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Enzymes and binding proteins affecting retinoic acid concentrations
J L Napoli1, M H Boerman, X Chai
1Department of Biochemistry, School of Medicine and Biomedical Sciences, SUNY-Buffalo 14214, USA.
Summary
Cellular binding proteins control retinoid metabolism in vivo, ensuring precise regulation of retinoic acid (RA) concentrations. This mechanism prevents uncontrolled synthesis by enzymes, maintaining RA homeostasis.
Area of Science:
- Biochemistry
- Molecular Biology
- Cell Biology
Background:
- Free retinoids undergo non-specific metabolism in vitro.
- In vivo, retinoids are bound by high-affinity proteins like cellular retinol-binding protein (CRBP) and cellular retinoic acid-binding protein (CRABP).
- These binding proteins are crucial for regulating retinoid homeostasis.
Purpose of the Study:
- To elucidate the role of retinoid-binding proteins in controlling retinoid metabolism and retinoic acid (RA) homeostasis in vivo.
- To investigate the enzymatic pathways involved in RA biosynthesis and metabolism mediated by CRBP and CRABP.
Main Methods:
- Enzyme kinetics and chemical crosslinking to characterize substrate recognition.
- Cloning, expression, and characterization of retinol dehydrogenase (RDH) and retinal dehydrogenase (RalDH) cDNAs.
- Purification and substrate specificity analysis of RalDH.
Main Results:
- An NADP-dependent microsomal retinol dehydrogenase (RDH) recognizes CRBP as a substrate.
- RDH catalyzes the conversion of retinol from holo-CRBP to retinal.
- An NAD-dependent retinal dehydrogenase (RalDH) synthesizes RA from retinal, and CRABP modulates RA levels by sequestering RA.
Conclusions:
- Retinoid-binding proteins confer specificity to RA homeostasis by restricting enzyme access.
- The CRBP-RDH-RalDH pathway and CRABP are critical for precise control of retinoid concentrations in vivo.
- This regulated pathway prevents opportunistic retinoid synthesis and maintains functional retinoid levels.