Subcellular localization of cyclic ADP-ribosyl cyclase and cyclic ADP-ribose hydrolase activities in porcine airway smooth muscle.

Recent studies have provided evidence for a role of cyclic ADP-ribose (cADPR) in the regulation of intracellular calcium in smooth muscles of the intestine, blood vessels and airways. We investigated the presence and subcellular localization of ADP-ribosyl cyclase, the enzyme that catalyzes the conversion of beta-NAD(+) to cADPR, and cADPR hydrolase, the enzyme that degrades cADPR to ADPR, in tracheal smooth muscle (TSM). Sucrose density fractionation of TSM crude membranes provided evidence that ADP-ribosyl cyclase and cADPR hydrolase activities were associated with a fraction enriched in 5'-nucleotidase activity, a plasma membrane marker enzyme, but not in a fraction enriched in either sarcoplasmic endoplasmic reticulum calcium ATPase or ryanodine receptor channels, both sarcoplasmic reticulum markers. The ADP-ribosyl cyclase and cADPR hydrolase activities comigrated at a molecular weight of approximately 40 kDa on SDS-PAGE. This comigration was confirmed by gel filtration chromatography. Investigation of kinetics yielded K(m) values of 30.4+/-1.5 and 695. 3+/-171.2 microM and V(max) values of 330.4+/-90 and 102.8+/-17.1 nmol/mg/h for ADP-ribosyl cyclase and cADPR hydrolase, respectively. These results suggest a possible role for cADPR as an endogenous modulator of [Ca(2+)](i) in porcine TSM cells.

ADP-ribosyl cyclase and CD38. Multi-functional enzymes in Ca+2 signaling.

Mobilization of internal Ca+2 is an important signaling mechanism in cells. In addition to the inositol trisphosphate pathway, cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide (NAADP) have been shown to mobilize Ca+2 via independent mechanisms. Although the structures of cADPR and NAADP are totally distinct, both nucleotides can be synthesized by ADP-ribosyl cyclase or CD38, a lymphocyte antigen. Both enzymes cyclize NAD to cADPR. In the presence of nicotinic acid the two enzymes catalyze a base exchange reaction resulting in the synthesis of NAADP from NADP. The switch between these two modes of catalysis is regulated by pH. Furthermore, both enzymes can also cyclize nicotinamide guanine dinucleotide (NGD) to produce a fluorescent product, cyclic GDP-ribose (cGDPR), which has a site of cyclization different from cADPR. A model is proposed to account for the multi-functionality of these enzymes. In order to be able to verify the model, a soluble ADP-ribosyl cyclase has been crystallized and X-ray diffraction shows that it is a dimer. Solution of the crystal structure of the cyclase should provide valuable insight into the structural features necessary for its multiple catalytic functions.

Possible role of cyclic nucleotides in the mechanism of the protective effect of methylprednisolone on the hypoxic rat heart.

The isolated isovolumic rat heart was used as a model of cardiac hypoxia. Force of cardiac contraction and cardiac cyclic nucleotide levels (cyclic GMP and cyclic AMP) were monitored in hearts subjected to hypoxia for 5 min and allowed to recover by reoxygenation. Hearts were obtained from both control animals and animals pretreated with methylprednisolone at 18 hr and 1 hr prior to sacrifice. Myocardial levels of cyclic GMP which were significantly (p less than 0.05) elevated above control during all periods of hypoxia were found to be lower when hearts were pretreated with methylprednisolone prior to hypoxic exposure. Hearts of animals pretreated with methylprednisolone also demonstrated better recovery during reoxygenation than did control hearts. These studies suggest that methylprednisolone may be beneficial in the prevention of myocardial failure following hypoxia via a modulation in myocardial cyclid GMP content.

GDP-ribosyl cyclase activity as a measure of CD38 induction by retinoic acid in HL-60 cells.

