Cyclic nucleotide phosphodiesterases associated with bovine retinal outer-segment fragments

Area of Science:

• Biochemistry
• Phototransduction
• Retinal Physiology

Background:

• Bovine retinal outer segments contain ATP-dependent cyclic GMP phosphodiesterase.
• Light stimulation is known to affect phosphodiesterase activity in some species.

Purpose of the Study:

• To investigate light-induced activation of cyclic GMP phosphodiesterase in bovine retinal outer segments.
• To compare the substrate specificity and light sensitivity of this enzyme with previous findings.

Main Methods:

• Preparation of bovine retinal outer-segment fragments.
• Assay of phosphodiesterase activity with cyclic GMP and cyclic AMP as substrates.
• Investigation of enzyme activity following light exposure and various treatments (sucrose washing, sonication, cold storage).

Main Results:

• Light significantly stimulated cyclic GMP phosphodiesterase activity (approximately tenfold).
• No light stimulation of phosphodiesterase activity was observed with cyclic AMP as a substrate.
• Enzyme activity was recovered after sucrose washing but was no longer light-sensitive.
• Kinetic analysis revealed two Km values for cyclic AMP (5 mM and 0.05 mM) and one for cyclic GMP (0.22 mM).
• Storage at -76°C preserved light sensitivity, while sonication or storage at -20°C abolished it.

Conclusions:

• Bovine retinal outer-segment phosphodiesterase is light-activated specifically for cyclic GMP.
• The mechanism of activation differs from previous reports on frog systems regarding rhodopsin bleaching levels.
• Enzyme stability and light responsiveness are dependent on preparation and storage conditions.