Activation of phosphodiesterase in frog rod outer segment by an intermediate of rhodopsin photolysis I

Adaptation in vertebrate photoreceptors

When light is absorbed within the outer segment of a vertebrate photoreceptor, the conformation of the photopigment rhodopsin is altered to produce an activated photoproduct called metarhodopsin II or Rh(*). Rh(*) initiates a transduction cascade similar to that for metabotropic synaptic receptors and many hormones; the Rh(*) activates a heterotrimeric G protein, which in turn stimulates an effector enzyme, a cyclic nucleotide phosphodiesterase. The phosphodiesterase then hydrolyzes cGMP, and the decrease in the concentration of free cGMP reduces the probability of opening of channels in the outer segment plasma membrane, producing the electrical response of the cell. Photoreceptor transduction can be modulated by changes in the mean light level. This process, called light adaptation (or background adaptation), maintains the working range of the transduction cascade within a physiologically useful region of light intensities. There is increasing evidence that the second messenger responsible for the modulation of the transduction cascade during background adaptation is primarily, if not exclusively, Ca(2+), whose intracellular free concentration is decreased by illumination. The change in free Ca(2+) is believed to have a variety of effects on the transduction mechanism, including modulation of the rate of the guanylyl cyclase and rhodopsin kinase, alteration of the gain of the transduction cascade, and regulation of the affinity of the outer segment channels for cGMP. The sensitivity of the photoreceptor is also reduced by previous exposure to light bright enough to bleach a substantial fraction of the photopigment in the outer segment. This form of desensitization, called bleaching adaptation (the recovery from which is known as dark adaptation), seems largely to be due to an activation of the transduction cascade by some form of bleached pigment. The bleached pigment appears to activate the G protein transducin directly, although with a gain less than Rh(*). The resulting decrease in intracellular Ca(2+) then modulates the transduction cascade, by a mechanism very similar to the one responsible for altering sensitivity during background adaptation.

Inhibitory effect of NZ-105, a 1,4-dihydropyridine derivative, on cyclic nucleotide phosphodiesterase activity

The effects of NZ-105, a 1,4-dihydropyridine calcium antagonist, on the intracellular cyclic nucleotide system were investigated in-vitro. In rabbit isolated aorta, both NZ-105 (1 and 10 microM) and nicardipine significantly and in a concentration-dependent manner increased intracellular cyclic AMP and cyclic GMP content. NZ-105 inhibited bovine cardiac phosphodiesterase activity (Ki 30 microM) by competitive antagonism. The concentration ranges for inhibition were consistent with the range of increases in cyclic nucleotides.

Characterization of the light-induced increase in the Michaelis constant of the cGMP phosphodiesterase in frog rod outer segments

Activation of cGMP phosphodiesterase in rod disk membrane in the light is thought to be an intermediary process of phototransduction. In various preparations of frog rod outer segments, the Michaelis constant (Km) of the phosphodiesterase was measured with pH assay method. On illumination, the Km increased from the value of the dark (130 microM) by about 8-fold (1 mM) in crude preparations, but did not change appreciably in purified disk membranes, confirming the previous finding by Robinson et al. (Robinson, P.R., Kawamura, S., Abramson, B. and Bownds, M.D. (1980) J. Gen. Physiol. 76, 631-645). The present work further showed that the light-induced Km increase is labile to various experimental manipulations such as sonication, freeze-thawing, etc. However, the Km in the light was relatively high in a crude disk membrane preparation and in a lyzed preparation. In addition, reconstitution experiments revealed that the Km increase does not require soluble components. Both proteolytic digestion and phospholipase treatment reduced the light Km of the phosphodiesterase in crude disk membranes. The above results suggest that there is a labile factor(s) responsible for the light-induced Km increase and that the factor is a membrane-bound protein and requires structural integrity of the disk membrane to exert its function. The latency of the Km increase after light stimulation was less than 2 s.

Studies on structure and function of rhodopsin by use of cyclopentatrienylidene 11-cis-locked-rhodopsin

The photochemical reaction of cyclopentatrienylidene 11-cis-locked-rhodopsin derived from cyclopentatrienylidene 11-cis-locked-retinal and cattle opsin was spectrophotometrically studied. The difference absorption spectrum between the cyclopentatrienylidene 11-cis-locked-rhodopsin and its retinal oxime had its maximum at 495 nm (P-495). Irradiation of P-495 at -196 degrees C with either blue light or orange light caused no spectral change, supporting the cis-trans isomerization hypothesis for formation of bathorhodopsin. Upon irradiation of P-495 at 0 degree C with orange light, however, its absorption spectrum shifted to a shorter wavelength owing to formation of a hypsochromic product. The difference absorption spectrum between this product (P-466) and its retinal oxime showed its maximum at 466 nm. Analysis of retinal isomers by high-performance liquid chromatography showed that this spectral shift was not accompanied by photoisomerization of the chromophore. P-466 could almost completely be photoconverted to the original pigment (P-495) by irradiation at 0 degree C with blue light with little formation of the other isomeric form of its chromophore. The alpha-band of the circular dichroism spectrum of P-495 was very small in comparison with that of rhodopsin, while that of P-466 was comparable to it. These facts suggest that P-495 has a planar conformation in the side chain of the chromophore and that P-466 has a twisted one, probably at the C8-C9 single bond. Cyclic-GMP phosphodiesterase in frog rod outer segment was activated by neither P-495 nor P-466. This result suggests that the isomerization of the retinylidene chromophore of rhodopsin is indispensable in the phototransduction process.

