INTRODUCTION In the bovine species, lysozyme has been described mainly by one research team. However, only minute amounts of this enzyme were detected in cow's milk: 13 µg/100 ml versus 30 mg/100 ml in human milk. Previous studies of biological fluids in ruminants, related to immunoglobulin synthesis, had shown a definite difference between the mammary gland and typical secretory organs such as the salivary, lacrimal, nasal or intestinal mucosa. These observations prompted us to investigate also secretions other than milk for the presence of lysozyme. Thus, a strong lytic activity against Micrococcus lysodeikticus cell walls was detected in commercial calf rennet.
METHODS The rate of lysis of M. lysodeikticus cell wells was determined according to Shugar. Identification of active fractions during purification was done with the lysoplate technique. Measurement of amino sugars released enzymatically was performed with an amino acid analyzer. The enzyme was purified by ion exchange, gel filtration and electrofocusing.
RESULTS The enzyme was mostly unadsorbed by DEAE-cellulose. This step was useful to retain most of the contaminating material, mainly the rennet proteases chymosin and pepsin. Filtration on Sephadex G-75 indicated a molecular size close to that of hen egg-white lysozyme, i.e. approximately 15,000 daltons. Electrofocusing revealed multiple enzyme forms with pl's ranging from 6.2 to 7.9. The predominant form (pl 7.5) represented more than 80% of the total lytic activity. The enzyme had an optimal pH of 5.0 and exhibited remarkable stability against heat and pH conditions ranging from 2 to 9. Hydrolysis of soluble and peptidoglycan by the purified enzyme released reducing groups but no free-NH2, indicating glycolytic activity exclusively. This glycosidase displayed endo-N-acetylmuramidase specificity, as demonstrated by analysis of the enzymatic reaction products, after reduction and acid hydrolysis. N-acetyl-ß-D-glucosamine competitively inhibited the enzyme (KI ~ 25 mM), while N-acetyl-muramic acid functioned as an activator. Significant chitinase activity on colloidal chitin was also observed at pH 5.0. By immunoelectrophoresis, specifie antisera revealed the multiple forms of lysozyme in the ß to y region, according to their pl. N-acetyl-hexosaminidase activity, carried by a rennet enzyme distinct from lysozyme, was detected using synthetic substrates. surprisingly, lysozyme was not found in other secretions tested in the same conditions.
CONCLUSION The physico-chemical and reactional properties of the calf rennet endo-N-acetyl-muramidase conform to criteria defined for lysozyme in other species.