Characterisation of the anti-microbial activity of bovine milk ribonuclease4 and ribonuclease5 (angiogenin)

Two members of the ribonucleaseA (RNaseA) family were identified in cows' milk. RNase5, already known to be present, was found at concentrations between 3 and 19 μg mL−1. RNase4, which was not previously known to be in milk, was found at concentrations between 1 and 4 μg mL−1. These proteins were purified from milk. Purified RNase5 suppressed the growth of Candida albicans, particularly its hyphal form, but not any other microbes tested. The concentrations required to suppress growth were higher than those found in milk. RNase4 had no growth suppression activity. Both RNases reduced the number of viable C. albicans cells, with RNase5 being the more potent. This activity was abrogated by physiological concentrations of NaCl. Although both RNases bound to C. albicans cell membrane, their mode of action was not through membrane lysis or permeabilization. Together, these results suggest that the bovine milk RNases do not have intrinsic anti-microbial activity in milk.