Properties of a magnesium- or calcium-dependent adenosine triphosphatase from frog rod photoreceptor outer segment disks and its inhibition by illumination

Are rod outer segment ATP-ase and ATP-synthase activity expression of the same protein?

Vertebrate retinal rod outer segments (OS) consist of a stack of disks surrounded by the plasma membrane, where phototransduction takes place. Energetic metabolism in rod OS remains obscure. Literature described a so-called Mg(2+)-dependent ATPase activity, while our previous results demonstrated the presence of oxidative phosphorylation (OXPHOS) in OS, sustained by an ATP synthetic activity. Here we propose that the OS ATPase and ATP synthase are the expression of the same protein, i.e., of F1Fo-ATP synthase. Imaging on bovine retinal sections showed that some OXPHOS proteins are expressed in the OS. Biochemical data on bovine purified rod OS, characterized for purity, show an ATP synthase activity, inhibited by classical F1Fo-ATP synthase inhibitors. Moreover, OS possess a pH-dependent ATP hydrolysis, inhibited by pH values below 7, suggestive of the functioning of the inhibitor of F1 (IF1) protein. WB confirmed the presence of IF1 in OS, substantiating the expression of F1Fo ATP synthase in OS. Data suggest that the OS F1Fo ATP synthase is able to hydrolyze or synthesize ATP, depending on in vitro or in vivo conditions and that the role of IF1 would be pivotal in the prevention of the reversal of ATP synthase in OS, for example during hypoxia, granting photoreceptor survival.

Partial characterization of a high affinity [Ca2+ + Mg2+]-dependent adenosinetriphosphatase from bovine retina

Examination of retinal tissue homogenates indicated the presence of a [Ca2+ + Mg2+]-dependent adenosinetriphosphatase activity that exhibited high affinity for Ca2+ (K0.5 = 0.17 microM) and moderately high affinity for Mg2+ and ATP (K0.5 = 12.5 microM and Km = 22.8 microM, respectively). Maximum ATP hydrolysis occurred at pH 7.4. Under conditions of optimal substrate, cation and hydrogen ion concentrations, specific activity ranged from 15 to 18 nmol phosphate released min-1 mg-1 protein. Although the retinal [Ca2+ + Mg2+] adenosinetriphosphatase hydrolyzes both ATP and dATP, other nucleotides (CTP, GTP, ITP and UTP) were not hydrolyzed to any great extent. The monovalent cations, Li+, K+ and Na+, had no effect upon hydrolysis of ATP; whereas Cs+ and NH4+ ions were moderately (approximately 30%) inhibitory. All divalent cations tested were stimulatory. With the exception of rotenone which inhibited ATP hydrolysis approximately 25%; retinal adenosinetriphosphatase activity was insensitive to mitochondrial inhibitors (NaN3, KCN, ruthenium red and oligomycin). Adenosinetriphosphatase activity was observed to be very sensitive to low concentrations (I50 approximately 2 microM) of vanadate; whereas, lanthanum administration resulted in no inhibition. Removal of calmodulin (80%) resulted in reducing adenosinetriphosphatase activity 60% but addition of exogenous calmodulin back to calmodulin deficient membranes did not restore activity to starting levels. Calmodulin antagonists trifluoperazine and calmidazolium reduced significantly Ca2+ stimulated, Mg2+ dependent ATP hydrolysis. We conclude that the [Ca2+ + Mg2+]-dependent adenosinetriphosphatase of bovine retina is a non-mitochondrial protein exhibiting very high affinity for Ca2+ and appears to require calmodulin for maximum activity. Because of its high affinity for Ca2+, this protein may play an important role in reducing intracellular Ca2+ to nanomolar levels.

Ultracytochemical study of Ca++-ATPase and K+-NPPase activities in retinal photoreceptors of the guinea pig

Ca++-ATPase activity was demonstrated histochemically at light- and electron-microscopic levels in inner and outer segments of retinal photoreceptor cells of the guinea pig with the use of a newly developed one-step lead-citrate method (Ando et al. 1981). The localization of ouabain-sensitive, K+-dependent p-nitrophenylphosphatase (K+-NPPase) activity, which represents the second dephosphorylative step of the Na+-K+-ATPase system, was studied by use of the one-step method newly adapted for ultracytochemistry (Mayahara et al. 1980). In retinal photoreceptor cells fixed for 15 min in 2% paraformaldehyde the electron-dense Ca++-ATPase reaction product accumulated significantly on the inner membranes of the mitochondria but not on the plasmalemma or other cytoplasmic elements of the inner segments. The membranes of the outer segments remained unstained except the membrane arrays in close apposition to the retinal pigment epithelium. The cytochemical reaction was Ca++- and substrate-dependent and showed sensitivity to oligomycin. When Mg++-ions were used instead of Ca++-ions, a distinct reaction was also found on mitochondrial inner membranes. In contrast to the localization of the Ca++-ATPase activity, the K+-NPPase activity was demonstrated only on the plasmalemma of the inner segments, but not on the mitochondria, other cytoplasmic elements or the outer segment membranes. This reaction was almost completely abolished by ouabain or by elimination of K+ from the incubation medium.

