Measurement of the reversibility of ATP binding to myosin in calcium-activated skinned fibers from rabbit skeletal muscle. Oxygen exchange between water and ATP released to the solution

We have measured the rate constant for ATP release from myosin heads of Ca2+-activated, demembranated muscle fibers using the technique of phosphate-water oxygen exchange. Single rabbit psoas fibers were held in an activating solution in [18O]water ([MgATP] = 8 mM, ionic strength = 0.2 M, pH = 7.0, 24 degrees C). After about 20% hydrolysis of ATP, product Pi and remaining ATP were isolated, and the distribution of 18O in both molecules was analyzed using a mass spectrometer. The exchange in Pi was similar to that previously reported (Hibberd, M. G., Webb, M. R., Goldman, Y. E., and Trentham, D. R. (1985) J. Biol. Chem. 260, 3496-3501). The amount of 18O in ATP gave a rate constant of about 4 s-1 for ATP release, if it is assumed that each rate constant in the pathway of ATP hydrolysis has the same value for all myosin ATPase sites. However, the distribution of 18O in both released Pi and ATP is not well explained by a single pathway for ATP hydrolysis. We present a model that indicates how such distributions could arise from a range of values for the rate constants for Pi and ATP release from actomyosin, and this range is determined by differences in the amounts of strain in attached crossbridges. The kinetic information obtained from these isotope exchange experiments is compared to show that they give a compatible set of rate constants for actomyosin in fibers.     

Reversible effects of glycerol on the metabolism of platelets kept at room temperature

The effects of the addition and removal of glycerol on the metabolic activities of human platelets were studied. Platelet concentrates (PC) with 20 ml plasma were stored with 3-7% (v/w) glycerol in 150-ml polyvinylchloride plastic bags for 2 days at 22 degrees C with constant agitation. Incubation of glycerol with platelets produced a dose-dependent inhibition of oxygen consumption. The inhibitions of glucose utilization and lactate production had reached the plateau level at 3% glycerol. The rate of adenosine triphosphate (ATP) generation of control platelets was 9.8 nmol/min/10(9) platelets, in which over 90% ATP generation was derived from oxidative phosphorylation. There was a dose-dependent decrease (up to 20%) by glycerol in the rate of platelet ATP generation. Glycerol inhibited glycolysis more than oxidative phosphorylation. However, the inhibition potency diminished with increasing concentrations of glycerol. The energy metabolism of platelets after removal of 5% glycerol was examined. Deglycerolized platelets after 1 hr incubation facilitated energy metabolism more strongly than that of 24 hr incubation. The platelet aggregation response to collagen was not impaired by a cycle of the addition and removal of glycerol. The results indicate that glycerol lowered the rate of ATP generation of platelets stored at 22 degrees C. However, the removal of glycerol reversed the decreased energy metabolism.     

Transfer of adenine nucleotides between the releasable and nonreleasable compartments of rabbit blood platelets

The metabolic pool of adenine nucleotides in platelets can be labeled by incubating platelets for 1 h in vitro with [14C]adenosine or [32P]orthophosphate. When these platelets are treated with thrombin, the adenine nucleotides released are not labeled. Because of this, Holmsen's suggestion of a metabolically inert pool of granule nucleotides has been generally accepted. We have found that upon incubation of labeled rabbit platelets for longer times (up to 6 h) in vitro, or upon reinjection and reharvesting at times up to 66 h, the releasable pool of adenine nucleotides becomes labeled. Because the rates of 32p and 14C incorporation into this releasable pool are similar, it seems likely that these labels enter the granules as ATP. Equilibrium between the metabolic and granule pools is complete by 18 h. When rabbit platelets are labeled in vivo by intravenous injection of [32P]orthophosphate, peak labeling occurs between 60 and 70 h; this corresponds to their maturation time. The platelets probably incorporate 32P during their production in the megakaryocytes. The specific radioactivity of the adenine nucleotides in the releasable (granule) pool of these platelets is the same as the specific radioactivity in the nonreleasable (metabolic) pool. Since inorganic phosphate in platelets (and undoubtedly in the megakaryocytes) exchanges with inorganic phosphate in plasma, and since the radioactivity of the latter decreases rapidly, the adenine nucleotides in the two pools must exchange to maintain the same specific radioactivity. Transfer of adenine nucleotides into storage granules may represent a general phenomenon because it has been observed in the chromaffin cells of the adrenal medulla also.     

