The human and mammalian N-acetylmuramyl-L-alanine amidase: distribution, action on different bacterial peptidoglycans, and comparison with the human lysozyme activities.

N-acetylmuramyl-L-alanine amidase (EC 3.5.1.28) specifically hydrolyzes the bacterial cell wall peptidoglycans (or mureins) and the muropeptides. The enzyme splits these molecules into two parts: the peptide subunits and the glycan strands or moieties. The bacterial peptidoglycans and their derived muropeptides display a number of biological properties. Removal of the glycosidic part of these molecules abolishes their beneficial as well as their detrimental properties. We report the high level of enzymatic activity found in all mammalian (including human) sera tested. The enzyme also occurred in human saliva, milk, cerebrospinal fluid, and synovial liquid. Mucosal tissue from different parts of the mammalian digestive tract exhibited enzymatic activity, but the enzyme was not detectable in the lumen content. The range of substrate specificity of the human enzyme was evaluated by measuring its action on the peptidoglycans extracted from several bacterial strains and representing different chemotypes and structures. Time course of the muramylalanine amidase and of the lysozyme (both of human origin) activities on some of these peptidoglycans are also reported, with the enzymes acting separately or together. From these data, we would speculate that a probable physiological role of the muramylalanine amidase is the maintenance of adequate ratios between the biologically active muropeptides and their inactive derivatives in the organism, the amidase activity antagonizing the production of biologically active molecules by lysozyme.

Purification and characterization of N-acetylmuramoyl-L-alanine amidase from human serum.

Purification to homogeneity of the N-acetylmuramoyl-L-alanine amidase (mucopeptide amidohydrolase, EC 3.5.1.28) from human serum has been achieved with a high yield. By molecular sieving chromatography, a molecular weight of 120,000-130,000 has been found for the native enzyme. Polyacrylamide gel electrophoresis under native conditions gave a unique band of Mr = 125,000. The same technique performed under denaturing conditions revealed that the protein is a dimer composed of one subunit of Mr = 57,000 and another of Mr = 70,000. In isoelectrofocalization assays, the amidase behaved as an acidic protein. Ethylenediaminetetraacetate inhibited the enzyme activity; the Mg2+ requirement was confirmed. The simultaneous presence of sulfhydryl groups and disulfide bonds in the protein was evidenced by the inhibitions produced by different thiol-blocking reagents and by several thiol-bearing substances. Direct measurements established the presence of two accessible thiol groups and the occurrence of nine disulfide bonds per protein molecule. Studies of substrate hydrolyzing capacities showed a marked preference for the muramoyl tripeptide derived from the Escherichia coli or Bacillus cereus mureins, the disaccharide tetrapeptide and the bis disaccharide tetra-tetrapeptide from E. coli were also good substrates. Activities on small muropeptides of other composition are also reported. Whole (insoluble) peptidoglycans representing the main bacterial chemotypes were submitted to the enzyme action; although with weak specific activities, the human amidase was nevertheless able to release soluble peptides from some of them. A bacteriolytic capacity on some microorganisms cannot be excluded. Results are discussed and the human enzyme is compared to presently known microbial muramoyl amidases.

Regulation of compartmentation of amino acid pools in Saccharomyces cerevisiae and its effects on metabolic control.

Compartmentation of intracellular amino acid pools has been studied under various growth conditions in wild-type strains as well as in mutants. Aspartate, glutamate, leucine and isoleucine pools are present in high concentrations in the cytoplasm, while all the other amino acids are more vacuolar. The nature of the nitrogen source for growth, the effectiveness of nitrogen assimilation, the rate of protein synthesis and the presence of high internal basic amino acid pools are important factors in the repartition of amino acid pools between the cytoplasm and the vacuole.