High molecular weight soluble neutral maltase-glucoamylases in the intestine of the suckling rat

Abstract

Previous work from our laboratory has shown that the intestine of the suckling rat, unlike adult rat intestine, contains abundant quantities of at least two soluble neutral maltase-glucoamylases. These enzymes are related antigenically to membrane-bound maltase-glucoamylase, which predominates in adult intestine, but are either more easily solubilized or occupy a different cellular locus. To study the soluble enzymes further, we attempted their isolation from the intestine of 11-day-old suckling rats. Initial attempts were complicated by proteolytic degradation, despite the addition of phenylmethylsulfonyl fluoride, N-ethylmaleimide, leupeptin, pepstatin, and EDTA to buffers used for homogenization and column chromatography. Addition of aprotinin, amastatin, bestatin, and phosphoramidone resulted, however, in the isolation of two stable, high molecular weight maltases (HM1 and HM2). Both enzymes eluted before a papain-solubilized membrane-derived maltase-glucoamylase on Sepharose 4B and were separable by DE-52 and Sepharose 6B – Tris affinity columns. They were further purified on a lentil lectin – Sepharose 4B column. Substrate specificities were almost the same and characteristic of maltase-glucoamylases. Hydrophobic binding properties and pH optima of HM1 and HM2 were also similar. HM1 was resolved by sodium dodecyl sulfate – polyacrylamide gel electrophoresis into approximately equal portions of an endo-β-N-acetylglucosaminidase H sensitive enzyme of molecular weight (MW) 200 000 and an endo-β-N-acetylglucosaminidase H resistant but endo-β-acetylglucosaminidase F sensitive enzyme of MW 400 000. In contrast, most of HM2 consisted of a doublet of MW 200 000 – 210 000 that was endo-β-N-acetylglucosaminidase H sensitive. The intestine of the suckling rat, therefore, contains two soluble maltase–glucoamylase fractions, with a major portion of high mannose rather than complex oligosaccharides. In HM1, it is possible that the high mannose derivative is transformed during trafficking into the complex endo-β-N-acetylglucosaminidase H resistant form. HM2 appears to undergo little oligosaccharide processing.Key words: soluble maltase-glucoamylases, suckling rats.