Magnitude of increase in retinal cGMP metabolic flux determined by 18O incorporation into nucleotide alpha-phosphoryls corresponds with intensity of photic stimulation

The property of cyclic nucleotide phosphodiesterases to catalyze 3'-P--O bond cleavage and the insertion of a single nonexchangeable atom of 18O from [18O]water into the phosphoryl of the 5'-nucleotide product has been utilized as a means for measuring the hydrolytic flux of cGMP and cAMP in isolated dark-adapted intact rabbit retinas. Without illumination 18O labeling of guanine nucleotide (GTP and GDP) alpha-phosphoryls proceeds linearly for at least 80 s at a rate of 3.3 nmol of 18O/s.g of retina (wet weight). This rate is estimated to be approximately 8 times greater in the rod outer segment layer where over 90% of retinal cGMP metabolic components reside. Photic stimulation during a 20-s incubation was provided by intermittent flashes of light representing 800 ms of total illumination. Light stimuli over a range of intensities of greater than 3 log units commencing with a minimally detectable intensity produce graded increments in the rate of 18O incorporation into guanine nucleotide alpha-phosphoryls to a maximum increase of 5-fold. On the basis of only the 800-ms period of illumination this maximum increase is 125-fold. Steady state levels of retinal cGMP are not altered appreciably over this greater than 3 log range of light intensities but a light stimulus exceeding this intensity range causes an approximate 50% decrease in retinal cGMP concentration and a relative decline in the maximal rate of 18O labeling of guanine nucleotide alpha-phosphoryls. No light-related increases were detected in 18O incorporation into adenine nucleotide alpha-phosphoryls nor the gamma-phosphoryls of GTP or ATP or Pi. These observations indicate that light stimuli over greater than 3 log of light intensity produce incremental increases in cGMP metabolic flux that result from comparable increases in the rates of both cGMP generation and cGMP hydrolysis. It is postulated that increases in cGMP metabolic flux rather than changes in cGMP steady state levels are integral to phototransduction by a mechanism that involves the coupling of cGMP synthesis and/or hydrolysis to either the release of calcium from disc membranes or the inhibition of Na+ conductance by the photoreceptor membrane. This is suggested to occur by an energy-linked process and/or the generation of protons.     

Use of actin-bound adenosine 5'-diphosphate as a method to determine the specific 32P-radioactivity of the gamma-phosphoryl group of adenosine 5'-triphosphate in a highly compartmentalized cell, the platelet

Determination of the specific 32P-radioactivity of cytoplasmic ATP in 32P-Pi-labeled platelets is complicated by the presence of a large pool of metabolically inactive, granule-stored nucleotides. Moreover, our data show that the specific 32P-radioactivity of cytoplasmic ATP is severely underestimated when determined in platelets after the complete secretion of granule-stored nucleotides, possibly due to isotopic dilution with granule-stored phosphate. As F-actin-bound ADP is ethanol-insoluble, this pool can be readily separated from the other nucleotide pools in platelets. Here we show that the specific 32P-radioactivity of F-actin-bound ADP accurately reflects that of the gamma-phosphoryl group of cytoplasmic ATP. During uptake of 32P-Pi by human platelets the specific 32P-radioactivity of F-actin-bound ADP equals that of the monoester phosphates of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate, which are in metabolic equilibrium with cytoplasmic ATP. Therefore, this method enables the determination of the specific 32P-radioactivity of the gamma-phosphoryl group of cytoplasmic ATP in platelets even under short-term labeling conditions.     

Balanced contribution of glycolytic and adenylate pool in supply of metabolic energy in platelets

When platelets are treated with H2O2 the metabolic ATP content decreases sharply (Holmsen, H., and Robkin, L. (1977) J. Biol. Chem. 252, 1752-1757). Here we report that the loss of metabolic energy is fully recovered in phosphorylated glycolytic intermediates. A mixture of antimycin A/2-deoxy-D-glucose/D-gluconic acid-1,5-lactone blocks mitochondrial ATP resynthesis and prevents the entry of sugars into the glycolytic sequence. The energy-rich phosphates in the adenylate and the glycolytic pool are then consumed in a specific order. First, the glycolytic pool is consumed at a rate of 4.5 mumol of ATP equivalents/min/10(11) cells, and metabolic ATP and ADP are kept stable; then the consumption of the glycolytic pool decreases and metabolic ATP and ADP are consumed, together keeping up with the same rate of energy consumption. Thrombin stimulation increases the energy consumption to about 17 mumol of ATPeq/min/10(11) cells which is now furnished by both the glycolytic and the adenylate pool, again with a preferential consumption of the former. The results show that H2O2 triggers a shift of energy-rich phosphates from the adenylate to the glycolytic pool and that the latter remains rapidly accessible to energy consumption thereby stabilizing the level of metabolic ATP. The adenylate energy charge is independent of the distribution of energy among the two pools, which extends its importance to the regulation of energy supply and demand beyond the adenylate pool.     

