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Phosphocholine phosphatase and alkaline phosphatase are different enzymes in hamster heart
1Department of Biochemistry, Faculty of Medicine, University of Manitoba, Winnipeg, Canada.
Insights
A cardiac phosphocholine phosphatase hydrolyzes phosphocholine back to choline in hamster hearts. This enzyme may regulate phosphocholine pool size, distinct from alkaline phosphatase.
Area of Science:
- Biochemistry
- Cardiovascular Science
- Cellular Metabolism
Background:
- The CDP-choline pathway is crucial for phosphatidylcholine synthesis in the heart.
- Phosphocholine formation from choline is the initial committed step.
- Cardiac phosphocholine pools are smaller than in other tissues, with significant hydrolysis observed.
Purpose of the Study:
- To characterize the phosphocholine phosphatase in the hamster heart.
- To differentiate cardiac phosphocholine phosphatase from alkaline phosphatase.
- To investigate the role of this phosphatase in regulating cardiac phosphocholine levels.
Main Methods:
- Enzyme localization studies (microsomal fraction).
- Enzyme kinetics (pH optima, heat sensitivity, substrate inhibition).
- Comparative analysis with alkaline phosphatase activity.
- Investigated effects of amino acids and p-nitrophenylphosphate on hydrolysis.
Main Results:
- A distinct phosphocholine phosphatase activity was identified in the cardiac microsomal fraction.
- This enzyme exhibited different pH optima and heat sensitivity compared to alkaline phosphatase.
- It was not inhibited by amino acids, unlike alkaline phosphatase.
- Kinetic analysis suggested separate enzymes for phosphocholine and p-nitrophenylphosphate hydrolysis.
Conclusions:
- Cardiac phosphocholine phosphatase is a distinct enzyme from alkaline phosphatase.
- The enzyme is not inhibited by amino acids.
- Cardiac phosphocholine phosphatase likely plays a regulatory role in maintaining phosphocholine pool size in the hamster heart.
Abstract:
The CDP-choline pathway is the major pathway for the synthesis of phosphatidylcholine in the hamster heart. The formation of phosphocholine from choline was regarded as the first committed reaction in this pathway. We demonstrated earlier that the phosphocholine pool in the heart was substantially less than that found in other tissues, and we observed that a substantial amount of the phosphocholine was hydrolyzed back to choline by a phosphatase. This phosphatase was located in the microsomal fraction of the heart, and unlike alkaline phosphatase, it was not inhibited by amino acids. The pH optima and heat sensitivity of phosphocholine phosphatase were also found to differ from alkaline phosphatase. Phosphocholine did not inhibit the hydrolysis of p-nitrophenylphosphate, but a "mixed type" inhibition of the hydrolysis of phosphocholine was observed in the presence of p-nitrophenylphosphate. Our data support the hypothesis that these two activities originate from separate and distinct enzymes, and we postulate that the cardiac phosphocholine phosphatase may play a role in the regulation of the phosphocholine pool size in the hamster heart.