Background

The carbohydrate fraction of mammalian milk is constituted of lactose and oligosaccharides, most of which contain a lactose unit at their reducing ends. Although lactose is the predominant saccharide in the milk of most eutherians, oligosaccharides significantly predominate over lactose in the milk of monotremes and marsupials.

Scope of review

This review describes the most likely process by which lactose and milk oligosaccharides were acquired during the evolution of mammals and the mechanisms by which these saccharides are digested and absorbed by the suckling neonates.

Major conclusions

During the evolution of mammals, c-type lysozyme evolved to α-lactalbumin. This permitted the biosynthesis of lactose by modulating the substrate specificity of β4galactosyltransferase 1, thus enabling the concomitant biosynthesis of milk oligosaccharides through the activities of several glycosyltransferases using lactose as an acceptor. In most eutherian mammals the digestion of lactose to glucose and galactose is achieved through the action of intestinal lactase (β-galactosidase), which is located within the small intestinal brush border. This enzyme, however, is absent in neonatal monotremes and macropod marsupials. It has therefore been proposed that in these species the absorption of milk oligosaccharides is achieved by pinocytosis or endocytosis, after which digestion occurs through the actions of several lysosomal acid glycosidases. This process would enable the milk oligosaccharides of monotremes and marsupials to be utilized as a significant energy source for the suckling neonates.

General significance

The evolution and significance of milk oligosaccharides is discussed in relation to the evolution of mammals.

中文翻译：

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乳寡糖的进化：哺乳动物乳寡糖与乳糖比例的起源和选择性

背景

哺乳动物乳汁的碳水化合物部分由乳糖和寡糖组成，其中大部分在其还原端含有乳糖单元。尽管乳糖是大多数eutherians 乳汁中的主要糖类，但寡糖在单孔类动物和有袋动物乳汁中的含量明显高于乳糖。

审查范围

这篇综述描述了哺乳动物进化过程中获得乳糖和乳寡糖的最可能的过程，以及这些糖类被哺乳新生儿消化和吸收的机制。

主要结论

在哺乳动物的进化过程中，c型溶菌酶进化为α-乳清蛋白。这允许通过调节 β4 半乳糖基转移酶 1 的底物特异性来生物合成乳糖，从而能够通过使用乳糖作为受体的几种糖基转移酶的活性同时生物合成乳寡糖。在大多数eutherian哺乳动物中，乳糖消化成葡萄糖和半乳糖是通过位于小肠刷状缘内的肠道乳糖酶（β-半乳糖苷酶）的作用实现的。然而，这种酶在新生儿单孔类动物和大型有袋类动物中不存在。因此有人提出，在这些物种中，牛奶寡糖的吸收是通过胞饮作用或胞吞作用实现的，然后通过几种溶酶体酸性糖苷酶的作用进行消化。

一般意义

牛奶寡糖的进化和意义与哺乳动物的进化有关。