Guanine nucleotides can activate the insulin-stimulated phosphodiesterase in liver plasma membranes

The insulin-stimulated cyclic AMP phosphodiesterase from liver plasma membranes is shown to be activated upon incubation with guanine nucleotides in the presence of ATP. The non-hydrolysable analogue of ATP, adenylyl imidodiphosphate failed to substitute for ATP in achieving activation. GTP, its non-hydrolysable analogues p[NH]ppG and GTP-gamma-S, as well as GDP, all elicited activation. It is suggested that guanine nucleotides, and probably insulin, exert their effect on this enzyme through a distinct species of guanine nucleotide regulatory protein.

Taurine-calcium interactions in frog rod outer segments: taurine effects on an ATP-dependent calcium translocation process

Frog rod outer segments (ROS) isolated in a Ca-free, EGTA-containing medium, showed a rapid ATP-dependent accumulation of 45Ca, GTP, CTP, ITP, UTP, GMP and beta-, gamma-methylene ATP did not substitute for ATP in energizing Ca uptake. This process required Mg, it was abolished in the presence of Ca ionophores A23187 and X537A and is not affected by external sodium. The Arrhenius activation energy was 7.9 kcal/mol and the pH optimum was approximately 7.2. The apparent Km for Ca uptake was 66 microM with a V max of 12.5 nmol/mg protein. 45Ca accumulation was reduced in illuminated ROS. The presence of 5-25 mM taurine, but not of GABA, glycine, histidine or proline, markedly enhanced Ca uptake by ROS.

Some considerations on the ion transport properties of the rod disc membrane

The ion transport properties of the disc membranes in rod outer segments are discussed on the basis of available data. The properties of an air-water interface film of spectroscopically intact and chemically regenerable rhodopsin are presented, and results of studies of ion binding to these films are reported.

Transmembrane channel formation in rhodopsin-containing bilayer membranes

Rhodopsin has been incorporated into planar lipid bilayer membranes. The effect of light is to increase the bilayer permeability in a pattern consistent with the formation of a transmembrane channel of about 10 A diameter. A model of visual excitation based on a light-activated and voltage-sensitive channel is presented.

Molecular aspects of photoreceptor function

The description of the molecular processes which underlie visual excitation is the fundamental problem in understanding vision at the level of a single photoreceptor. Thus far only a general outline of photoreceptor function has emerged with little known about actual biochemical and biophysical mechanisms.

Calcium-binding properties and ATPase activities of rat liver plasma membranes

Plasma membranes from rat liver purified according to the procedure of Neville bind calcium ions by a concentration-dependent, saturable process with at least two classes of binding sites. The higher affinity sites bind 45 nmol calcium/mg membrane protein with a K(D) of 3 microM. Adrenalectomy increases the number of the higher affinity sites and the corresponding K(D). Plasma membranes exhibit a (Na(+)-K(+))-independent-Mg(2+)-ATPase activity which is not activated by calcium between 0.1 microM and 10 mM CaCl(2). Calcium can, with less efficiency, substitute for magnesium as a cofactor for the (Na(+)-K(+))-independent ATPase. Both Mg(2+)- and Ca(2+)-ATPase activities are identical with respect to pH dependence, nucleotide specificity and sensitivity to inhibitors. But when calcium is substituted for magnesium, there is no detectable membrane phosphorylation from [gamma-(32)P] ATP as it is found in the presence of magnesium. The existence of high affinity binding sites for calcium in liver plasma membranes is compatible with a regulatory role of this ion in membrane enzymic mechanisms or in hormone actions. Plasma membranes obtained by the procedure of Neville are devoid of any Ca(2+)-activated-Mg(2+)-ATPase activity indicating the absence of the classical energy-dependent calcium ion transport. These results would suggest that the overall calcium-extruding activity of the liver cell is mediated by a mechanism involving no direct ATP hydrolysis at the membrane level.