Retinoic acid (RA) treatment of HL-60 cells induces surface expression of CD38. This lymphocytic antigen is also a novel bifunctional enzyme catalyzing the synthesis and hydrolysis of cyclic ADP-ribose (cADPR), a Ca2+ mobilizing metabolite of NAD+. The synthetic activity of CD38 is very difficult to detect because of the concurrent hydrolytic activity. In this study, a Ca2+ release assay capable of detecting submicromolar concentrations of cADPR was used to demonstrate the induction of ADP-ribosyl cyclase activity in HL-60 cells by RA. Concomitantly, cADPR hydrolase activity was also increased. The results were further substantiated by using a newly developed assay for GDP-ribosyl cyclase activity. This assay uses NGD+ as substrate instead of NAD+. The resulting fluorescent product, cyclic GDP-ribose, is resistant to hydrolysis and accumulates, making it a highly sensitive and convenient assay for CD38-like enzymes.

The dynamics of cGMP metabolism in neuroblastoma N1E-115 cells determined by 18O labeling of guanine nucleotide alpha-phosphoryls.

The rates of phosphodiesterase-promoted hydrolysis of cGMP and cAMP have been measured in intact neuroblastoma N1E-115 cells by determining rates of 18O incorporation from 18O-water into the alpha-phosphoryls of guanine and adenine nucleotides. The basal rate of guanine nucleotide alpha-phosphoryl labeling ranged from 180 to 244 pmol X mg protein-1 X min-1. Sodium nitroprusside (SNP) caused a sustained 3.4-fold increase in this 18O-labeling rate in conjunction with 28- and 50-fold increases in cellular cGMP concentration at 3 and 6 min, respectively. This 18O-labeling rate (795 pmol X mg protein-1 X min-1) corresponded with the sum of the low (1.7 microM) and high (34 microM) Km phosphodiesterase activities assayable in cell lysates which exhibited a combined maximum velocity of 808 pmol X mg protein-1 X min-1 to which the high Km species contributed 84%. This information and the characteristics of the profile of 18O-labeled molecular species indicate that cGMP metabolism was restricted to a very discrete cellular compartment(s) of approximately 12% of the cell volume. Carbachol (1 mM) produced a transient increase (6-fold) in cellular cGMP concentration and a transient increase (90%) in the rate of 18O labeling of alpha-GTP during the first minute of treatment which translates into 30 additional cellular pools of cGMP hydrolyzed in this period. IBMX (1 mM) produced a relatively rapid increase in cellular cGMP (3- to 5-fold) and cAMP (2-fold) concentrations and a delayed inhibition of 18O labeling of guanine and adenine nucleotide alpha-phosphoryls without further elevation of cyclic nucleotide levels. These results indicate that besides inhibiting cyclic nucleotide hydrolysis, IBMX also imparts a time-dependent inhibitory influence on the generation of cyclic nucleotides. The data obtained show that measurement of 18O labeling of guanine and adenine nucleotide alpha-phosphoryls combined with measurements of cyclic nucleotide steady state levels provides a means to assess the rates of cyclic nucleotide synthesis and hydrolysis within intact cells and to identify the site(s) of action of agents that alter cellular cyclic nucleotide metabolism.

Sensitization of calcium-induced calcium release by cyclic ADP-ribose and calmodulin.

Cyclic ADP-ribose (cADPR) is emerging as an endogenous regulator of Ca2+-induced Ca2+ release (CICR), and we have recently demonstrated that its action is mediated by calmodulin (CaM) (Lee, H. C., Aarhus, R., Graeff, R., Gurnack, M. E., and Walseth, T. F. (1994) Nature 370, 307-309). In this study we show by immunoblot analyses that the protein factor in sea urchin eggs responsible for conferring cADPR sensitivity to egg microsomes was CaM. This was further supported by the fact that bovine CaM was equally effective as the egg factor. In contrast, plant CaM was only partially active even at 10-20-fold higher concentrations. This exquisite specificity was also shown by binding studies using 125I-labeled bovine CaM. The effectiveness of various CaMs (bovine > spinach > wheat germ) in competing for the binding sites was identical to their potency in conferring cADPR sensitivity to the microsomes. A comparison between bovine and wheat germ CaM in competing for the sites suggests only 10-14% of the total binding was crucial for the activity. Depending on the CaM concentration, the sensitivity of the microsomes to cADPR could be changed by several orders of magnitude. The requirement for CaM could be alleviated by raising the divalent cation concentration with Sr2+. Results showed that CaM, cADPR, and caffeine all act synergistically to increase the divalent cation sensitivity of the CICR mechanism. The combined action of any of the three agonists was sufficient to sensitize the mechanism so much that even the nanomolar concentration of ambient Ca2+ was enough to activate the release. Unlike the CICR mechanism, the microsomal inositol 1,4,5-trisphosphate-sensitive Ca2+ release showed no dependence on CaM. Using an antagonist of CaM, W7, it was demonstrated that the cADPR-but not the inositol 1,4,5-trisphosphate-dependent release mechanism could be blocked in live sea urchin eggs. These results indicate cADPR can function as a physiological modulator of CICR and, together with CaM, can alter the sensitivity of the release mechanism to divalent cation by several orders of magnitude.

