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A rat liver lysosomal membrane flavin-adenine dinucleotide phosphohydrolase: purification and characterization
Abstract:
An enzyme hydrolyzing flavin-adenine dinucleotide (FAD) to flavin mononucleotide and AMP was identified and purified from rat liver lysosomal (Tritosomal) membranes. The purified enzyme showed a single band on silver-stained denaturing gels with an apparent Mr 70,000. Periodate-Schiff staining after denaturing gel electrophoresis of whole membrane preparations revealed that this enzyme is one of the major glycoproteins in lysosomal membranes. FAD appeared to be the preferred substrate for the purified enzyme; equivalent concentrations of NAD or CoA were hydrolyzed at about one-half of the FAD rate. Negligible activity (less than or equal to 16%) was noted with ATP, TTP, ADP, AMP, FMN, pyrophosphate, or p-nitrophenylphosphate. The enzyme was inhibited by EDTA or dithiothreitol. It was stimulated by Zn, and was not affected by Ca or Mg ions, nor by p-chloromercuribenzoate. The pH optimum for FAD hydrolysis was 8.5-9 with an apparent Km of 0.125 mM. Antibodies prepared against the purified enzyme partially (50%) inhibited FAD phosphohydrolase activity in lysosomal membrane preparations but had no effect on the soluble lysosomal acid pyrophosphatase known to hydrolyze FAD. This enzyme could not be detected immunochemically in preparations of microsomes, Golgi, plasma membranes, mitochondrial membranes, or the soluble lysosomal fraction, suggesting that the enzyme is different from either soluble lysosomal acid pyrophosphatase or other FAD hydrolyzing activities in the liver cell.
Insights
Researchers identified and purified a novel enzyme from rat liver lysosomal membranes that specifically hydrolyzes flavin-adenine dinucleotide (FAD). This FAD phosphohydrolase enzyme is distinct from other known FAD-hydrolyzing activities in liver cells.
Area of Science:
- Biochemistry
- Cell Biology
- Enzymology
Background:
- Flavin adenine dinucleotide (FAD) is a crucial coenzyme involved in various metabolic processes.
- Understanding the enzymes that regulate FAD metabolism is essential for comprehending cellular function.
- Lysosomal membranes contain various enzymes, but their specific roles in nucleotide metabolism are not fully elucidated.
Purpose of the Study:
- To identify and characterize an enzyme responsible for hydrolyzing flavin-adenine dinucleotide (FAD) in rat liver lysosomal membranes.
- To determine the substrate specificity, kinetic properties, and cellular localization of the purified enzyme.
- To differentiate this enzyme from other known FAD-hydrolyzing enzymes in liver cells.
Main Methods:
- Enzyme purification from rat liver lysosomal (tritosomal) membranes.
- Characterization of enzyme properties using denaturing gel electrophoresis, substrate specificity assays, and kinetic analysis (pH optimum, Km).
- Immunochemical analysis using antibodies against the purified enzyme to assess its localization and differentiate it from other enzymes.
Main Results:
- A novel FAD-hydrolyzing enzyme (FAD phosphohydrolase) was purified from rat liver lysosomal membranes, showing a single band at Mr 70,000.
- The enzyme preferentially hydrolyzed FAD over other nucleotides (NAD, CoA, ATP, etc.) and exhibited optimal activity at pH 8.5-9.
- Immunochemical studies confirmed the enzyme's presence in lysosomal membranes and its absence in other cellular fractions, distinguishing it from soluble lysosomal acid pyrophosphatase.
Conclusions:
- A distinct FAD phosphohydrolase is localized in rat liver lysosomal membranes.
- This enzyme plays a specific role in FAD metabolism within the lysosomal compartment.
- The findings suggest a unique function for this glycoprotein in lysosomal membrane-associated nucleotide catabolism.