In connection with possible "markers" for identification of attenuated strains of measles virus, we have been interested in searching for enzymatic alterations of normal and measles-infected tissue culture cells under closely controlled conditions. In general, enzymatic alterations initiated in tissue culture by virus infection might offer a practical and qualitative means of evaluating attenuated measles strains.
It was considered of interest to screen the amino acid amidase enzymes, since various tissue cultures have an absolute requirement for amide-nitrogen. Earlier studies in bovine kidney cultures infected with low- and high-passage strains of infectious bovine rhinotracheitis (IBR) virus showed fundamentally different amino acid amidase activities. For example, leucinamidase activity exhibited a rapid increase, reaching a maximum toward the end of a one-hour absorption period in cells infected with the high-passage virus strain. This activity decreased sharply to a low activity within 2 to 3 hours. While the peak activity of the low-passage