Human small intestinal maltase-glucoamylase cDNA cloning - Homology to sucrase-isomaltase

It has been hypothesized that human mucosal glucoamylase (EC 3.2.1.20 and 3.2.1.3) activity serves as an alternate pathway for starch digestion when luminal cu-amylase activity is reduced because of immaturity or malnutrition and that maltase-glucoamylase plays a unique role in the digestion of malted dietary oligosaccharides used in food manufacturing, As a first step toward the testing of this hypothesis, we have cloned human small intestinal maltase-glucoamylase cDNA to permit study of the individual catalytic and binding sites for maltose and starch enzyme hydrolase activities in subsequent expression experiments, Human maltase-glucoamylase was purified by immunoisolation and partially sequenced, Maltase-glucoamylase cDNA was amplified from human intestinal RNA using degenerate and gene-specific primers with the reverse transcription-polymerase chain reaction. The 6,513-base pair cDNA contains an open reading frame that encodes a 1,857-amino acid protein (molecular mass 209,702 Da). Maltase-glucoamylase has two catalytic sites identical to those of sucrase-isomaltase, but the proteins are only 59% homologous, Both are members of glycosyl hydrolase family 31, which has a variety of substrate specificities, Our findings suggest that divergences in the carbohydrate binding sequences must determine the substrate specificities for the four different enzyme activities that share a conserved catalytic site.