Dobson D E, Prager E M, Wilson A C
J Biol Chem. 1984 Sep 25;259(18):11607-16.
A major regulatory shift affecting the expression of lysozyme c may have been involved in the origin of two groups of mammals whose nutrition depends on foregut bacteria. A survey of 23 mammalian species reveals that the lysozyme c activity per g of stomach mucosa is many times higher for ruminants and a leaf-eating monkey than for animals lacking a foregut. The implication is that stomach lysozyme c functions as a major digestive enzyme in ruminant-like mammals, helping to make those bacterial which enter the stomach from the foregut available for hydrolysis by conventional digestive enzymes. The high level of stomach lysozyme is due to more enzyme molecules rather than to an increase in the activity of each molecule. This was shown for the cow by purifying the three, non-allelic lysozymes c that account for the lysozyme activity in gastric mucosa and measuring their specific activities and for other foregut fermenters by immunological titration. Lysozyme appears in the stomach mucosa before birth and reaches adult levels before weaning. Other tissues tested from cattle lack lysozyme c and may instead have low levels of another lysozyme that could belong to the g class, the first indication that lysozyme g may be present in mammals. The lysozymes of eight ruminants, four Old World monkeys, and 12 other animals were compared as regards the ability to lyse bacterial cells under various conditions and to resist inactivation by pepsin. There are differences among these species in the dependence of the rate of bacterial lysis on time, pH, and ionic strength. Although not every lysozyme was tested in all of these catalytic respects, there were no exceptions to the following generalizations. First, at ionic strengths above 0.1 and pH values above 5, the rate of lysis by ruminant and monkey lysozymes c rose with the time of reaction, whereas the rate was more nearly constant for the other animal lysozymes. Second, the lytic activity at neutral pH is lower than at pH 5 for the ruminant and monkey lysozymes c when the ionic strength is over 0.1; by contrast, for other lysozymes c under these conditions the activity at neutral pH is about as high as at pH 5. This latter property, which may be viewed as an adaptation for functioning as a digestive enzyme in the stomach, can be explained in part by differences in electrostatic interactions between lysozyme and the substrate due to the relatively non-basic nature of ruminant and monkey lysozymes compared to other lysozymes c.(ABSTRACT TRUNCATED AT 400 WORDS)
一个影响溶菌酶c表达的重大调控转变可能与两类营养依赖前肠细菌的哺乳动物的起源有关。对23种哺乳动物的调查显示，每克胃黏膜的溶菌酶c活性，反刍动物和一种食叶猴比没有前肠的动物高出许多倍。这意味着胃溶菌酶c在类似反刍动物的哺乳动物中作为一种主要的消化酶发挥作用，有助于使从前肠进入胃的细菌能被常规消化酶水解。胃溶菌酶的高含量是由于酶分子增多，而非每个分子的活性增加。通过纯化牛胃黏膜中三种非等位的溶菌酶c并测定它们的比活性，证明了这一点；对于其他前肠发酵动物，则通过免疫滴定法证明。溶菌酶在出生前出现在胃黏膜中，并在断奶前达到成年水平。对牛的其他测试组织缺乏溶菌酶c，可能反而含有低水平的另一种溶菌酶，可能属于g类，这首次表明溶菌酶g可能存在于哺乳动物中。比较了8种反刍动物、4种旧世界猴和12种其他动物的溶菌酶在不同条件下裂解细菌细胞的能力以及抵抗胃蛋白酶失活的能力。这些物种在细菌裂解速率对时间、pH值和离子强度的依赖性方面存在差异。尽管并非每种溶菌酶都在所有这些催化方面进行了测试，但以下概括无一例外。首先，在离子强度高于0.1且pH值高于5时，反刍动物和猴的溶菌酶c的裂解速率随反应时间增加，而其他动物的溶菌酶的裂解速率则更接近恒定。其次，当离子强度超过0.1时，反刍动物和猴的溶菌酶c在中性pH下的裂解活性低于在pH 5时；相比之下，在这些条件下，其他溶菌酶c在中性pH下的活性与在pH 5时大致相同。后一种特性可被视为在胃中作为消化酶发挥作用的一种适应，部分原因可以用反刍动物和猴的溶菌酶与其他溶菌酶c相比相对非碱性的性质导致溶菌酶与底物之间静电相互作用的差异来解释。（摘要截选至400字）
