BIOSYNTHESIS, PROCESSING & SECRETION OF NEUROPEPTIDES & PITUITARY PEPTIDE HORMONE

参与蛋白质前体加工的分泌囊泡酶 阿黑皮素（POMC，促肾上腺皮质激素原/内啡肽）。 70，000mol wt. 成对碱性残基特异性阿黑皮素原转化酶 最近被鉴定为一种独特的钙激活天冬氨酸 蛋白酶 这种酶的克隆正在进行中。 然而，PCE 不裂解ACTH的四碱基残基形成α-MSH（NAc ACTH 1 - 13 NH2）。 一种酸性、钙激活的丝氨酸蛋白酶已从 牛中叶分泌囊泡，是高度具体的 ACTH的四碱基残基位点。 这种酶富含中间体 与垂体前叶相比， 促肾上腺皮质激素转化为α-MSH 此外，钙激活 中性内切蛋白酶，切割POMC的成对碱性残基和 ACTH的四碱基残基在中间和前部 裂片分泌小泡膜。 它的最适pH值是中性的 可能不参与分泌囊泡中ACTH的裂解， 具有酸性的内部pH，但可能使POMC在 高尔基 抑制剂特性和底物特异性表明， 一种PC2样酶，一种与枯草杆菌蛋白酶相关的丝氨酸蛋白酶的哺乳动物成员 与酵母前α-交配因子同源的蛋白酶家族， 配对碱性残基特异性加工酶KEX-2。PC3，另一个成员 从大鼠中克隆了该枯草杆菌蛋白酶家族的成员和PC2， 显示与中叶POMC mRNA协同调节 溴隐亭或氟哌啶醇治疗，调节 POMC衍生肽的分泌和POMC生物合成。 这表明 PC2和PC3可能是POMC的候选加工酶。 信号 参与靶向POMC进入调节分泌途径的是 也调查了。 将截短的POMC/CAT构建体转染入 内分泌AtT-20细胞 细胞内定位的表达 POMC-CAT融合蛋白的免疫荧光组织化学和毛喉素 刺激分泌研究表明N-POMC的前26个氨基酸， 形成发夹环结构，由二硫键稳定 含有将激素原靶向分泌囊泡的信号。

Pituitary secretory vesicle enzymes involved in the processing of pro- opiomelanocortin (POMC, pro-ACTH/endorphin) were studied. A 70,000 mol wt. paired basic residue-specific pro-opiomelanocortin converting enzyme (PCE) has recently been characterized as a unique calcium activated aspartic protease. Cloning of this enzyme is now in progress. However, PCE does not cleave the tetrabasic residues of ACTH to form alpha-MSH (NAc ACTH 1-13 NH2). An acidic, calcium-activated serine protease has been isolated from bovine intermediate lobe secretory vesicles that is highly specific for the tetrabasic residue site of ACTH. This enzyme is enriched in intermediate versus anterior pituitary and may account for the differential processing of ACTH to alpha-MSH in these two lobes. In addition, a calcium activated neutral endoprotease that cleaves both paired basic residues of POMC and the tetrabasic residues of ACTH was detected in intermediate and anterior lobe secretory vesicle membranes. It's neutral pH optimum suggests that it may not be involved in the cleavage of ACTH in secretory vesicles which have an acidic internal pH, but may make the early cleavages of POMC in the Golgi. The inhibitor profile and substrate specificity indicate that it is a PC2-like enzyme, a mammalian member of the subtilisin-related serine protease family that is homologous to the yeast pro-alpha-mating factor, paired basic residue specific processing enzyme KEX-2. PC3, another member of this subtilisin family and PC2 were cloned from rat and their mRNAs were shown to be coordinately regulated with POMC mRNA in the intermediate lobe with bromocryptine or haloperidol treatment, agents which regulate secretion of POMC-derived peptides and POMC biosynthesis. This suggests that PC2 and PC3 may be candidate processing enzymes for POMC. The signal involved in the targeting of POMC into the regulated secretory pathway was also investigated. Truncated POMC/CAT constructs were transfected into endocrine AtT-20 cells. Intracellular localization of the expressed POMC-CAT fusion proteins by immunofluoreseence histochemistry and forskolin stimulated secretion studies indicate the first 26 amino acids of N-POMC, which forms a hairpin loop structure, stabilized by disulfide bridges contains the signal for targeting the prohormone to secretory vesicles.