Distinctions between intestinal and placental isoenzymes of alkaline phosphatase

Abstract

1. 1. Placental and intestinal alkaline phosphatases are equally inhibited by 0.005 M l-phenylalanine, a fact which complicates the interpretation of the values for l-phenylalanine-sensitive alkaline phosphatase of pregnancy serum which can be expected to have both phosphatases. 2. 2. Contrary to (human and rat) intestinal alkaline phosphatases (orthophosphoric monoester phosphohydrolase, EC 3.1.3.1) the isoenzymes of human and rat placental tissues are split by neuraminidase ( N-acetyl neuraminate glycohydrolase, EC 3.2.1.18) i.e. the anodic mobilities of the placental enzymes are appreciably reduced by prior incubation with neuraminidase. 3. 3. Advantage is also taken of the marked heat-stability of placental alkaline phosphatase and thus starch gel electrophoretic patterns of the placental and intestinal enzymes in heated and unheated specimens are different from each other. 4. 4. The pH optima of human intestinal and placental alkaline phosphatases are 9.8 and 10.6 with 18 m M phenylphosphate as substrate. The ratio of the enzyme activity at pH 10.6 to that at pH 9.8 is ten times higher for placenta than that for intestine (1.1 versus 0.11) with 2 m M phenylphosphate as substrate. 5. 5. Although the Michaelis constants of placental and intestinal alkaline phosphatase at pH 10.6 are the same, the K m-values of placental alkaline phosphatase at lower pH's are considerably higher than those of the intestinal isoenzymes. 6. 6. The above electrophoretic and kinetic differences may be employed to distinguish from each other human intestinal and placental alkaline phosphatase isoenzymes and may be utilized further to investigate the intestinal and placental origins of serum alkaline phosphatase.