Oxidative stress in myelin sheath: The other face of the extramitochondrial oxidative phosphorylation ability

Oxidative phosphorylation (OXPHOS) is not only the main source of ATP for the cell, but also a major source of reactive oxygen species (ROS), which lead to oxidative stress. At present, mitochondria are considered the organelles responsible for the OXPHOS, but in the last years we have demonstrated that it can also occur outside the mitochondrion. Myelin sheath is able to conduct an aerobic metabolism, producing ATP that we have hypothesized is transferred to the axon, to support its energetic demand. In this work, spectrophotometric, cytofluorimetric, and luminometric analyses were employed to investigate the oxidative stress production in isolated myelin, as far as its respiratory activity is concerned. We have evaluated the levels of malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE), markers of lipid peroxidation, as well as of hydrogen peroxide (H2O2), marker of ROS production. To assess the presence of endogenous antioxidant systems, superoxide dismutase, catalase, and glutathione peroxidase activities were assayed. The effect of certain uncoupling or antioxidant molecules on oxidative stress in myelin was also investigated. We report that isolated myelin produces high levels of MDA, 4-HNE, and H2O2, likely through the pathway composed by Complex I-III-IV, but it also contains active superoxide dismutase, catalase, and glutathione peroxidase, as antioxidant defense. Uncoupling compounds or Complex I inhibitors increase oxidative stress, while antioxidant compounds limit ROS generation. Data may shed new light on the role of myelin sheath in physiology and pathology. In particular, it can be presumed that the axonal degeneration associated with myelin loss in demyelinating diseases is related to oxidative stress caused by impaired OXPHOS.

Extra-mitochondrial aerobic metabolism in retinal rod outer segments: new perspectives in retinopathies

Vertebrate retinal rods are photoreceptors for dim-light vision. They display extreme sensitivity to light thanks to a specialized subcellular organelle, the rod outer segment. This is filled with a stack of membranous disks, expressing the proteins involved in visual transduction, a very energy demanding process. Our previous proteomic and biochemical studies have shed new light on the chemical energy processes that supply ATP to the outer segment, suggesting the presence of an extra-mitochondrial aerobic metabolism in rod outer segment, devoid of mitochondria, which would account for a quantitatively adequate ATP supply for phototransduction. Here the functional presence of an oxidative phosphorylation in the rod outer limb is examined for its relationship to many physiological and pathological data on the rod outer segment. We hypothesize that the rod outer limb is at risk of oxidative stress, in any case of impairment in the respiratory chain functioning, or of blood supply. In fact, the electron transfer chain is a major source of reactive O(2) species, known to produce severe alteration to the membrane lipids, especially those of the outer segment that are rich in polyunsaturated fatty acids. We propose that the disk membrane may become the target of reactive oxygen species that may be released by the electron transport chain under pathologic conditions. For example, during aging reactive oxygen species production increases, while cellular antioxidant capacity decreases. Also the apoptosis of the rod observed after exposure to bright or continuous illumination can be explained considering that an overfunctioning of phototransduction may damage the disk membrane to a point at which cytochrome c escapes from the intradiskal space, where it is presently supposed to be, activating a putative caspase 9 and the apoptosome. A pathogenic mechanism for many inherited and acquired retinal degenerations, representing a major problem in clinical ophthalmology, is proposed: a number of rod pathologies would be promoted by impairment of energy supply and/or oxidative stress in the rod outer segment. In conclusion we suppose that the damaging role of oxygen, be it hypoxia or hyperoxia invoked in most of the blinding diseases, acquired and even hereditary is to be seeked for inside the photoreceptor outer segment that would conceal a potential for cell death that is still to be recognized.

Creatine ethyl ester: a new substrate for creatine kinase

The creatine kinase/phosphocreatine system plays a key role in cell energy buffering and transport, particularly in cells with high or fluctuating energy requirements, like neurons, i.e. it participates in the energetic metabolism of the brain. Creatine depletion causes several nervous system diseases, alleviated by phosphagen supplementation. Often, the supplementation contains both creatine and creatine ethyl ester, known to improve the effect of creatine through an unknown mechanism. In this work we showed that purified creatine kinase is able to phosphorilate the creatine ethyl ester. The K(m) and V(max) values, as well as temperature and pH optima were determined. Conversion of the creatine ethyl ester into its phosphorylated derivative, sheds light on the role of the creatine ethyl ester as an energy source in supplementation for selected individuals.