Oxygen exchange between phosphate and water accompanies calcium-regulated ATPase activity of skinned fibers from rabbit skeletal muscle

The extent of oxygen exchange between phosphate and water has been measured for the calcium-regulated magnesium-dependent ATPase activity of chemically skinned fibers from rabbit skeletal muscle. The oxygen exchange was determined for isometrically held fibers by measuring with a mass spectrometer the distribution of 18O atoms in the product inorganic phosphate when ATP hydrolysis was carried out in H2(18)O. The extent of exchange was much greater in relaxed muscle (free Ca2+ less than 10(-8) M) than in calcium-activated muscle (free Ca2+ approximately equal to 3 X 10(-5) M). Activated fibers had an ATPase activity at least 30-fold greater than the relaxed fibers. These results correlate well with the extents of oxygen exchange accompanying magnesium-dependent myosin and unregulated actomyosin ATPase activities, respectively. In relaxed fibers, comparison of the amount of exchange with the ATPase activity suggests that the rate constant for the reformation of myosin-bound ATP from the myosin products complex is about 10 s-1 at 20 degrees C and pH 7.1. In each experiment the distribution of 18O in the Pi formed was incompatible with a single pathway for ATP hydrolysis. In the case of the calcium-activated fibers, the multiple pathways for ATP hydrolysis appeared to be an intrinsic property of the actomyosin ATPase in the fiber. These results indicate that in muscle fibers, as in isolated actomyosin, cleavage of protein-bound ATP is readily reversible and that association of the myosin products complex with actin promotes Pi release.     

ATP binding/hydrolysis by and phosphorylation of peroxisomal ATP-binding cassette proteins PMP70 (ABCD3) and adrenoleukodystrophy protein (ABCD1)

The 70-kDa peroxisomal membrane protein (PMP70) and adrenoleukodystrophy protein (ALDP), half-size ATP-binding cassette transporters, are involved in metabolic transport of long and very long chain fatty acids into peroxisomes. We examined the interaction of peroxisomal ATP-binding cassette transporters with ATP using rat liver peroxisomes. PMP70 was photoaffinity-labeled at similar efficiencies with 8-azido-[alpha-32P]ATP and 8-azido-[gamma-32P]ATP when peroxisomes were incubated with these nucleotides at 37 degrees C in the absence Mg2+ and exposed to UV light without removing unbound nucleotides. The photoaffinity-labeled PMP70 and ALDP were co-immunoprecipitated together with other peroxisomal proteins, which also showed tight ATP binding properties. Addition of Mg2+ reduced the photoaffinity labeling of PMP70 with 8-azido-[gamma-32P]ATP by 70%, whereas it reduced photoaffinity labeling with 8-azido-[alpha-32P]ATP by only 20%. However, two-thirds of nucleotide (probably ADP) was dissociated during removal of unbound nucleotides. These results suggest that ATP binds to PMP70 tightly in the absence of Mg2+, the bound ATP is hydrolyzed to ADP in the presence of Mg2+, and the produced ADP is dissociated from PMP70, which allows ATP hydrolysis turnover. Properties of photoaffinity labeling of ALDP were essentially similar to those of PMP70. Vanadate-induced nucleotide trapping in PMP70 and ALDP was not observed. PMP70 and ALDP were also phosphorylated at a tyrosine residue(s). ATP binding/hydrolysis by and phosphorylation of PMP70 and ALDP are involved in the regulation of fatty acid transport into peroxisomes.     

