Characterization of Iraqi sheep milk lysozyme with respect to molecular weight and hydrolytic activity

Abstract

Lysozyme is an enzyme that can be found in egg white, milk of various species, as well as in animal tissues and secretions and it is antimicrobial to Gram-positive bacteria. The objective of this work was to purify and characterize lysozyme from Iraqi sheep milk with respect to molecular weight and the influence of pH and temperature on the hydrolytic activity. The purification procedure comprised of enzymatic separation of the casein from the whey proteins and filtration of the whey through a 100-kDa membrane. The permeate was applied to a size exclusion column, Sephadex G-75, and fractions were collected and assayed for lysozyme activity. Isoelectric precipitation of alpha-lactalbumin from the Sephadex fractions containing lysozyme activity resulted in a 28.6-fold purification from crude whey with a 21% yield. The molecular weight as determined by size exclusion was between 30 and 34 kDa and was 14.3 kDa as determined by SDS-PAGE analysis. Sheep milk lysozyme was active from 25 to 80 °C with the highest activity at 55–65 °C. The influence of pH on the activity revealed the highest activity at pH 7.5 and the lysozyme was active at a pH range between pH 4.5 and 9.5. This work demonstrated that Iraqi sheep lysozyme was similar to bovine milk lysozyme with respect to hydrolytic activity and molecular weight.