Organic pyrophosphates as substrates for human alkaline phosphatases

Area of Science:

• Biochemistry
• Enzymology

Background:

• Alkaline phosphatases are crucial enzymes involved in phosphate metabolism.
• Understanding the substrate specificity and regulatory mechanisms of alkaline phosphatases is essential for comprehending cellular phosphate homeostasis.

Purpose of the Study:

• To investigate the pyrophosphatase activity of purified human liver and small-intestinal alkaline orthophosphatases.
• To determine if orthophosphatase and pyrophosphatase activities reside in the same enzyme molecule.
• To compare the relative pyrophosphatase and orthophosphatase activities between liver and intestinal alkaline phosphatases.

Main Methods:

• Enzymatic assays using various organic pyrophosphate substrates.
• Enzyme kinetics studies, including mixed-substrate experiments.
• Denaturation studies using heat and urea.
• Starch-gel electrophoresis for enzyme characterization.

Main Results:

• Purified human liver and intestinal alkaline orthophosphatases demonstrate significant inorganic phosphate release from pyrophosphate substrates.
• Magnesium ions inhibited the pyrophosphatase activity at concentrations that activated orthophosphate hydrolysis.
• Evidence from multiple experimental approaches indicated that both activities are intrinsic properties of a single enzyme.
• The intestinal alkaline phosphatase exhibited a higher relative pyrophosphatase activity compared to the liver enzyme.

Conclusions:

• Human liver and intestinal alkaline orthophosphatases possess dual enzymatic functions, acting as both orthophosphatases and pyrophosphatases.
• The observed inhibition by magnesium suggests a complex regulatory mechanism for pyrophosphatase activity.
• These findings highlight distinct biochemical properties between liver and intestinal alkaline phosphatases, with implications for their physiological roles.