Oxygen-exchange studies on the pathways for magnesium adenosine 5'-triphosphate hydrolysis by actomyosin

At an intermediate stage in the hydrolysis of magnesium adenosine 5'-phosphate (MgATP) by myosin or actomyosin, there is an exchange of oxygen between water and the P gamma group of enzyme-bound nucleotide. Starting with [P gamma-18O]ATP as substrate, the exchange is revealed in the [18O]Pi species that are ultimately released as product into the reaction medium. An analysis of the distribution of these labeled Pi species, which contain 3, 2, 1, or none of the 18O atoms originally on the P gamma of ATP, is used to probe intermediate stages of the hydrolytic mechanism. In recent years, studies of this kind by several groups have shown that more than one pathway of hydrolysis operates. The work reported here demonstrates that two of these pathways are spurious; one is a "nonexchanging MgATPase" that is present in fresh myosin preparations; the other is an induced slow exchange that develops in myosin during storage (-20 degrees C) and subsequent aging (4 degrees C). However, after correction for these artifacts, two normal pathways for actomyosin hydrolysis remain. These normal pathways differ in the mode of interaction between actin and myosin in the course of hydrolysis; one is the Lymn-Taylor pathway where oxygen exchange occurs at a stage when actin and myosin are dissociated; the other is a pathway in which actin and myosin are associated during oxygen exchange. Each of these two pathways contributes an equal amount of Pi to the product pool. Thus, on average, each myosin head uses each of these pathways half the time. The findings suggest, e.g., that during contraction, myosin can dissociate from the actin filament only during every other cycle of MgATP hydrolysis or that only half the heads, at any one time, can exchange oxygen while free of the actin filament.     

Composition and Distribution of Adenylates in Soybean (Glycine max L.) Nodule Tissue

Adenylates (ATP, ADP, and AMP) may play a central role in the regulation of the O2-limited C and N metabolism of soybean nodules. To be able to interpret measurements of adenylate levels in whole nodules and to appreciate the significance of observed changes in adenylates associated with changes in O2-limited metabolism, methods were developed for measuring in vivo levels of adenylate pools in the cortex, plant central zone, and bacteroid fractions of soybean (Glycine max L. Merr cv Maple Arrow x Bradyrhizobium japonicum strain USDA 16) nodules. Intact nodulated roots were either frozen in situ by flushing with prechilled Freon-113(-156[deg]C) or by rapidly (<1 s) uprooting plants and plunging them into liquid N2. The adenylate energy charge (AEC = [ATP + 0.5 x ADP]/[ATP + ADP + AMP]) of whole-nodule tissue (0.65 [plus or minus] 0.01, n = 4) was low compared to that of subtending roots (0.80 [plus or minus] 0.03, n = 4), a finding indicative of hypoxic metabolism in nodules. The cortex and central zone tissues were dissected apart in lyophilized nodules, and AEC values were 0.84 [plus or minus] 0.04 and 0.61 [plus or minus] 0.03, respectively. Although the total adenylate pool in the lyophilized nodules was only 41% of that measured in hydrated tissues, the AEC values were similar, and the lyophilized nodules were assumed to provide useful material for assessing adenylate distribution. The nodule cortex contained 4.4% of whole-nodule adenylates, with 95.6% being located in the central zone. Aqueous fractionation of bacteroids from the plant fraction of whole nodules and the use of marker enzymes or compounds to correct for recovery of bacteroids and cross-contamination of the bacteroid and plant fractions resulted in estimates that 36.2% of the total adenylate pool was in bacteroids, and 59.4% was in the plant fraction of the central zone. These are the first quantitative assessments of adenylate distribution in the plant and bacteroid fractions of legume nodules. These estimates were combined with theoretical calculations of rates of ATP consumption in the cortex (9.5 nmol g-1 fresh weight of nodule s-1), plant central zone (38 nmol g-1 fresh weight of nodule s-1), and bacteroids (62 nmol g-1 fresh weight of nodule s-1) of soybean nodules to estimate the time constants for turnover of the total adenylate pool and the ATP pool within each nodule fraction. The low values for time constant (1.6-5.8 s for total adenylate, 0.9-2.5 s for ATP only) in each fraction reflect the high metabolic activity of soybean nodules and provide a background for further studies of the role of adenylates in O2-limited nodule metabolism.     

