Further studies on the lysozyme-like activity of alpha-lactalbumin: development of alternative methods of assay

Earlier, a sensitive turbidimetric method was reported (H.A. McKenzie and F.H. White, Jr. Biochem. Int. 14, 347-356, 1987), with which evidence was found for weak lysozyme-like activity in alpha-lactalbumin (alpha-LA) against Micrococcus luteus. Alternative methods have been developed for the further study of trace cell-lytic activity, and the results are compared with those of the turbidimetric technique. These methods involve (1) determination of weight loss from a suspension of bacterial cells after exposure to the protein under investigation, and (2) viability studies on the exposed cells. In addition, exposed and control cells were subjected to microscopic examination. Results from all studies were consistent with lysis of cells by alpha-LA as well as by lysozyme. Activities of alpha-LA from the three methods of assay, expressed as ratios to those of lysozyme, were 2.2-5.2 x 10(-5) (mean = 3.6 x 10(-5). The methods were assessed with respect to sources of error characteristic of each and to protein dose requirements for a specified level of cell killing. The turbidimetric approach remains useful for measuring cell-lytic activities as described here. However, caution is urged in its general use.