1. Department of Animal Science, University of Illinois, 205 MSL, 1503 S. Maryland Dr., Urbana, IL 61801, U.S.A.
2. Department of Biochemistry, Biosciences West, University of Arizona, Tucson, AZ 85721, U.S.A.
3. Department of Physiology and Biophysics, University of Illinois 524 Burrill Hall, 407 S. Goodwin Avenue, Urbana, Illinois 61801, U.S.A.