Relation between energy production and adenine nucleotide metabolism in human blood platelets

The relation between ATP production and adenine nucleotide metabolism was investigated in human platelets which were starved by incubation in glucose-free, CN^?-containing medium and subsequently incubated with different amounts of glucose. In the absence of mitochondrial energy production (blocked by CN^?) and glycogen catabolism (glycogen almost completely consumed during starvation), lactate production increased proportionally with increasing amounts of glucose. The generated ATP was almost completely consumed in the various ATP-consuming processes in the cell except for a fixed portion (about 7%) that was reserved for restoration of the adenylate energy charge. During the first 10 min after glucose addition, the adenine nucleotide pool remained constant. Thereafter, when the glycolytic flux, measured as lactate formation, was more than 3.5 μmol · min^?1 · 10^?11 cells, the pool increased slightly by resynthesis from hypoxanthine-inosine and then stabilized; at a lower flux the pool decreased and metabolic ATP and energy charge declined to values found during starvation. Between moments of rising and falling adenylate energy charges, periods of about 10 min remained in which the charge was constant and ATP supply and demand had reached equilibrium. This enabled comparison between the adenylate energy charge and ATP regeneration velocity. A linear relation was obtained for charge values between 0.4 and 0.85 and ATP regeneration rates between 0.6 and 3.5 ATP equiv. · min^?1 · 10^?11 cells. These data indicate that in starved platelets ATP regeneration velocity and energy charge are independent and that each appears to be subject to the availability of extracellular substrate.     

Effects of meal size,meal type,body temperature,and body size on the specific dynamic action of the marine toad,Bufo marinus

Specific dynamic action (SDA), the accumulated energy expended on all physiological processes associated with meal digestion, is strongly influenced by features of both the meal and the organism. We assessed the effects of meal size, meal type, body temperature, and body size on the postprandial metabolic response and calculated SDA of the marine toad, Bufo marinus. Peak postprandial rates of O(2) consumption (.V(O2)) and CO(2) production (.V(CO2)) and SDA increased with meal size (5%-20% of body mass). Postprandial metabolism was impacted by meal type; the digestion of hard-bodied superworms (Zophobas larva) and crickets was more costly than the digestion of soft-bodied earthworms and juvenile rats. An increase in body temperature (from 20 degrees to 35 degrees C) altered the postprandial metabolic profile, decreasing its duration and increasing its magnitude, but did not effect SDA, with the cost of meal digestion remaining constant across body temperatures. Allometric mass exponents were 0.69 for standard metabolic rate, 0.85 for peak postprandial .V(O2), and 1.02 for SDA; therefore, the factorial scope of peak postprandial .V(O2) increased with body mass. The mass of nutritive organs (stomach, liver, intestines, and kidneys) accounted for 38% and 20% of the variation in peak postprandial .V(O2) and SDA, respectively. Toads forced to exercise experienced 25-fold increases in .V(O2) much greater than the 5.5-fold increase experience during digestion. Controlling for meal size, meal type, and body temperature, the specific dynamic responses of B. marinus are similar to those of the congeneric Bufo alvarius, Bufo boreas, Bufo terrestris, and Bufo woodhouseii.     

The ATPase reaction cycle of yeast DNA topoisomerase II. Slow rates of ATP resynthesis and P(i) release

DNA topoisomerase II catalyzes the transport of one DNA duplex through a transient break in a second duplex using a complex ATP hydrolysis mechanism. Two key rates in the ATPase mechanism, ATP resynthesis and phosphate release, were investigated using 18O exchange and stopped-flow phosphate release experiments, respectively. The 18O exchange results showed that the rate of ATP resynthesis on the topoisomerase II active site was slow compared with the rate of phosphate release. When topoisomerase II was bound to DNA, phosphate was released slowly, with a lag. Since each of the preceding steps is known to occur rapidly, phosphate release is apparently a rate-determining step. The length of the lag phase was unaffected by etoposide, indicating that inhibiting DNA religation inhibits the ATPase reaction cycle at some step following phosphate release. By combining the 18O exchange and phosphate release results, the rate constant for ATP resynthesis can be calculated as approximately 0.5 s(-1). These data support the mechanism of sequential hydrolysis of two ATP by DNA topoisomerase II.     

Adenine nucleotide metabolism of blood platelets. IX. Time course of secretion and changes in energy metabolism in thrombin-treated platelets.

