[1] Ctr. of Res. on Infectious Diseases, Instituto Nacional de Salud Publica, Av. Universidad 655, Cuernavaca, Morelos 62508, Mexico
[2] Department of Biochemistry, Faculty of Medicine, Univ. Nacional Autonoma de Mexico, University City, Mexico D.F. 04510, Mexico
[3] Olivar de los Padres 941, México D. F. 01780, Mexico