Rhodopsin phosphorylation occurs at metarhodopsin II level

Photolyzed rhodopsin was phosphorylated in bovine rod outer segments incubated at -10 degrees C. In the experiment in which urea-treated outer segments and rhodopsin kinase were incubated with ATP in the presence of 30% glycerol, the extent of phosphate incorporation at -10 degrees C was about 30% of that at 37 degrees C. Separation of phosphorylated rhodopsin by isoelectric focusing indicated that a limited number of sites were phosphorylated at -10 degrees C. The partially phosphorylated pigment incorporated more phosphates when the temperatures was raised to 37 degrees C. This was partly due to decreased inhibition of phosphorylation by glycerol at higher temperature. Since the maximum phosphorylation at -10 degrees C (at which metarhodopsin II is stable) occurred at a pH value (6.0) lower than the pKa for metarhodopsin I-metarhodopsin II transition, metarhodopsin II was suggested to be the preferred substrate for rhodopsin kinase at -10 degrees C. Limited proteolysis with thermolysin of rhodopsin phosphorylated at 37 degrees C released peptides containing about 50% of the total phosphate incorporated. In contrast, proteolytic digestion of rhodopsin phosphorylated at -10 degrees C released negligible amounts of phosphate-containing peptides. The results were taken to suggest that the incorporation of phosphates at metarhodopsin II level under the present condition occurred in the residues other than those removed by thermolysin digestion.

Activation of phosphodiesterase by rhodopsin and its analogues

Activation of guanosine 3',5'-cyclic monophosphate (cGMP) phosphodiesterase (EC 3.1.4.35.) in frog rod outer segment membrane by rhodopsin and its analogues was investigated. The Schiff-base linkage between opsin and retinal in rhodopsin was not always necessary for the phosphodiesterase activation. The binding of beta-ionone ring of retinal to a hydrophobic region of opsin was not enough to induce the enzyme activation. A striking photo-activation of the enzyme was induced by photo-isomerization of rhodopsin analogues from cis to trans form. It seems probable that an "expanded" conformation of opsin around the retinylidene chromophore induced by the cis to trans isomerization may be the trigger for the activation of phosphodiesterase. On the other hand, the phosphodiesterase in frog rod outer segment was activated by warming of bathorhodopsin to -12 degrees C and then incubating it at the same temperature. Thus, metarhodopsin II or an earlier intermediate than metarhodopsin II should be a direct intermediate for the enzyme activation.

Activation of phosphodiesterase by chicken iodopsin

Activation of guanosine 3',5'-cyclic monophosphate phosphodiesterase in outer-segment membrane of chicken retina was investigated. Irradiation of dark-adapted chicken outer segment membrane for bleaching of iodopsin increased the enzyme activity twice as much as that in the dark in the presence of GTP. Further irradiation of the sample for bleaching of rhodopsin in the membrane induced some additional activation of the enzyme. However, chicken iodopsin activated the enzyme in frog rod outer segment membrane without irradiation, while chicken rhodopsin did not. Irradiation of chicken iodopsin increased the enzyme activity twice as much as that in the dark.

Activation of phosphodiesterase in frog rod outer segment by rhodopsin analogues

Activation of guanosine 3',5'-cyclic monophosphate (cGMP) phosphodiesterase (EC 3.1.4.35) in frog rod outer segment membrane by rhodopsin analogues has been investigated. A rhodopsin analogue modified at the Schiff-base linkage (N-retinyl-opsin) or the beta-ionone ring (3-dehydro-rhodopsin) in the retinylidene chromophore of rhodopsin has some ability in activation of the enzyme. In consideration of our previous observation that opsin including a retinal-oxime can activate the enzyme, it seems likely that the Schiff-base linkage is not always necessary for the phosphodiesterase activation. On the other hand, a change in the length of the side chain of retinal (complex of opsin and beta-ionone, beta-ionylideneacetaldehyde or retinylideneacetaldehyde) or dissection of the conjugate double-bond system of the side chain (retro-gamma-rhodopsin) remarkably reduces the activation ability. However, 5,8-epoxy-rhodopsin having a similar dissected conjugate double-bond system induces some enzyme activation because of its rigid conformation around C7-C8-C9 single bonds. Consequently, it is suggested that the necessary portion of rhodopsin chromophore for the activation of the enzyme is the rigid conjugate double-bond system between the beta-ionone ring and the Schiff-base linkage in its all-trans form.

Activation of phosphodiesterase in frog rod outer segment by an intermediate of rhodopsin photolysis. II

Frog (Rana catesbeiana) rod outer segment membrane contains cyclic GMP phosphodiesterase (EC 3.1.4.1). Irradiation of dark-adapted rod outer segment membrane increased the enzyme activity by 5-20-fold in the presence of GTP. The phosphodiesterase in rod outer segment membrane is also activated by mixing a photo-product of 11-cis (regenerated), 9-cis or 7-cis rhodopsin which is stable at 0 degrees C. However, neither opsin in the membrane nor all-trans retinal activates the enzyme. The phosphodiesterase in rod outer segment membrane is also activated by irradiation at -4 degrees C. Thus, we conclude that the phosphodiesterase in activated by a common photolysis intermediate of these rhodopsin isomers, perhaps before metarhodopsin II decays.