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Extracellular cGMP phosphodiesterase related to the rod outer segment phosphodiesterase isolated from bovine and
Abstract:
A phosphodiesterase (PDE) has been characterized in the interphotoreceptor matrix (IPM) of light-adapted fresh bovine retinas. It is obtained through a gentle rinsing of the retinal surface under conditions where the light-activated rod outer segment (ROS) enzyme remains attached. The enzyme has an apparent native molecular weight of 350 000 by gel filtration and appears as a doublet at Mr 47 000 and 45 000 on sodium dodecyl sulfate-polyacrylamide gels. It has an apparent Km value for cGMP of 33 microM and an apparent Km value for cAMP of 2200 microM. It is activated 3-6-fold by protamine and over 40-fold by trypsin. Protamine has no effect on the Km for cGMP while trypsin decreases the Km for cGMP by a factor of 2. The enzyme occurs in at least two forms as evidenced by two distinct peaks of activity after gel electrophoresis under nondenaturing conditions. A heat-stable inhibitor is tightly bound to the enzyme. The inhibitor obtained from the IPM PDE inhibits 98% of the activity of the trypsin-activated ROS PDE: conversely, the inhibitor obtained by boiling the ROS PDE completely inhibits the trypsin-activated IPM enzyme. A high-affinity monoclonal antibody to the active site of the ROS PDE, ROS 1 [Hurwitz, R., Bunt-Milan, A.H., & Beavo, J. (1984) J. Biol. Chem. 259, 8612-8618], quantitatively absorbs the IPM PDE. These observations indicate a clear relationship between these two PDEs even though their location, sizes, and specific functions in the retina appear to be distinct.
Insights
Researchers identified a phosphodiesterase (PDE) in bovine retinal interphotoreceptor matrix (IPM). This IPM PDE shares a relationship with the rod outer segment (ROS) PDE, despite distinct locations and functions.
Area of Science:
- Biochemistry
- Retinal Physiology
- Enzymology
Background:
- The interphotoreceptor matrix (IPM) is a unique extracellular space in the retina.
- Phosphodiesterases (PDEs) play critical roles in visual signal transduction.
- Characterizing enzymes within the IPM provides insight into retinal function.
Purpose of the Study:
- To isolate and characterize a phosphodiesterase (PDE) from the bovine retinal interphotoreceptor matrix (IPM).
- To compare the properties of the IPM PDE with the well-characterized rod outer segment (ROS) PDE.
- To investigate potential relationships and distinctions between these two retinal PDEs.
Main Methods:
- Isolation of IPM PDE from light-adapted bovine retinas via gentle rinsing.
- Biochemical characterization including gel filtration and SDS-PAGE for molecular weight determination.
- Enzyme kinetic analysis (Km values for cGMP and cAMP) and activity assays with activators (protamine, trypsin).
- Analysis of enzyme forms using non-denaturing gel electrophoresis.
- Investigation of inhibitor interactions and cross-reactivity with a monoclonal antibody against ROS PDE.
Main Results:
- A PDE was successfully isolated from the IPM with a native molecular weight of 350,000 Da and subunits of 47,000/45,000 Da.
- The IPM PDE exhibited distinct kinetic properties (Km for cGMP: 33 µM, Km for cAMP: 2200 µM) compared to ROS PDE.
- Enzyme activity was modulated by protamine and trypsin, with trypsin significantly decreasing Km for cGMP.
- The enzyme exists in multiple forms and is associated with a heat-stable inhibitor that cross-reacts with ROS PDE components.
- A monoclonal antibody specific to ROS PDE also bound the IPM PDE, indicating structural similarities.
Conclusions:
- The IPM contains a distinct phosphodiesterase (PDE) with unique biochemical and kinetic properties.
- Despite differences in location and size, the IPM PDE shares significant antigenic and inhibitory relationships with the ROS PDE.
- These findings suggest a complex interplay of PDEs within the retina, potentially involving shared origins or regulatory mechanisms.