A Ca2+-linked increase in coupled cAMP synthesis and hydrolysis is an early event in cholinergic and beta-adrenergic stimulation of parotid secretion.

The dynamics and compartmental characteristics of cAMP metabolism were examined by 18O labeling of cellular adenine nucleotide alpha phosphoryls in rat parotid gland stimulated to secrete with beta-adrenergic and cholinergic agents. The secretory response occurred in association with a rapidly increased rate of cAMP hydrolysis apparently coordinated with an equivalent increase in the rate of cAMP synthesis, since the cellular concentration of cAMP remained unchanged. The magnitude of this metabolic response was equivalent to the metabolism of 10-75 times the cellular content of cAMP within the first minute of stimulation. This increased metabolic rate occurred only during the early (1-3 min) period of stimulation, in what appeared to be an exclusive cellular compartment distinguished by a unique distribution of 18O among adenine nucleotide alpha phosphoryls. This 18O distribution contrasted with that produced by forskolin, which increased cellular cAMP concentration and elicited only a delayed response missing the early secretory component. The early acceleration of cAMP metabolism appeared linked to a stimulus-induced increase in intracellular Ca2+ concentration, since the Ca2+ ionophore ionomycin produced the same metabolic response in association with secretion. These observations suggest that cAMP metabolism is involved in stimulus-secretion coupling by a Ca2+-linked mechanism different from that in which cAMP plays the role of a second messenger.

Enzymatic synthesis and characterizations of cyclic GDP-ribose. A procedure for distinguishing enzymes with ADP-ribosyl cyclase activity.

Cyclic nucleotides such as cAMP and cGMP are second messengers subserving various signaling pathways. Cyclic ADP-ribose (cADPR), a recently discovered member of the family, is derived from NAD+ and is a mediator of Ca2+ mobilization in various cellular systems. The synthesis and degradation of cADPR are, respectively, catalyzed by ADP-ribosyl cyclase and cADPR hydrolase. CD38, a differentiation antigen of B lymphocytes, has recently been shown to be a bifunctional enzyme catalyzing both the formation and hydrolysis of cADPR. The overall reaction catalyzed by CD38 is the formation of ADP-ribose and nicotinamide from NAD+, identical to that catalyzed by NADase. The difficulties in detecting the formation of cADPR have led to frequent identification of CD38 as a classical NADase. In this study, we show that both ADP-ribosyl cyclase and CD38, but not NADase, can cyclize nicotinamide guanine dinucleotide (NGD+) producing a new nucleotide. Analyses by high performance liquid chromatography and mass spectroscopy indicate the product is cyclic GDP-ribose (cGDPR) with a structure similar to cADPR except with guanine replacing adenine. Compared to cADPR, cGDPR is a more stable compound showing 2.8 times more resistance to heat-induced hydrolysis. These results are consistent with a catalytic scheme for CD38 where the cyclization of the substrate precedes the hydrolytic reaction. Spectroscopic analyses show that cGDPR is fluorescent and has an absorption spectrum different from both NGD+ and GDPR, providing a very convenient way for monitoring its enzymatic formation. The use of NGD+ as substrate for assaying the cyclization reaction was found to be applicable to pure enzymes as well as crude tissue extracts making it a useful diagnostic tool for distinguishing CD38-like enzymes from degradative NADases.

Non-identity of cGMP as the guanine nucleotide stimulated to bind to ROS by light and ATP.