Changes in the energy metabolism of washed human platelets were compared with the kinetics of secretion induced by thrombin (5 units/ml). A 50% decrease in the level of metabolic ATP (3H-labelled), which was essentially complete in 30s, was matched in rate by adenine nucleotide secretion from storage in dense granules. Incubation of platelets with antimycin before thrombin addition increased the rate of fall in metabolic ATP, but did not affect the rate of adenine nucleotide secretion. beta-N-Acetylglucosaminidase secretion, which was slower than adenine nucleotide secretion in control platelets, was noticeably inhibited by antimycin, confirming previous reports that different regulatory mechanisms exist for dense and alpha-granule secretion. The rates of rephosphorylation of metabolic ADP to ATP via glycolysis and oxidative phosphorylation were estimated by measuring lactate production and O2 consumption in resting and thrombin-stimulated platelets and compared to the level of metabolic ATP (9-10 nmol/mg of platelet protein in the resting state). The rate of ATP production was stimulated at least two fold from 12 nmol to 24 nmol/min/mg within seconds of thrombin addition. This increased rate was maintained over the observed period of 5 min although the level of metabolic ATP had decreased to 4-5 nmol/mg within 30 s; the turnover of the remaining metabolic ATP thus increased four fold over the resting state although the actual stimulation of energy production was only two fold.     

Oxygen exchange reaction during ATP hydrolysis by glycerinated muscle fibers, myofibrils, and synthetic actomyosin filaments

The oxygen exchange during ATP hydrolysis by glycerinated muscle fibers, myofibrils, and synthetic actomyosin filaments was studied from the distribution of the [18O]Pi species produced by the hydrolysis of [gamma-18O]ATP. The products were mixtures of two species, one with a low extent of oxygen exchange and the other with a high extent. The low and high extents of oxygen exchange in these two Pi species were the same as those of the acto-S-1 ATPase reaction through the routes with and without the dissociation of actomyosin, respectively (Yasui, M., Ohe, M., Kajita, A., Arata, T., & Inoue, A. [1988] J. Biochem. 104, 550-559). During isometric contraction of glycerinated muscle fibers at 20 degrees C, the fraction of ATP hydrolysis with low extent of oxygen exchange was 0.83 and 0.70, respectively, in 0 and 120 mM KCl. In myofibrils, the fraction of ATP hydrolysis with a low extent of oxygen exchange was 0.72-0.88 in 0-120 mM KCl at 20 degrees C. Therefore, in glycerinated muscle fibers and myofibrils ATP seems to be mainly hydrolyzed through a route without the dissociation of actomyosin, especially at low ionic strength and at room temperature when the tension development is high. ATP hydrolysis through this route may be coupled with muscle contraction.     

Kinetics of energy consumption in human platelets with blocked ATP regeneration

Abrupt arrest of ATP resynthesis in blood platelets induces a rapid decline in metabolic ATP-ADP. This decline is biexponential with a 7-fold difference in the rate-constants of the two components. Stimulation with thrombin increases both rate-constants, and raises the relative contribution of the rapid component from 60 to 90% of total. The initial decline can be approximated by a single exponential term, yielding the rate-constant for initial ATP hydrolysis. Since this initial decline reflects energy consumption of undisturbed platelets, this approach offers a sensitive means to determine energy consumption and ATP turnover within short time intervals.     

ADP release is rate limiting in steady-state turnover by the dynein adenosinetriphosphatase

The kinetics of the product release steps in the pathway of ATP hydrolysis by dynein were investigated by examining the rate and partition coefficient of phosphate-water 18O exchange under equilibrium and steady-state conditions. Dynein catalyzed both medium and intermediate phosphate-water oxygen exchange with a partition coefficient of 0.30. The dependence of the rate of loss of the fully labeled phosphate species on the concentration of ADP was hyperbolic, with an apparent Kd for the binding of ADP to dynein of 0.085 mM. The apparent second-order rate constant for phosphate binding to the dynein-ADP complex was 8000 M-1 s-1. The time course of medium phosphate-water oxygen exchange during net ATP hydrolysis was examined in the presence of an ATP regeneration system. The observed rate of loss of P18O4 was comparable to the rate observed at saturating ADP which implies that ADP release is rate limiting for dynein in the steady state. Product inhibition of the dynein ATPase was also examined. ADP inhibited the enzyme competitively with a Ki of 0.4 mM. Phosphate was a linear noncompetitive mixed-type inhibitor with a Ki of 11 mM. These data were fit to a model in which phosphate release is fast and is followed by rate-limiting release of ADP, allowing us to define each rate constant in the pathway. A discrepancy between the total free energy calculated compared to the known free energy of ATP hydrolysis suggests that there is an additional step in the pathway, perhaps involving a change in conformation of the enzyme-ADP state preceding ADP release.     

