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 digest ion when luminal cu-amylase activity is reduced because of immaturity or malnutrition and that maltase-glucoamylase plays a unique role in t he digestion of malted dietary oligosaccharides used in food manufactu ring, As a first step toward the testing of this hypothesis, we have c loned 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, Huma n maltase-glucoamylase was purified by immunoisolation and partially s equenced, Maltase-glucoamylase cDNA was amplified from human intestina l RNA using degenerate and gene-specific primers with the reverse tran scription-polymerase chain reaction. The 6,513-base pair cDNA contains an open reading frame that encodes a 1,857-amino acid protein (molecu lar mass 209,702 Da). Maltase-glucoamylase has two catalytic sites ide ntical to those of sucrase-isomaltase, but the proteins are only 59% h omologous, Both are members of glycosyl hydrolase family 31, which has a variety of substrate specificities, Our findings suggest that diver gences in the carbohydrate binding sequences must determine the substr ate specificities for the four different enzyme activities that share a conserved catalytic site.