Sachs George, Scott David R., Weeks David L., Rektorscheck Marina, Melchers Klaus
University of California, Los Angeles, Los Angeles, CA, 90073
Byk Gulden, Konstanz, Germany
shows a remarkable acid-resistance response to the gastric environment by a specialized means of utilizing an intracellular, neutral pH optimum urease. Within the urease gene cluster it has a gene encoding an acid-activated urea channel, UreI. Instead of an acidic pH optimum urease suitable only for passage through the stomach, it has a neutral pH optimum urease that allows the cytoplasm to remain relatively neutral while the organism protects itself against acid by enhancing urea access to intrabacterial urease via Urel and achieving buffering of the periplasm. This maintains an adequate PMF for survival and for growth in an otherwise damaging environment. There are undoubtedly other subtleties involved in the regulation of urease activity inside the organism as well as the NH efflux across the inner membrane to buffer the periplasm. For example, at low rates of intrabacterial urease activity, the NH produced will not result in significant cytoplasmic alkalinization. This is the situation in the absence of acid activation of Urel. However, at a pH where urea transport is fully activated, NH production intracellularly probably increases by more than two orders of magnitude. This rapid production of NH could alkalinize the cytoplasm even in the presence of NH efflux through a bilayer with permeability of 10 cm s. However, if NH efflux were enabled through the urea channel, the combination of NH efflux across the bilayer and through UreI could maintain intrabacterial pH near neutrality in the face of high intrabacterial urease activity. Further, as soon as the periplasmic pH reaches 6.5, urea entry is switched off, thus inactivating intra-cellular urease. It would seem possible that this mechanism of adaptation to life in the human stomach may form the basis for methods designed to eradicate the organism. Presumably, drugs could be developed targeting the periplasmic domain of UreI and preventing acid activation of urea transport. If the bacterium is exposed, even for a short time, to a pH of less than 4.0, interference with urea uptake at these acidic pH values could prove lethal to the organism.
通过利用细胞内最适pH为中性的脲酶这一特殊方式，对胃部环境表现出显著的耐酸性反应。在脲酶基因簇中，它有一个编码酸激活尿素通道UreI的基因。它没有仅适用于通过胃部的最适pH为酸性的脲酶，而是有一个最适pH为中性的脲酶，该脲酶可使细胞质保持相对中性，同时生物体通过增强尿素经Urel进入细菌内脲酶的过程并实现周质的缓冲来保护自身免受酸的侵害。这为在其他方面具有破坏性的环境中的生存和生长维持了足够的质子动力势。毫无疑问，生物体内部脲酶活性的调节以及氨穿过内膜以缓冲周质还涉及其他微妙之处。例如，在细菌内脲酶活性较低时，产生的氨不会导致显著的细胞质碱化。这是在Urel未被酸激活的情况下的情形。然而，在尿素转运完全被激活的pH值下，细胞内氨的产生可能会增加两个以上数量级。即使存在通过渗透率为10⁻⁷ cm/s的双层膜的氨外流，氨的这种快速产生仍可能使细胞质碱化。然而，如果氨能够通过尿素通道外流，那么氨通过双层膜和通过UreI外流的结合可以在细菌内脲酶活性较高的情况下使细菌内pH维持在接近中性。此外，一旦周质pH达到6.5，尿素进入就会关闭，从而使细胞内脲酶失活。似乎这种适应人类胃部生活的机制可能构成旨在根除该生物体的方法的基础。据推测，可以开发针对UreI周质结构域并阻止尿素转运酸激活的药物。如果该细菌即使短时间暴露于pH小于4.0的环境中，在这些酸性pH值下对尿素摄取的干扰可能会证明对该生物体是致命的。
