Danielsen E M, Cowell G M
Biochem J. 1986 Dec 15;240(3):777-82. doi: 10.1042/bj2400777.
Castanospermine, an inhibitor of glucosidase I, the initial enzyme in the trimming of N-linked carbohydrate, was used to study the importance of carbohydrate processing in the biosynthesis of microvillar enzymes in organ-cultured pig intestinal explants. For aminopeptidase N (EC 3.4.11.2), aminopeptidase A (EC 3.4.11.7), sucrase-isomaltase (EC 3.2.1.48-10) and maltase-glucoamylase (EC 3.2.1.20), castanospermine caused the formation of novel transient forms of higher Mr than corresponding controls, indicating a blocked removal of glucose residues. For the first three enzymes, the 'mature' (Golgi-processed) forms were similar in size to or slightly smaller than corresponding controls and were, as shown for aminopeptidase N, endoglycosidase-H-sensitive, evidence of a blocked attachment of complex sugars. Maltase-glucoamylase did not undergo conversion into a 'mature' form, suggesting that, unlike other microvillar enzymes, it does not receive post-translational O-linked carbohydrate. Castanospermine suppressed the synthesis of the four enzymes, but did not block their transport to the microvillar membrane, showing that processing of N-linked carbohydrate is not required for microvillar expression. The proteinase inhibitor leupeptin partially restored the suppressed synthesis, indicating that the majority of the wrongly processed enzymes, probably because of conformational instability, become degraded soon after synthesis rather than being transported to the microvillar membrane.
栗精胺是一种葡糖苷酶I（N-连接碳水化合物修剪过程中的起始酶）的抑制剂，被用于研究碳水化合物加工在器官培养的猪肠道外植体微绒毛酶生物合成中的重要性。对于氨肽酶N（EC 3.4.11.2）、氨肽酶A（EC 3.4.11.7）、蔗糖酶-异麦芽糖酶（EC 3.2.1.48 - 10）和麦芽糖酶-葡糖淀粉酶（EC 3.2.1.20），栗精胺导致了比相应对照具有更高相对分子质量的新型瞬时形式的形成，这表明葡萄糖残基的去除受阻。对于前三种酶，“成熟”（高尔基体加工的）形式在大小上与相应对照相似或略小，并且如氨肽酶N所示，对内切糖苷酶H敏感，这是复杂糖附着受阻的证据。麦芽糖酶-葡糖淀粉酶没有转化为“成熟”形式，这表明与其他微绒毛酶不同，它不接受翻译后O-连接的碳水化合物。栗精胺抑制了这四种酶的合成，但没有阻止它们向微绒毛膜的转运，这表明微绒毛表达不需要N-连接碳水化合物的加工。蛋白酶抑制剂亮肽素部分恢复了被抑制的合成，这表明大多数加工错误的酶可能由于构象不稳定，在合成后不久就被降解，而不是被转运到微绒毛膜。
