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Definition

Endorphins are small neuropeptides that are produced by the body and act to reduce pain hence, the name endorphin (a shortened version of endogenous morphine). The term "enkephalin" (meaning literally "in the head") is also applied to endorphins, but usually refers to smaller molecules that have pain-relieving properties 1.

Related Peptides

There are 3 types of Endorphins:

Enkephalins: Met- and Leu-
Endorphins
Dynorphins
Endorphins are neuropeptides that can range from 2 to 39 amino acids in length. Neuropeptides are peptide molecules produced and released in the nervous system that act like transmitters 2. There are three different neuropeptide sequences including enkephalins, endorphins, and dynorphins 3

Discovery

In 1975, John Hughes and Hans W. Kosterlitz of the University of Aberdeen isolated two naturally occurring peptides in the brain that bound tightly to the opiate receptors and named them enkephalins. The endorphin molecule was subsequently isolated from the pituitary gland 4.

Structural Characteristics

Four distinct groups of endorphins have been identified to date. They have been termed: a-endorphin, a polypeptide with 16 residues; ß-endorphin, a polypeptide with 31 residues; ?-endorphin, a polypeptide with 17 residues; and S-endorphin, a polypeptide with 27 residues. These different types of endorphins, like all known polypeptide hormones, are synthesized in a "prepro" form that is one gigantic polypeptide with a signal sequence and additional sequences that are cleaved out during posttranslational maturation of the polypeptide. The most interesting example of this is the pituitary multihormone precursor termed pro-opiomelanocortin that contains the sequences for ß-lipotropin, melanocyte-stimulating hormone (MSH), endorphins, enkephalins, and adrenocorticotropic hormone (ACTH). After synthesis, this peptide is cleaved in the pituitary to generate ACTH and ß-lipotropin, while processing in the central nervous system produces endorphins and enkephalins, along with some other products 5.

Mode of Action

Receptors enable endorphins to perform their specific function. Opioid receptors are large protein molecules embedded in the semi-fluid matrix of the cell membrane of the receiving neuron. The surface of the receptor protein contains a region that is the precise size and shape to match the structure of the endorphin molecule. The endorphin molecule precisely fits into the specific receptor site. The binding of the neuropeptide with its specific receptor (opioid receptor) alters the three-dimensional shape of the receptor protein, thereby causing a neuron to be excited or inhibited6. As in the case of endorphins, inhibition of the neuron will reduce the release of substance P. In other words, the opioid receptor translates the precise messages encoded by the molecular structure of the endorphin molecule into a specific physiological response. Thus, receptors act as a control mechanism thereby regulating the function of endorphins 7.

Functions

Endorphins are not considered to be neurotransmitter molecules, but are instead classified as neuromodulatory, that is, they modify the action of neurotransmitters through a number of effects associated with pain or pleasure. Endorphins exhibit a number of neurological effects associated with the relief of pain. The administration of exogenous endorphins (those prepared outside the body) stimulates the release of many other hormones including prolactin, ACTH, and antidiuretic hormone. The analgesic effects of morphine are commonly believed to be caused by binding to receptor sites for endorphins, but few beneficial effects of treatment with exogenous endorphins have been reported. Early speculations concerning the function of endorphins suggested that they were natural painkillers that the body produced to alleviate pain in circumstances requiring an individual to continue functioning in spite of injury or stress. Examples of such situations might include childbirth, exercise, and combat. Several procedures that treat chronic pain (acupuncture, direct electrical stimulation of the brain and even hypnosis) may act by inducing the release of enkephalins or endorphins in the brain and spinal cord. This hypothesis is based on the finding that the effectiveness of treating pain implemented by these procedures is blocked by administration of naloxone, a drug that specifically blocks the binding of morphine to the opiate receptor 1.

References

Book: Textbook of Biochemistry: With Clinical Correlations by Devlin TM.
Book: Animal Physiology by Eckert R.
Book: Neurobiology by Shepherd GM.
Book: The Brain by Iverson L.
Book: Molecular Expressions: Exploring the World of Optics and Microscopy Michael WD.
Book: Neural and Integrative Animal Physiology by Prosser CL.
Book:. Neuroscience by Barker RA

多肽H2N-Tyr-Gly-Gly-Phe-Leu-Arg-Lys-Tyr-Pro-COOH的合成步骤：

1、合成CTC树脂：称取0.59g CTC Resin（如初始取代度约为1.04mmol/g）和0.74mmol Fmoc-Pro-OH于反应器中，加入适量DCM溶解氨基酸（需要注意，此时CTC树脂体积会增大好几倍，避免DCM溶液过少），再加入1.84mmol DIPEA（Mw：129.1,d：0.740g/ml），反应2-3小时后，可不抽滤溶液，直接加入1ml的HPLC级甲醇，封端半小时。依次用DMF洗涤2次，甲醇洗涤1次，DCM洗涤一次，甲醇洗涤一次，DCM洗涤一次，DMF洗涤2次（这里使用甲醇和DCM交替洗涤，是为了更好地去除其他溶质，有利于后续反应）。得到  Fmoc-Pro-CTC Resin。结构图如下：

