We studied the substrate-dependence of pH activity of porcine pancreatic alpha-amylase by using a series of p-nitrophenyl maltooligosaccharides. The mechanism controlling the optimum pH of mammalian alpha-amylase involved the reception and recognition of a substrate component at some other substrate binding sites, in addition to those at subsite 5 that were reported previously [K. Ishikawa et al., Biochemistry, 32, 6259-6265 (1993)].