The effect of sucrose and other carbohydrates on human alkaline phosphatase isoenzyme activity.

The modification of alkaline phosphatase (ALP) activity by ten carbohydrates (d-isomers) was studied. These included three disaccharides : sucrose, lactose and maltose; four hexoses: glucose, galactose, mannose and fructose; and three pentoses: arabinose, ribose and xylose.In all cases the inhibition of placental isoenzyme activity was in proportion to the concentration of carbohydrate. Intestinal and hepatic ALP was activated at low carbohydrate concentrations and inhibited at high concentration. The exception was maltose which inhibited hepatic alkaline phosphatase markedly at all concentrations.Lineweaver-Burk plots generated by kinetic studies at pH 9.8, 10.15, 10.5 and 10.7 showed a mixed-type inhibition.With the use of sucrose in the assay medium it was possible to distinguish two types of term placental-like ALP isoenzymes of tumor origin, which differ in their sensitivity to inhibition by l-leucine. The Nagao type (d-leucine sensitive) was inhibited more by sucrose than was the non-d-leucine sensitive isoenzyme.The inhibitory effect of carbohydrates on placental ALP activity was independent of pH and substrate concentration, unlike the modifying effect on the activity of intestinal and hepatic enzyme by the carbohydrates studied. It is suggested that carbohydrate-rich media may accentuate conformational differences of the various isoenzymes with consequent effects on the availability of the active sites to the substrate.