Description of an acylpeptide hydrolase from lens

Acylpeptide hydrolase, which catalyses the hydrolysis of blocked N-terminal amino acids from peptide substrates, has been identified in the extracts from beef, human, rabbit and rat lens. In bovine lens sections, lower activity was observed in nuclear and inner cortical regions compared to the outer cortical region. The enzyme from bovine lens showed a high molecular weight nature, eluting between alpha and beta crystallins during Sephadex G-200 chromatography. The activity has a pH optimum around 7.8 when assayed with N-acetyl-Ala-p-NA as substrate. The enzyme was capable of hydrolyzing a variety of blocked peptides including N-acetyl-(Ala)2, Me-O-Suc-Ala-Ala-Pro-Val-p-NA, N-Acetyl-Met-Leu-Phe, Acetyl-Ser-Gln-Asn-Tyr and N-formyl-Met-p-NA. In each case the enzyme released an N-blocked amino acid and exposed a free amino group as judged by thin layer chromatography. Neither Ala-p-NA nor N-acetyl-Ala were hydrolysed by the same enzyme preparation. The enzyme activity from human and bovine lens was completely inhibited by DFP, and partially inhibited by PMSF, penicillin-G and ampicillin. These preliminary results show that lens tissue has an active acylpeptide hydrolase, however, a partially purified enzyme preparations was not able to cleave the acetyl-Met- from native alpha A-crystallin in vitro suggesting that the N-terminus of native crystallins is not accessible to the enzymes.