Related Experiment Videos
Analysis of recombinant Phex: an endopeptidase in search of a substrate
1Department of Medicine, Duke University Medical Center, Durham, North Carolina 27710, USA.
Abstract:
X-linked hypophosphatemia (XLH) is caused by inactivating mutations of Phex, a phosphate-regulating endopeptidase. Further advances in our knowledge of the pathogenesis of XLH require identification of the biological function of Phex and its physiologically relevant substrates. We evaluated several potential substrates using mouse recombinant wild-type Phex proteins (rPhex-WT) and inactive mutant Phex proteins (rPhex-3'M) lacking the COOH-terminal catalytic domain as controls. By Western blot analysis, we demonstrated that Phex is a membrane-bound 100-kDa glycosylated monomer. Neither casein, a substrate for the related endopeptidase thermolysin, human stanniocalcin 1 (hSTC-1), an osteoblast-derived phosphate-regulating factor, nor FGF-23 peptide (amino acid 172-186), comprising the region mutated in autosomal dominant hypophosphatemia, was cleaved by rPhex-WT. In addition, membranes expressing rPhex-WT, rPhex-3'M, and the empty vector hydrolyzed parathyroid hormone-(1-34), indicating the lack of Phex-specific cleavage of parathyroid hormone. In contrast, rPhex-WT did display an EDTA-dependent cleavage of the neutral endopeptidase substrate [Leu]enkephalin. Further studies with wild-type and mutant rPhex proteins should permit the identification of physiologically relevant substrates involved in the pathogenesis of XLH.
Insights
X-linked hypophosphatemia (XLH) results from Phex mutations. Researchers used recombinant Phex proteins to test potential substrates, finding Phex cleaves [Leu]enkephalin but not hSTC-1 or FGF-23.
Area of Science:
- Biochemistry
- Genetics
- Molecular Biology
Background:
- X-linked hypophosphatemia (XLH) is a genetic disorder characterized by impaired phosphate regulation.
- The precise biological function of Phex, the gene mutated in XLH, and its substrates remain largely unknown.
- Identifying Phex substrates is crucial for understanding XLH pathogenesis.
Purpose of the Study:
- To investigate the enzymatic activity of Phex and identify its physiologically relevant substrates.
- To characterize the biochemical properties of Phex, including its cellular localization and post-translational modifications.
Main Methods:
- Utilized mouse recombinant wild-type Phex proteins (rPhex-WT) and inactive mutant Phex proteins (rPhex-3'M).
- Employed Western blot analysis to assess Phex properties and cleavage assays for potential substrates.
- Tested casein, human stanniocalcin 1 (hSTC-1), FGF-23 peptide, parathyroid hormone-(1-34), and [Leu]enkephalin as substrates.
Main Results:
- Phex was identified as a membrane-bound, 100-kDa glycosylated monomer.
- rPhex-WT did not cleave casein, hSTC-1, or FGF-23 peptide.
- EDTA-dependent cleavage of [Leu]enkephalin by rPhex-WT was observed, suggesting Phex acts as a neutral endopeptidase.
- Non-specific hydrolysis of parathyroid hormone-(1-34) occurred in membranes expressing Phex variants and empty vector controls.
Conclusions:
- Phex exhibits enzymatic activity, specifically cleaving [Leu]enkephalin in an EDTA-dependent manner.
- This study rules out casein, hSTC-1, and FGF-23 as direct Phex substrates.
- Further investigation using wild-type and mutant Phex proteins is needed to identify substrates directly involved in XLH pathogenesis.