Oligomerization studies of Leuconostoc mesenteroides G6PD activity after SDS-PAGE and blotting

Glucose-6-phosphate 1 dehydrogenase (G6PD) is a ubiquitous enzyme catalyzing the oxidation of D-glucose 6-phosphate to D-glucono-lactone 6-phosphate, in the first step of the pentose phosphate pathway. Based on the currently available structural information on Leuconostoc mesenteroides G6PD, it is believed that the enzyme only works as a homodimer. Here we show that both after non-denaturing and after denaturing electrophoretic separation (SDS-PAGE) and blotting L. mesenteroides G6PD retains its complete catalytic activity. In the two latter cases the molecular weight of the band corresponded to that of a G6PD monomer. Conversely, when the same technique was applied to G6PD from Saccharomyces cerevisiae, another fermentative organism, the monomer activity was not detectable after SDS-PAGE and blotting. The results are discussed in terms of molecular evolution of the oligomeric state in the various G6PD sources.

Effects of extremely low frequency electromagnetic fields on membrane-associated enzymes

The effects of extremely low frequency electromagnetic fields of 75 Hz were studied on different membrane-associated enzymes. Only the activities of three enzymes out of seven exposed to the field decreased approximately of about 54-61% with field amplitudes above a threshold of 73-151 microT depending on the enzyme. The same field had no effect on the activities of either integral membrane enzymes such as Ca,ATPase, Na/K,ATPase, and succinic dehydrogenase or peripheral membrane enzymes such as photoreceptor PDE. The decrease in enzymatic activity of the field-sensitive enzymes was independent of the time of permanence in the field and was completely reversible. When these enzymes were solubilized with Triton, no effect of the field was obtained on the enzymatic activity, suggesting the crucial role of the membrane in determining the conditions for enzyme inactivation. The role of the particular linkage of the field-sensitive enzymes to the membranes is also discussed.

ATP synthesis in the disk membranes of rod outer segments of bovine retina

ATP is synthesized on the disk membrane isolated from rod outer segments of the bovine retina. Together with a slow component which accounted for a constant rate of about 22 nmol ATP/min/mg of protein and which was due to the adenylate kinase activity, a fast component with a maximal activity of about 58 nmol ATP/min/mg of protein was measured at physiological calcium concentrations. This fast activity disappeared in the presence of the Ca(2+) ionophore A23187, was inhibited by vanadate or thapsigargin but not by oligomycin, suggesting that this ATP synthesis is due to the reversal functioning of the Ca(2+)-ATPase previously found on the disk membranes.

Free radical-dependent Ca2+ signaling: role of Ca2+-induced Ca2+ release

Previously we have shown that Fe3+/ascorbate-induced Ca2+ release from scallop sarcoplasmic reticulum (SR) is due to Ca2+-channel gating by free radicals. This study is aimed at demonstrating that Ca2+-induced Ca2+ release (CICR) plays a role in this kind of Ca2+ release. Scallop SR vesicles were incubated with fluo-3 and exposed to Fe3+/ascorbate. Fluorimetric recordings showed massive Ca2+ release, with maximum rate and 50% release occurring at 30 min after exposure. Conversely, the use of the probe for reactive oxygen species dihydrorhodamine or the assay of malondialdehyde allowed oxyradical production to be traced for approximately 5 min only. Hence, although Ca2+ release started just after exposure to Fe3+/ascorbate, most release occurred after free radical exhaustion. Ruthenium red addition after Fe3+/ascorbate slowed down the Ca2+ release, whereas cyclic adenosine 5'-diphosphoribose addition accelerated it, indicating that the free radical-induced Ca2+ release from SR vesicles triggers a mechanism of CICR that dramatically increases the initial effect.

Effects of heavy metals on phospholipase C in gill and digestive gland of the marine mussel Mytilus galloprovincialis Lam

We studied the in vivo and in vitro effects of Hg2+ and Cu2+ on the activity of phospholipase C (PLC), specific for phosphatidylinositol 4,5-bisphosphate, in the mussel (Mytilus galloprovincialis Lam). The enzyme activity was assayed in tissue homogenates from gills and digestive gland. The toxic effect of Hg2+ appeared to be stronger than that of Cu2+ both in vitro and in vivo, especially for the digestive gland. In in vitro tests, Hg2+ was able to inhibit PLC activity when added directly to the reaction mixture. Conversely, Cu2+ was effective only after preincubation, suggesting that the effect of the metal may be derived from lipid peroxidation due to Cu2+-induced oxyradical production. Treatment of mussels with sublethal concentrations of Hg2+ or Cu2+ in vivo produced significant PLC inhibition after 1 or 4 days, respectively. A recovery was reached after 7 days of in vivo metal incubation. Data indicate that in mussel gills and digestive gland heavy metals impair PLC activity, thereby affecting IP3-dependent Ca2+ signaling.

