Smyth D G, Darby N J, Maruthainar K
Laboratory of Peptide Chemistry, National Institute for Medical Research, Mill Hill, London, UK.
Neuroendocrinology. 1988 Apr;47(4):317-22. doi: 10.1159/000124931.
Lipotropin and peptides related to beta-endorphin were extracted from the anterior pituitary and the pars intermedia of porcine pituitary and were resolved by gel exclusion and ion exchange chromatography. Possible heterogeneity in the structure of the lipotropin was investigated by identifying the C-terminal fragment released by limited proteolysis with trypsin; the cleavage was restricted to the carboxyl group of arginine residues by employing citraconylation to protect the epsilon-NH2 groups of lysine. The lipotropin obtained from both regions of the pituitary gave rise to the same C-terminal peptide which contained the 31-residue sequence of beta-endorphin; none of the 26- and 27-residue forms was detected. In contrast, the beta-endorphin-related peptides that were isolated directly from the pars intermedia exhibited a high degree of C-terminal proteolysis: they were present principally as the 26- and 27-residue peptides. The results demonstrate that lipotropin differs from beta-endorphin in that it occurs exclusively in the form that contains the full C-terminal sequence. It is concluded that during biosynthesis lipotropin undergoes conversion to beta-endorphin before proteolysis takes place at the C-terminus. The processing reactions that convert lipotropin to beta-endorphin 1-31 and beta-endorphin 1-31 to beta-endorphin 1-27 are thus ordered and not competitive. The results also indicate that glycylglutamine, the bioactive C-terminal dipeptide of lipotropin, is formed from beta-endorphin and not from lipotropin.
促脂素及与β-内啡肽相关的肽类是从猪垂体前叶和垂体中间部分提取的，并通过凝胶排阻色谱和离子交换色谱进行分离。通过鉴定用胰蛋白酶有限水解释放的C末端片段，研究了促脂素结构中可能存在的异质性；通过采用柠康酰化保护赖氨酸的ε-NH2基团，使裂解仅限于精氨酸残基的羧基。从垂体两个区域获得的促脂素产生相同的C末端肽，该肽包含β-内啡肽的31个氨基酸残基序列；未检测到26和27个氨基酸残基的形式。相反，直接从垂体中间部分分离的与β-内啡肽相关的肽表现出高度的C末端蛋白水解：它们主要以26和27个氨基酸残基的肽形式存在。结果表明，促脂素与β-内啡肽的不同之处在于，它仅以包含完整C末端序列的形式存在。得出的结论是，在生物合成过程中，促脂素在C末端发生蛋白水解之前先转化为β-内啡肽。因此，将促脂素转化为β-内啡肽1-31以及将β-内啡肽1-31转化为β-内啡肽1-27的加工反应是有序的，而非竞争性的。结果还表明，促脂素的生物活性C末端二肽甘氨酰谷氨酰胺是由β-内啡肽形成的，而不是由促脂素形成的。
