Identification of serum N-acetylmuramoyl- l-alanine amidase as liver peptidoglycan recognition protein 2

Abstract

N-acetylmuramoyl- l-alanine amidase (NAMLAA) hydrolyzes bacterial peptidoglycan and is present in human serum. A peptidoglycan-recognition protein 2 (PGLYRP2) is expressed in human liver and has N-acetylmuramoyl- l-alanine amidase activity. Here, we determined the amino acid sequences of human serum NAMLAA and liver PGLYRP2 and tested the hypothesis that serum NAMLAA and PGLYRP2 are the same protein. Liver PGLYRP2 and serum NAMLAA had the same mass determined by mass spectrometry and polyacrylamide gel electrophoresis, and both proteins and recombinant PGLYRP2 reacted with polyclonal anti-NAMLAA and anti-PGLYRP2 antibodies, and with monoclonal anti-NAMLAA antibodies. Digestion of serum NAMLAA with trypsin, chymotrypsin, or trypsin plus V8 protease, or with CNBr yielded, respectively, 37, 40, and 3 overlapping peptides that matched 100% and covered 81% of the deduced amino acid sequence of mature PGLYRP2. These peptides overlapped all exon–intron junctions indicating no alternative splice forms. Digestion of liver PGLYRP2 with trypsin yielded 23 peptides that matched 100% and covered 44% of the deduced amino acid sequence of mature PGLYRP2. Serum NAMLAA had a C398–C404 disulfide, partial phosphorylation of S218, and deamidation of N253 and N301. These results indicate that serum NAMLAA and liver PGLYRP2 are the same protein encoded by the pglyrp2 gene.