Interference of heavy metal cations with fluorescent Ca2+ probes does not affect Ca2+ measurements in living cells

In studies about the effects of heavy metals on intracellular Ca2+, the use of fluorescent probes is debated, as metal cations are known to affect the probe signal. In this study, spectrofluorimetric experiments in free solution, using Fluo-3 and Fura-2, showed that Zn2+ and Cd2+ enhanced the probe signal, Cu2+ quenched it, and Hg2+ had no effect. Addition of GSH prevented most of these effects, suggesting the occurrence of a similar protective role in living cells. Digital imaging of living mussel haemocytes loaded with Fura-2/AM or Fluo-3/AM showed that Hg2+, Cu2+ and Cd2+ induced a rise in probe fluorescence, whereas up to 200 microM Zn2+ had no effect. In particular, Cd2+ produced the strongest probe signal rise in free solution, but the lowest fluorescence increase in cells. Probe calibration yielded [Ca2+]i values characteristic of resting levels in control and Zn2+-exposed cells, and, as expected, indicated Ca2+ homeostasis impairment in cells exposed to Cd2+, Cu2+ and Hg2+. Our results show that Ca2+ probe responses to heavy metals in living cells are completely different from those obtained in free solution, indicating that fluorescent probes can be a suitable tool to record the effects of heavy metals on [Ca2+]i.

Antioxidant role of metallothioneins: a comparative overview

Metallothioneins (MTs) are sulfhydryl-rich proteins binding essential and non-essential heavy metals. MTs display in vitro oxyradical scavenging capacity, suggesting that they may specifically neutralize hydroxyl radicals. Yet, this is probably an oversimplified view, as MTs represent a superfamily of widely differentiated metalloproteins. MT antioxidant properties mainly derive from sulfhydryl nucleophilicity, but also from metal complexation. Binding of transition metals displaying Fenton reactivity (Fe,Cu) can reduce oxidative stress, whereas their release exacerbates it. In vertebrates, MT gene promoters contain metal (MRE) and glucocorticoid response elements (GRE), Sp and AP sequences, but also antioxidant response elements (ARE). MT neosynthesis is induced by heavy metals, cytokines, hormones, but also by different oxidants and prooxidants. Accordingly, MT overexpression increases the resistance of tissues and cells to oxidative stress. As for invertebrates, data from the mussel show that MT can actually protect against oxidative stress, but is poorly inducible by oxidants. In yeast, there is a Cu(I)-MT that in contrast to mammalCu-MT exhibits antioxidant activity, possibly due to differences in metal binding domains. Finally, as the relevance of redox processes in cell signaling is becoming more and more evident, a search for MT effects on redox signaling could represent a turning point in the understanding of the functional role of these protein.

ATP synthesis in rod outer segments of bovine retina by the reversal of the disk Ca(2+) pump

Purified disk membranes from rod outer segments of the bovine retina were able to synthesize ATP with a maximal activity (about 52 nmoles ATP/min/mg of protein) at physiological calcium concentrations. This activity was inhibited by vanadate or thapsigargin but not by oligomycin, suggesting the reversal functioning of the disk Ca(2+)-ATPase, which would act as a ATP synthesizer at the expense of the calcium gradient between the disks and the cytoplasm of the rod outer segment. The results are discussed in terms of the need of an immediate source of ATP on the disk membranes where the energy is required to supply the rapid reactions of the photoreception processes.

Cyclic ADP-ribose-dependent Ca2+ release is modulated by free [Ca2+] in the scallop sarcoplasmic reticulum