Light, in the presence of ATP, has been reported to stimulate cGMP binding to a 58 kDa protein in ROS (rod outer segments, Fesenko and Krapivinsky, 1986b, Photobiochem. Photobiophys. 13 345-58). This apparent light-related redistribution of ROS cGMP has been suggested to eliminate any requirement for phosphodiesterase-promoted hydrolysis of cGMP in the mechanism subserving phototransduction. Using conditions identical to those previously reported, this effect of light and ATP was examined further by characterizing the metabolic products that arise and the nucleotides that become liganded. The increased binding of radiolabeled guanine nucleotide upon illumination of ROS in the presence of ATP was confirmed, but the species of guanine nucleotide that were stimulated to bind under these conditions were identified as [32P]GDP and [32P]GTP rather than [32P]cGMP. The precautions to prevent enzymic hydrolysis of cGMP, which included conducting the reactions at 0 degrees and the addition of 3-isobutyl-l-methylxanthine (250 microM) to the reaction mixture did not prevent about a 20-fold increase in the rate of phosphodiesterase-catalyzed hydrolysis of radiolabeled cGMP by light when ATP was also present. This stimulation of phosphodiesterase activity is undoubtedly related to transphosphorylation by exogenous ATP of endogenous GMP and GDP involving catalytic actions of guanylate kinase and nucleoside diphosphate kinase in isolated ROS. These enzymes can also serve to generate [32P]GDP and [32P]GTP, which subsequently bind to ROS components. Such a mechanism involving ATP as phosphoryl donor was supported by observing that an analog of ATP (beta,gamma-methyleneadenosine 5'-triphosphate), which cannot serve as a phosphoryl donor, did not increase radiolabeled guanine nucleotide binding. Although several ROS proteins can form filter-retainable complexes with GDP and GTP, the properties of the 58 kDa protein found to be photoaffinity labeled with radioactive guanine nucleotide are most characteristic of those attributable to tubulin. The previous report that illumination in the presence of ATP stimulates the binding of cGMP to ROS components finds no support from the data obtained in the present studies.

Fluorescent analogs of cyclic ADP-ribose: synthesis, spectral characterization, and use.

Cyclic ADP-ribose (cADPR) is a Ca(2+)-mobilizing cyclic nucleotide derived from NAD+. Accumulating evidence indicates that it is an endogenous modulator of the Ca(2+)-induced Ca2+ release mechanism in cells. In this study, we show that ADP-ribosyl cyclase catalyzes the cyclization of not only NAD+ but also several of its analogs with various purine bases (guanine, hypoxanthine, or xanthine) substituting for adenine. Unlike cADPR, the resulting cyclic products are fluorescent. Comparisons with various model compounds indicate that only 7-methyl substituted purine nucleosides and nucleotides are fluorescent, and the pH-dependence of their UV spectra is most similar to that of the fluorescent cADPR analogs, indicating that the site of cyclization of these analogs is at the N7-position of the purine ring. This finding is novel since the site of cyclization is at the N1-position for cADPR as determined by X-ray crystallography. That a single enzyme can cyclize a variety of substrates at two different sites has important implications mechanistically, and a model is proposed to account for these novel catalytic properties. Among the analogs synthesized, cyclic GDP-ribose is highly resistant to hydrolysis, while cyclic IDP-ribose can be readily hydrolyzed by CD38, a bifunctional enzyme involved in the metabolism of cADPR. These unique properties of the analogs can be used to develop fluorimetric assays for monitoring separately the cyclization and hydrolytic reactions catalyzed by the metabolic enzymes of cADPR. The convenience of the method in measuring kinetic parameters, pH-dependence, and modulator activity of the metabolic enzymes of cADPR is illustrated.

ADP-ribosyl cyclase and CD38 catalyze the synthesis of a calcium-mobilizing metabolite from NADP.

ADP-ribosyl cyclase catalyzes the cyclization of NAD+ to produce cyclic ADP-ribose (cADPR), which is emerging as an endogenous regulator of the Ca(2+)-induced Ca2+ release mechanism in cells. CD38 is a lymphocyte differentiation antigen which has recently been shown to be a bifunctional enzyme that can synthesize cADPR from NAD+ as well as hydrolyze cADPR to ADP-ribose. In this study, we show that both the cyclase and CD38 can also catalyze the exchange of the nicotinamide group of NADP+ with nicotine acid (NA). The product is nicotinic acid adenine dinucleotide phosphate (NAADP+), a metabolite we have previously shown to be potent in Ca2+ mobilization (Lee, H. C., and Aarhus, R. (1995) J. Biol. Chem. 270, 2152-2157). The switch of the catalysis to the exchange reaction requires acidic pH and NA. The half-maximal effective concentration of NA is about 5 mM for both the cyclase and CD38. In the absence of NA or at neutral pH, the cyclase converts NADP+ to another metabolite, which is identified as cyclic ADP-ribose 2'-phosphate. Under the same conditions, CD38 converts NADP+ to ADP-ribose 2'-phosphate instead, which is the hydrolysis product of cyclic ADP-ribose 2'-phosphate. That two different products of ADP-ribosyl cyclase and CD38, cADPR and NAADP+, are both involved in Ca2+ mobilization suggests a crucial role of these enzymes in Ca2+ signaling.