2、脱Fmoc：加3倍树脂体积的20%Pip/DMF溶液，鼓氮气30分钟，然后2倍树脂体积的DMF 洗涤5次。得到 H2N-Pro-CTC Resin 。（此步骤脱除Fmoc基团，茚三酮检测为蓝色，Pip为哌啶）。结构图如下：

3、缩合：取1.84mmol Fmoc-Tyr(tBu)-OH 氨基酸，加入到上述树脂里，加适当DMF溶解氨基酸，再依次加入3.68mmol DIPEA，1.75mmol HBTU。反应30分钟后，取小样洗涤，茚三酮检测为无色。用2倍树脂体积的DMF 洗涤3次树脂。（洗涤树脂，去掉残留溶剂，为下一步反应做准备）。得到Fmoc-Tyr(tBu)-Pro-CTC Resin。氨基酸：DIPEA：HBTU：树脂=3：6：2.85：1（摩尔比）。结构图如下：

4、依次循环步骤二、步骤三，依次得到

H2N-Tyr(tBu)-Pro-CTC Resin

Fmoc-Lys(Boc)-Tyr(tBu)-Pro-CTC Resin

H2N-Lys(Boc)-Tyr(tBu)-Pro-CTC Resin

Fmoc-Arg(Pbf)-Lys(Boc)-Tyr(tBu)-Pro-CTC Resin

H2N-Arg(Pbf)-Lys(Boc)-Tyr(tBu)-Pro-CTC Resin

Fmoc-Leu-Arg(Pbf)-Lys(Boc)-Tyr(tBu)-Pro-CTC Resin

H2N-Leu-Arg(Pbf)-Lys(Boc)-Tyr(tBu)-Pro-CTC Resin

Fmoc-Phe-Leu-Arg(Pbf)-Lys(Boc)-Tyr(tBu)-Pro-CTC Resin

H2N-Phe-Leu-Arg(Pbf)-Lys(Boc)-Tyr(tBu)-Pro-CTC Resin

Fmoc-Gly-Phe-Leu-Arg(Pbf)-Lys(Boc)-Tyr(tBu)-Pro-CTC Resin

H2N-Gly-Phe-Leu-Arg(Pbf)-Lys(Boc)-Tyr(tBu)-Pro-CTC Resin

Fmoc-Gly-Gly-Phe-Leu-Arg(Pbf)-Lys(Boc)-Tyr(tBu)-Pro-CTC Resin

H2N-Gly-Gly-Phe-Leu-Arg(Pbf)-Lys(Boc)-Tyr(tBu)-Pro-CTC Resin

Fmoc-Tyr(tBu)-Gly-Gly-Phe-Leu-Arg(Pbf)-Lys(Boc)-Tyr(tBu)-Pro-CTC Resin

以上中间结构，均可在专肽生物多肽计算器-多肽结构计算器中，一键画出。

最后再经过步骤二得到 H2N-Tyr(tBu)-Gly-Gly-Phe-Leu-Arg(Pbf)-Lys(Boc)-Tyr(tBu)-Pro-CTC Resin，结构如下：

5、切割：6倍树脂体积的切割液（或每1g树脂加8ml左右的切割液），摇床摇晃 2小时，过滤掉树脂，用冰无水乙醚沉淀滤液，并用冰无水乙醚洗涤沉淀物3次，最后将沉淀物放真空干燥釜中，常温干燥24小试，得到粗品H2N-Tyr-Gly-Gly-Phe-Leu-Arg-Lys-Tyr-Pro-COOH。结构图见产品结构图。

切割液选择：1）TFA：H2O=95%：5%、TFA：H2O=97.5%：2.5%

2）TFA：H2O：TIS=95%：2.5%：2.5%

3）三氟乙酸：茴香硫醚：1，2-乙二硫醇：苯酚：水=87.5%：5%：2.5%：2.5%：2.5%

（前两种适合没有容易氧化的氨基酸，例如Trp、Cys、Met。第三种适合几乎所有的序列。）

6、纯化冻干：使用液相色谱纯化，收集目标峰液体，进行冻干，获得蓬松的粉末状固体多肽。不过这时要取小样复测下纯度 是否目标纯度。

7、最后总结：

杭州专肽生物技术有限公司（ALLPEPTIDE https://www.allpeptide.com）主营定制多肽合成业务，提供各类长肽，短肽，环肽，提供各类修饰肽，如：荧光标记修饰（CY3、CY5、CY5.5、CY7、FAM、FITC、Rhodamine B、TAMRA等），功能基团修饰肽（叠氮、炔基、DBCO、DOTA、NOTA等），同位素标记肽（N15、C13），订书肽（Stapled Peptide）,脂肪酸修饰肽（Pal、Myr、Ste），磷酸化修饰肽（P-Ser、P-Thr、P-Tyr），环肽（酰胺键环肽、一对或者多对二硫键环），生物素标记肽，PEG修饰肽，甲基化修饰肽

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