Cyclic ADP-ribose (cADPR) elicits calcium-induced calcium release (CICR) in a variety of cell types. We studied the effect of cADPR on Ca2+ release in muscle cells by incubating SR vesicles from scallop (Pecten jacobaeus) adductor muscle in the presence of the Ca2+ tracer fluo-3. Exposure of SR to cADPR (20 microM) produced Ca2+ release, which was a function of free [Ca2+] in a range between about 150 and 1000 nM, indicating an involvement of ryanodine-sensitive Ca2+ channels. This Ca2+ release was not significantly enhanced by calmodulin (7 micrograms/ml), but it was enhanced by equimolar addition of noncyclic ADPR. Also, the Ca2+ release elicited by cADPR/ADPR was a function of free [Ca2+] in a range between about 150 and 3000 nM, over which Ca2+ was inhibitory. cADPR self-inactivation was observed at low free [Ca2+] (about 150 nM), but it tended to disappear upon [Ca2+] elevation (about 250 nM). Caffeine or ryanodine induced a Ca2+ release which was ruthenium red (2.5 microM) sensitive at low [Ca2+]. However, the Ca2+ release induced by either ryanodine or cADPR was no longer ruthenium red sensitive when free [Ca2+] was increased. Based on these data, a model is proposed for Ca2+ signaling in muscle cells, where a steady-state cADPR level would trigger Ca2+ release when free [Ca2+] does reach a threshold slightly above its resting level, hence producing cascade RyR recruitment along SR cisternae from initial Ca2+ signaling sites.

Measurements of (Na+,K+)ATPase after in vitro hypoxia and reoxygenation are affected by methods of membrane preparation

(Na+,K+ )ATPase activity was evaluated in membranes from rat hippocampal slices after in vitro hypoxia and reoxygenation. Membranes were prepared with two different methods, one using an isotonic medium and another using a hypotonic one. The changes that were found after hypoxia went into opposite directions in the two cases. Membranes prepared in a hypotonic medium are probably more suitable for these measurements. Using these membranes, hypoxia results in a slight decrease of (Na+,K+)ATPase activity and in a further decrease after reoxygenation. We also found that expressing (Na+,K+)ATPase activity as a percent of total ATPase activity is appropriate for membranes prepared under hypotonic conditions and can unveil (by reducing variability between experiments) significant changes that may be masked in small samples like ours.

Ca(2+)-ATPase pump forms and an endogenous inhibitor in bovine brain synaptosomes

Two forms of Ca(2+)-pump were identified in bovine brain synaptic membranes as aspartylphosphate intermediates and were characterized. The 140 kDa and 97 kDa phosphoproteins were digested by calpain, producing two phosphorylated fragments, of M.W. 124 and 80 kDa respectively, not inhibited by thapsigargin, and displayed a trypsin digestion pattern with the formation of one phosphorylatable fragment of about 80 kDa. These results suggest that both pumps belong to the Plasma Membrane-type of Ca2+ ATPases, differing from the Sarco- or Endoplasmic Reticulum kind. A plasma membrane Ca(2+)-ATPase proteinaceous inhibitor with molecular weight between 6,000 and 10,000 Da was resolved from synaptic terminal cytosol, where it is enriched by fourfold with respect to frontal cortex brain cytosol. Such enrichment is already evident in the correspondent crude fractions. The presence of calcium pump and its proteinaceous inhibitor inside the synaptic terminals from bovine brain is discussed in terms of free calcium level regulation in neuron synaptoplasm.

Endoplasmic reticulum Ca(2+)-ATPase in microsomal vesicles isolated from bovine retinae

Active Ca2+ transport was measured in microsomal vesicles prepared from bovine retinae and was compared with that in disk membranes of the photoreceptor cells of the same retina. The 45Ca uptake was dependent on the presence of Mg(2+)-ATP and was inhibited by vanadate or when GTP substituted for ATP. The dependence of calcium uptake on the external free Ca2+ concentration gave a KM = 13 microM or a KM = 0.1 microM for disks and microsomal vesicles, respectively. A phosphorylated intermediate (E-P) of Ca(2+)-ATPase of about 100 kDa was isolated in microsomal vesicles. The E-P formation was strongly inhibited by thapsigargin and partially by 2,5-di-(-butyl)benzohydroquinone. Digestion of disks or microsomes with calpain had no effect on the phosphorylated intermediate, while digestion with trypsin produced two fragments of approximately 55 kDa and 35 kDa. These results suggest that bovine retinal microsomes contain a calcium pump belonging to the SERCA family.

Characterization of Ca(2+)-ATPase in rod outer segment disk membranes

The Ca(2+)-pump isolated in rod outer segment disk membranes as aspartylphosphate intermediate E-P has been characterized: the 100 kDa phosphoprotein was completely inhibited by thapsigargin, was not sensitive to digestion by calpain, and displayed a tryptic digestion pattern with the formation of two autophosphorylatable fragments of about 55 and 35 kDa. These results are typical of the calcium pumps of sarcoplasmic or endoplasmic reticulum calcium and differ from those of plasma membrane, such as the Ca(2+)-ATPase of the red blood cells, here shown as controls. The physiological role of calcium pump in disk membranes of vertebrate photoreceptors is discussed in terms of intracellular calcium buffering.

