Papaya Lysozyme: Terminal sequences and enzymatic properties

Abstract

Abstract The amino-terminal sequence of papaya lysozyme was determined by a modified Edman degradation procedure to be Gly-Ile-Ser-Lys-Ile. The carboxyl-terminal sequence was determined by carboxypeptidase A degradation to be Ser-Phe-Gly. The presence of carboxyl-terminal glycine was verified by hydrazinolysis. Papaya lysozyme was shown to release reducing groups concomitantly with the lysis of Micrococcus lysodeikticus cell walls, and the reducing group was found to be that of N-acetylmuramic acid, as previously shown for egg white lysozyme by other investigators. However, papaya lysozyme initially hydrolyzed more glycosidic bonds than egg white lysozyme for an equal amount of lysis. The enzyme exhibited only 10% of maximal activity at an ionic strength of 0.15 and was maximally active at an ionic strength of 0.05. Papaya lysozyme was stable from pH 1.8 to 10 at room temperature and remained active for 5 min up to 95° at pH 4.6. Whereas papaya lysozyme exhibited 200 to 400 times higher chitinase activity toward tetra-N-acetyl-d-glucosamine than egg white lysozyme, with whole chitin as substrate, it was only 10 times as active as the egg white enzyme. The papaya enzyme exhibited a sharp pH activity profile with M. lysodeikticus cells. The enzyme had maximal activity at pH 4.6 and was virtually inactive above pH 6.5. With whole chitin as substrate, the pH profile was broad with an optimum from pH 4.5 to 6.0. The enzyme was still partly active at pH 7.5. The lysis of esterified cell walls from M. lysodeikticus yielded the same pH rate profile as that obtained for chitin. Thus, the difference in the pH dependence of hydrolysis of chitin and cell walls appears to result from the presence of carboxyl groups in the latter substrate. The mechanism of attack upon the bacterial cell wall by egg white and papaya lysozymes is discussed. Papaya lysozyme was strongly inhibited by histamine and histidine. No evidence was obtained of the involvement of tryptophan residues in the substrate-binding site.