Tight coupling of thrombin-induced acid hydrolase secretion and phosphatidate synthesis to receptor occupancy in human platelets.

Human platelets incubated with [32P]Pi and [3H]arachidonate were transferred to a Pi-free Tyrode's solution by gel filtration. The labile phosphoryl groups of ATP and ADP as well as Pi in the metabolic pool of these platelets had equal specific radioactivity which was identical to that of[32P]phosphatidate formed during treatment of the cells with thrombin for 5 min. Therefore, the 32P radioactivity of phosphatidate was a true, relative measure for its mass. The thrombin-induced formation of[32P]-phosphatidate had the same time course and dose-response relationships as the concurrent secretion of acid hydrolases. 125I-alpha-Thrombin bound maximally to the platelets within 13s and was rapidly dissociated from the cells by hirudin; readdition of excess 125I-alpha-thrombin caused rapid rebinding of radioligand. This binding-dissociation-rebinding sequence was paralleled by a concerted start-stop-restart of phosphatidate formation and acid hydrolase secretion. [3H]Phosphatidylinositol disappearance was initiated upon binding but little affected by thrombin dissociation and rebinding. ATP deprivation caused similar changes in the time courses for [32P]-phosphatidate formation and acid hydrolase secretion which were different from those of [3H]phosphatidylinositol disappearance. The metabolic stress did not alter the magnitude (15%) of the initial decrease in phosphatidylinositol-4,5-bis[32P]phosphate, but did abolish the subsequent increase of phosphatidylinositol-4,5-bis[32P]-phosphate in the thrombin-treated platelets. It is concluded that in thrombin-treated platelets (1) phosphatidate synthesis, but not phosphatidylinositol disappearance, is tightly coupled to receptor occupancy and acid hydrolase secretion in platelets, (2) successive phosphorylations to phosphatidylinositol-4,5-bisphosphate is unlikely to be the main mechanism for phosphatidylinositol disappearance, and (3) only a small fraction (15%) of phosphatidylinositol-4,5-bisphosphate is susceptible to hydrolysis.     

Free fatty acids, diacyl- and triacylglycerols and total phospholipids in vertebrate retina: comparison with brain, choroid and plasma

Abstract— A comparative study of the concentration and fatty acid distribution in diacyl- and triacylgly-cerols. free fatty acids and total phospholipids from rabbit, cattle and toad retina is presented. With respect to the toad, a comparison is made with brain, choroid and plasma lipids. Marked differences in diacylglycerol composition and levels between mammalian and toad retina are found: in the mammal arachidonate predominates (25 per cent), in the toad docosahexaenoate is the main fatty acid (42 per cent). The total phospholipid composition parallels that of the diacylglycerols only in the toad, whereas in the mammalian retina the phospholipids are richer in docosahexaenoate than are the diacylglycerols. It is suggested that there is a relationship between diacylglycerols and phosphoglyceride metabolism in the toad; in the retinas of other species the diacylglycerols may be related to specific phosphatides. In the three species, triacylglycerols show high levels of unsaturation; however, marked differences are found in the distribution of polyunsaturated acyl groups: in the cattle and toad retina docosahexaenoate predominates. whereas in the rabbit a higher proportion of 22:4 is found. Retina free fatty acid pools also show different features in the three species: the cattle retina contains the highest proportion of free 20:4 and 22:6. The triacylglycerol concentration is much higher in the toad choroid than in the retina, although the fatty acid compositions are similar. A possible relationship between these choroid lipids and those of the retina is suggested.     

Temperature Coefficients for the Oxidative Metabolic Responses to Electrical Stimulation in Cerebral Cortex

Temperature coefficients of both cat and toad brain have been calculated for the active metabolic state induced by electrical stimulation. Values are higher than most of the values previously reported for "rest" metabolism, whether calculated from Arrhenius plots or from linear graphs. Relative rates of oxidative metabolism were obtained by measuring the time course of the transient changes in NADH fluorescence and cytochrome aa3 absorption by reflectance techniques directly from the surface of the exposed cat cerebral cortex in vivo and from the isolated intact toad brain mounted in a cuvet. These findings demonstrate that such optical methods accurately record metabolic processes.