Glycosphingolipid glycosyl hydrolases and glycosidases of synchronized human KB cells

KB cells were synchronized by a double thymidine block procedure. An investigation was made of the activities of αL fucosidase (EC 3.2.1.51), αD galactosidase (EC 3.2.1.22), βD galactosidase (EC 3.2.1.23), αD glucosidase (EC 3.2.1.20), βD glucosidase (EC 3.2.1.21), αD mannosidase (EC 3.2.1.24), βD N acetylgalactosaminidase (EC 3.2.1.53), BD D N acetylglucosaminidase (EC 3.2.1.52) from synchronized cultures, using appropriate artificial substrates. Ceramide glucosidase (EC 3.2.1.45) and ceramide trihexosidase levels (EC 3.2.1.47) were also investigated at various stages in the cell cycle, using appropriate glycosphingolipid substrates. Whereas each of these enzymes exhibited some activity throughout the cell cycle, peak activity (2 to 6 fold increase) occurred late in the S phase. Two molecular forms of ceramide glucosidase (optimal activity at pH 4.0 and pH 6.0) and two forms of ceramide trihexosidase (pH 4.0 and pH 7.5) were identified. Peak levels of the forms that preferred the relatively acid pH occurred earlier in the S phase of the cell cycle than those of the forms that were more active at the higher pH. The possibility that the forms with optimal activity at pH 4 are precursors of those with optimal activity at pH 6 to 7.5 is discussed. Precipitation of β galactosidase of synchronized KB cells with specific antibody revealed that changes in the activity of this enzyme during the cell cycle were the result of fluctuations in the amount of the enzyme.