Magnesium ions but not ATP inhibit cyclic ADP-ribose-induced calcium release.

The pharmacology of the cyclic ADP-ribose (cADPR)-dependent Ca2+ release mechanism is very similar to that of the ryanodine receptor (RyR). Here we showed that MgCl2, a known inhibitor of RyR, blocked cADPR-induced Ca+2 release in sea urchin egg homogenates with a half maximal concentration of about 2.5 mM. The effect was specific since up to 10 mM Mg+2 had no effect on the Ca+2 release induced by inositol trisphosphate. K2ATP, another known modulator of RyR, at up to 10 mM did not affect the half-maximal concentration of cADPR, which remained at about 96 nM. These results indicate cADPR is a specific Ca+2 release activator and not merely an adenine nucleotide acting on the ATP-site. The inhibitory effects of Mg+2 further demonstrate the similarity between RyR and the cADPR-dependent Ca+2 release system.

Cyclic GMP-dependent and -independent effects on the synthesis of the calcium messengers cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate.

Cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) have been shown to mobilize intracellular Ca2+ stores by totally independent mechanisms, which are pharmacologically distinct from that activated by inositol trisphosphate. Although cADPR and NAADP are structurally and functionally different, they can be synthesized by a single enzyme having ADP-ribosyl cyclase activity. In this study, three different assays were used to measure the metabolism of cADPR in sea urchin egg homogenates including a radioimmunoassay, a Ca2+ release assay, and a thin layer chromatographic assay. Soluble and membrane-bound ADP-ribosyl cyclases were identified and both cyclized NAD to produce cADPR. The soluble cyclase was half-maximally stimulated by 5.3 microM cGMP, but not by cAMP, while the membrane-bound form was independent of cGMP. The two forms of the cyclase were also different in the pH dependence of utilizing nicotinamide guanine dinucleotide (NGD), a guanine analog of NAD, as substrate, indicating they are two separate enzymes. The stimulatory effect of cGMP required ATP or ATPgammaS (adenosine 5'-O-(3-thiotriphosphate)) and a cGMP-dependent kinase activity was shown to be present in the soluble fraction. The degradation of cADPR to ADP-ribose was catalyzed by cADPR hydrolase, which was found to be predominantly associated with membranes. Similar to the membrane-bound cyclase, the cADPR hydrolase activity was also independent of cGMP. Both the soluble and membrane fractions also catalyzed the synthesis of NAADP through exchanging the nicotinamide group of NADP with nicotinic acid (NA). The base-exchange activity was independent of cGMP and the half-maximal concentrations of NADP and NA needed were about 0.2 mM and 10 mM, respectively. The exchange reaction showed a preference for acidic pH, contrasting with the neutral pH optimum of the cyclase activities. The complex metabolic pathways characterized in this study indicate that there may be a multitude of regulatory mechanisms for controlling the endogenous concentrations of cADPR and NAADP.

Regulation of cyclic GMP metabolism in toad photoreceptors. Definition of the metabolic events subserving photoexcited and attenuated states.