Calcium pump in the disk membranes isolated from bovine retinal rod outer segments

The existence of a Ca2+ pump in rod outer segment disks of bovine retina is strongly suggested by the isolation on sodium dodecyl sulfate polyacrylamide gel electrophoresis of a hydroxylamine-sensitive phosphorylated intermediate (E-P) of molecular mass of about 100 kDa as well as by measurements of active calcium transport and adenosine 5'-triphosphate (ATP) hydrolysis. Active Ca2+ uptake by disks was dependent on the presence of Mg(2+)-ATP, was inhibited by vanadate or lanthanum and appeared poorly sensitive to calmodulin. ATP hydrolysis by disk membranes was a function of free Ca2+ concentration in the absence of exogenous Mg2+. The presence of a Ca2+ pump on disk membranes is discussed in terms of its possible role in Ca2+ ion buffering during photoreceptor cell functioning.

Biochemical characterization of a phosphatidylinositol 4,5-bisphosphate-specific phospholipase C activity in gills and digestive gland of the marine mussel Mytilus galloprovincialis Lam

1. Polyphosphoinositide-specific phosphodiesterase (phospholipase C, PLC) activity against phosphatidylinositol 4,5-bisphosphate, present in gill and digestive gland homogenates of mussel (Mytilus galloprovincialis Lam.), has been biochemically characterized. 2. The enzyme was strictly modulated by free calcium ion concentration in both tissues and maximally activated at 10(-5) M Ca2+ (19 +/- 4 and 11 +/- 2 nmol phosphatidylinositol 4,5-bisphosphate hydrolysed/min/mg of protein for gill and digestive gland PLC, respectively, at 19 degrees C). Optimum pH at 10(-5) M Ca2+ was around 7.0 in both cases. The Ca(2+)-stimulated PLC activity showed high specificity for PIP2; the KMa for PIP2 were 150 and 170 microM for the gills and digestive gland, respectively. 3. Good substrate dispersion was obtained in the presence of sodium deoxycholate; the concentration routinely used in the assay (0.08%) produced a 9-fold activation of both gill and digestive gland PLC, consistent with previous reports. 4. The possible biochemical and physiological role of the enzyme in mussel tissues is discussed.

Calcium adenosinetriphosphatase of bovine retinal rod outer segment disks

A Ca(2+)-pumping ATPase activity is present in bovine retinal rod outer segment purified disks. The ATPase has a high Ca2+ affinity (KM = 25 microM). Low Ca2+ (n-microM) concentrations stimulate an ATP-dependent Ca2+ uptake and the ATP hydrolysis in the absence of exogenous Mg2+. Electrophoretic analysis of disk proteins after treatment with (gamma-32P)ATP shows the existence of the enzyme-phosphate acid-stable, hydroxylamine-sensitive intermediate complex of molecular mass of about 135 kDa. The results would indicate the presence of an inwardly directed Ca(2+)-ATPase pump acting on the disk membrane, that could be involved in the regulation of cytosolic free Ca2+ levels inside ROS.

Resolution and characterization of two forms of phosphoinositide-specific phospholipase C from bovine rod outer segments

Two forms (I and II) of phospholipase C, specific for phosphatidyl inositol 4,5-bisphosphate, were resolved from bovine retinal rod outer segment (ROS) cytosol by DEAE-Sepharose column chromatography. The two isozymes showed reproducible differences in their catalytic properties in spite of similar substrate specificity and hydrolyzed specifically inositol 4,5-bisphosphate in a Ca(2+)-dependent fashion. In the presence of deoxycholate (DOC), pH optima were at 6.5 and 7.0 for phospholipase C I and II, respectively. Maximal phosphatidylinositol 4,5-bisphosphate hydrolysis rates were obtained at 10(-4) and 10(-5)M Ca2+ for phospholipase C I and II, respectively. Treatment with cAMP-dependent protein kinase did not alter either isozyme activity. Further purification steps were prevented by the extreme lability of the isozymes.

