Binding and sequestration of poison frog alkaloids by a plasma globulin

Alkaloids are important bioactive molecules throughout the natural world, and in many animals, they serve as a source of chemical defense against predation. Dendrobatid poison frogs bioaccumulate alkaloids from their diet to make themselves toxic or unpalatable to predators. Despite the proposed roles of plasma proteins as mediators of alkaloid trafficking and bioavailability, the responsible proteins have not been identified. We use chemical approaches to show that a ~50 kDa plasma protein is the principal alkaloid binding molecule in blood from poison frogs. Proteomic and biochemical studies establish this plasma protein to be liver-derived alkaloid-binding globulin (ABG) that is a member of the serine-protease inhibitor (serpin) family. In addition to alkaloid binding activity, ABG sequesters and regulates the bioavailability of "free" plasma alkaloids in vitro. Unexpectedly, ABG is not related to saxiphilin or albumin but instead exhibits sequence and structural homology to mammalian hormone carriers and amphibian biliverdin binding proteins. Alkaloid-binding globulin (ABG) represents a new small molecule binding functionality in serpin proteins, a novel mechanism of plasma alkaloid transport in poison frogs, and more broadly points towards serpins acting as tunable scaffolds for small molecule binding and transport across different organisms.

生物碱是自然界中广泛分布的重要生物活性分子，在诸多动物体内可作为化学防御物质抵御捕食。箭毒蛙科（Dendrobatidae）毒蛙可从食物中生物富集生物碱，使自身对捕食者产生毒性或降低适口性。尽管已有假说提出血浆蛋白介导生物碱的运输与生物利用度，但负责该过程的具体蛋白至今尚未被鉴定。本研究通过化学手段证实，约50千道尔顿（kDa）的血浆蛋白是毒蛙血液中主要的生物碱结合分子。蛋白质组学与生化实验证实，该血浆蛋白为肝源性生物碱结合球蛋白（alkaloid-binding globulin, ABG），属于丝氨酸蛋白酶抑制剂（serine-protease inhibitor, serpin）家族。除具备生物碱结合活性外，ABG在体外可螯合并调控“游离”血浆生物碱的生物利用度。出乎意料的是，ABG与沙昔菲林（saxiphilin）或白蛋白并无同源关系，但其序列与结构却与哺乳动物激素载体及两栖动物胆绿素结合蛋白具有同源性。生物碱结合球蛋白（ABG）代表了丝氨酸蛋白酶抑制剂家族中一种全新的小分子结合功能类型，为毒蛙体内血浆生物碱运输提供了全新机制，更广泛地表明丝氨酸蛋白酶抑制剂可作为可调控支架，在不同生物类群中介导小分子的结合与运输。