for hematoporphyrin and deuteroporphyrin, respectively. Protonation of one inner nitrogen leading to the monocationic form is sufficient to prevent incorporation into the hydrophobic bilayer. On the other hand, deprotonation of the peripheral propionic chains leading to anionic forms is less effective. These interactions between vesicles and porphyrins lead to shifts of the apparent pK of nitrogens and carboxylic groups, the latter one being now in the range of physiological pH. These results are discussed with regards to the hypothesis of a possible role of pH in the preferential uptake of porphyrins by tumors.
研究了二羧酸卟啉的疏水性和电荷对其与膜模型系统相互作用的影响。通过对血卟啉 IX 及其缺乏醇链的疏水性更强的母体化合物、去氢卟啉 IX 的荧光发射研究，证实了四个质子化步骤。它们归因于卟啉环内氮原子的连续质子化（血卟啉的 pK = 4.7 和 2.9，去氢卟啉的 pK = 4.4 和 2.7）以及丙酸基团的连续去质子化（血卟啉的 pK 约等于 5.0 和 5.5，去氢卟啉的 pK 约等于 5.4 和 6.0）。这些卟啉及其二甲酯形式显示以单体形式掺入卵磷脂酰胆碱小单层囊泡的疏水双层中，尽管酯化形式在水溶液中高度聚集。对于非酯化形式，卟啉掺入脂质双层是可逆的且强烈依赖于 pH。提出了一个理论模型，该模型考虑了各种质子化步骤以及卟啉在囊泡脂质相和水介质之间的分配平衡。卟啉的中性形式（即羧基质子化）具有更高的亲和力，血卟啉和去氢卟啉相对于脂质浓度的常数分别约为 K 约等于 2×10⁵ 和 K 约等于 6×10⁶ M⁻¹。一个内环氮原子质子化导致单阳离子形式足以阻止其掺入疏水双层。另一方面，外围丙酸链去质子化导致阴离子形式的效果较差。囊泡与卟啉之间的这些相互作用导致氮原子和羧基的表观 pK 发生变化，后者现在处于生理 pH 范围内。针对 pH 在肿瘤对卟啉的优先摄取中可能起作用的假设，对这些结果进行了讨论。