Phosphatidyl inositol 4,5-bisphosphate-specific phospholipase C in bovine rod outer segment membranes

Polyphosphoinositide-specific phosphodiesterase (phospholipase C) activity against phosphatidylinositol 4,5-bisphosphate has been examined in disrupted bovine retinal rod outer segments. The enzyme was strictly modulated by free calcium ion concentration and maximally activated at 10(-5) M Ca2+ (91 +/- 4 nmoles phosphatidylinositol 4,5-bisphosphate hydrolyzed/min/mg of protein). Guanine nucleotides did not affect in vitro phospholipase C activity either in the presence or absence of light, carbachol or epinephrine. The pH optimum at 10(-5) M Ca2+ in the presence of sodium deoxycholate was 6.5. The enzyme of bovine rod outer segments was concluded to be indirectly regulated by the phototransduction events.

Calcium ion-regulated phospholipase C activity in bovine rod outer segments

Bovine retinal rod outer segment membranes are enriched in a phosphoinositide-specific phosphodiesterase (phospholipase C) activity strictly modulated by free calcium ion concentration. The enzyme(s) was highly active on phosphatidylinositol 4,5-bisphosphate: maximal hydrolysis rate was attained at 10(-5)M Ca2+ and accounted for 91 +/- 4 nmoles hydrolyzed/min/mg of protein. The results support the notion that in vivo the enzyme(s) is regulated so as to conform to the phototransduction events.

Detection of novel guanine nucleotide binding proteins in bovine retinal rod outer segments

In the rod outer segment membranes of the bovine retina at least two members of the small molecular weight guanine nucleotide binding proteins were identified by means of the technique of binding radiolabeled GTP to nitrocellulose Western blots of proteins separated by sodium dodecyl sulphate gel electrophoresis. Such proteins, of 23 and 25 kDa, are able to specifically bind guanine nucleotides after denaturing treatments, and are not labeled by pertussis or cholera toxin-catalyzed ADP-ribosylation. The binding site is specific for GTP.

Biophysical approach to the determination of the secondary structure of the histone H1 globular region

A possible secondary structure of the globular part of the histone H1 was obtained with a statistical approach based on the GOR method. The results of circular dichroism measurements on the protein were taken into account in order to choose between theoretically equivalent structures.

Proteins of rod outer segments of toad retina: binding with calmodulin and with GTP

Proteins of purified rod outer segments from toad retina were analysed by electrophoresis on polyacrylamide gel containing sodium dodecyl sulfate. The binding of proteins with calmodulin and with guanosine triphosphate was studied by electroblotting the proteins resolved by electrophoresis onto nitrocellulose sheets and by incubating the blots with labelled ligands. The results indicate that rod outer segments from toad retina contain nine proteins which bind to calmodulin and one protein, different from transducin, that binds to guanosine triphosphate.

Visual transduction in vertebrate photoreceptors. Light activation of guanylate cyclase

Light activation of guanylate cyclase at different calcium concentrations was studied in the rod outer segments of the toad retina. The enzyme becomes sensitive to calcium ions after a flash of light, showing an enhancement of its activity when Ca2+ concentration is lowered from 10(-4) M to 10(-8) M. A possible pathway of guanylate cyclase activation by light was also investigated by means of the antibody 4A to transducin. When added in excess to transducin, the antibody inhibits light activation of phosphodiesterase as well as of cyclase, suggesting a possible coupling of the two enzymes.

Effect of light and calcium on cyclic GMP synthesis in rod outer segments of toad retina

The rod outer segments of toad retina contain a guanylate cyclase activity of about 3 +/- 1 nmol of cGMP formed/min per mg protein. In darkness this value is largely independent of the Ca2+ concentration, although it is enhanced by light upon lowering the Ca2+ concentration from 10(-5) to 10(-8) M. The activating effect of light on cyclase at low Ca2+ concentrations is enlarged upon increasing the light intensity. With a flash of light bleaching 7 X 10(-2) percent of rhodopsin, cyclase activity increased by a factor of 30 when Ca2+ levels dropped from 10(-5) to 10(-8) M. In view of recent observations that shortly after a flash of light the calcium activity inside the photoreceptor cell decreases, it seems likely that Ca2+ plays a regulatory role on cGMP metabolism in visual excitation.

Guanylate cyclase in rod outer segments of the toad retina. Effect of light and Ca2+

Guanylate cyclase activity was measured in disrupted rod outer segments of the toad retina. The experiments showed that cGMP is synthesized from GTP at a rate of 3 +/- 1 nmol/min per mg protein. In darkness this value is largely independent of the Ca2+ concentration, while it is enhanced by flashes of light of increasing intensity upon lowering Ca from 10-5 to 10-8 M. In view of recent observations that shortly after a flash of light calcium activity inside the photoreceptor cell decreases, it seems likely that calcium plays a regulatory role in cGMP metabolism in visual excitation.