A neutral beta-galactosidase from the hepatopancreas of the shrimp Penaeus monodon (Crustacea: Decapoda): dimeric and sialyated

Abstract

1. 1. A β-galactosidase was purified ca 245-fold to homogeneity from Penaeus monodon, with a final spec. act. of 61.3 U/mg of protein. 2. 2. By using SDS-polyacrylamide gel electrophoresis, the monomers of shrimp β-galactosidase were discovered to have mol. wts of 31,000 and those of human placental enzyme, 32,000. Since the active shrimp β-galactosidase was found to have a mol. wt of 66,000 by AcA 34 gel filtration chromatography, it was concluded that the purified shrimp enzyme was dimeric. 3. 3. In contrast to the discovery of thermostability with human placental β-galactosidase, the shrimp enzyme was found to be unstable to heating at 45°C for 10 min. Both enzyme activities were inhibited by Mn 2+ and Zn 2+ ions. 4. 4. The shrimp β-galactosidase has an isoelectric point (pI) of 7.0, but the human placental enzyme has a pI of 5.5. Both enzymes were sialyated. 5. 5. The shrimp β-galactosidase has a pH optimum at 7.0 and its K m was 1.9 μM with 4-methylumbelliferyl-β- d-galactoside as substrate. The human enzyme has pH optimum at 7.0 or 4.0, and its K m was 9.8 μM.