Photoreceptor metabolism of cGMP and its regulation were characterized in isolated toad retinas by determining the intensity and time dependence of light-induced changes in the following metabolic parameters: cGMP hydrolytic flux determined by the rate of 18O incorporation from 18O-water into retinal guanine nucleotide alpha-phosphoryls; changes in the total (protein-bound and unbound) concentrations of the guanine nucleotide metabolic intermediates; and changes in the concentration of metabolic (unbound) GDP calculated from the fraction of the alpha-GDP that undergoes labeling with 18O. The latter is interpreted to reflect the state of the equilibrium between GDP- and GTP-complexed forms of G-protein. With narrow band 500 nm light that preferentially stimulates red rod photoreceptors, a range of intensities covering approximately 5 log units produced increases of over 10-fold in cGMP metabolic flux. However, the characteristics of the cGMP metabolic response over the first 2.5 log units of intensity are readily distinguishable from those at higher intensities which exhibit progressive attenuation by an intensity- and time-dependent process. Over the range of low intensities (0.6-3 log photons.micron-2.s-1) the metabolic response is characterized by 1) increases in cGMP hydrolytic flux of up to 8-fold as a logarithmic function of intensity of photic stimulation that are sustained for at least 200 s; 2) small increases or no change in the concentration of total cGMP; 3) large increases of up to 10-fold in the concentration of metabolically active GDP as a linear function of intensity with no significant change in the tissue concentrations of total GDP or GTP; and 4) amplification of the photosignal by the metabolism of approximately 10,000 molecules of cGMP per photoisomerization with the major site of amplification at the level of the interaction of bleached rhodopsin with G-protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Light-induced increases in cGMP metabolic flux correspond with electrical responses of photoreceptors.

The metabolism of photoreceptor cGMP and the relationship of its light-sensitive regulation to rhodopsin photoisomerization and to the photoreceptor electrical response was examined in isolated, intact rabbit retinas. The dynamics of cGMP metabolism were assessed by measuring the rate of 18O incorporation from 18O-water into the alpha-phosphoryls of the guanine nucleotides. The photoreceptor electrical response was determined by measuring the aspartate-isolated mass receptor potential. Basal cGMP flux in dark-adapted retinas was 33 pmol cGMP X mg protein-1 X s-1 which translates into a metabolic rate in the rod outer segment (ROS) of 1.7 mM/min in ATP equivalents. Photic stimulation increased this flux as much as 4.5-fold. With continuous illumination, increasing intensity caused increments in cGMP metabolic flux to a maximum of 4.5-fold, with corresponding increases in the electrical response over the same 3-log unit intensity range. Tight coupling between activation of guanylate cyclase and phosphodiesterase was indicated by either no changes in cGMP steady state concentrations or relatively small fluctuations represented by increases of 50% at lower light intensities and a 12% decrease at one of the highest intensities. A stoichiometry of about 10,000 molecules of cGMP generated and hydrolyzed per photon absorbed was calculated for the lowest light intensity when the increment in cGMP metabolic flux per photon was maximal. Flashing light caused an increase in flux in proportion to frequency up to 1 Hz and a nearly proportional increase in the voltage time integral of the electrical response up to 0.5 Hz. This indicates that the temporal resolution, or "on"/"off" rate, of the cGMP metabolic response was as fast or faster than the temporal resolution of the electrical response. The concentration of cGMP remained relatively stable in spite of the marked acceleration of cGMP flux that occurred over the 32-fold range of frequencies tested. Taken together these results show that the light-accelerated rate of cGMP synthesis tightly coupled to hydrolysis becomes a primary energy-utilizing system in the photoreceptor and represents a response that fulfills certain of the fundamental criteria required of a metabolic event playing an essential role in phototransduction.

Adenosine triphosphate utilization rates and metabolic pool sizes in intact cells measured by transfer of 18O from water.

The hydrolytic rates and metabolic pool sizes of ATP were determined in intact cells by monitoring the time courses of 18O incorporation from 18O-water into the gamma-phosphoryl of ATP and orthophosphate. To calculate the rate of ATP hydrolysis, a kinetic model is used to fit the time course of the 18O labeling. The size of the metabolic pool of ATP is calculated from the 18O distribution after isotopic equilibrium has been achieved. Metabolic pools have a binomial distribution of 18O whereas nonmetabolic pools exhibit negligible 18O labeling. The application and limitations of this approach are illustrated with data from isolated toad retinas and human platelets. At 22 degrees C, the time constant of ATP hydrolysis in the dark-adapted toad retina is about 30 s. Under these conditions, over 80% of the retinal ATP is involved in high-energy phosphate metabolism. It is calculated that when cGMP metabolic flux in the photoreceptors is maximally stimulated by light, it accounts for 10% of the ATP utilization by the entire retina. The time constant of ATP hydrolysis in human platelets at 37 degrees C is approximately 1 s, and 60% of the platelet ATP is involved in energy metabolism.