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Glycogenesis and glyconeogenesis in human platelets: Incorporation of , pyruvate, and citrate into platelet ; glycogen synthetase and -1,6-diphosphatase activity

Simon Karpatkin, … , Arthur Charmatz, Richard M. Langer

J Clin Invest. 1970;49(1):140-149. https://doi.org/10.1172/JCI106212.

Washed human platelets are capable of depositing 1-4 as well as probable 1-6 glucosyl linkages onto preexistent glycogen primer. They are also capable of degrading () newly synthesized 1-4 as well as probable 1-6 glucosyl linkages. A higher rate of glycogen synthesis was found in platelet suspensions containing lower concentrations of platelets. This was shown to result from decreased glycogen degradation and consequent increased residual glycogen primer in low platelet suspensions. The increased glycogen content of low platelet suspensions was not a result of platelet washing, removal of platelets from plasma, or release of platelet metabolites into the media. The glycogen synthetase was found to be present at a rate of 5.2 μmoles of diphosphate (UDP) glucose incorporated into glycogen per gram platelets per hour at 37°C. The Km for UDP glucose was 6.6 mmoles/liter. At optimum concentration of glucose 6-, the Km was reduced 4.6 fold and Vmax was increased 4.3-fold.

Human platelets contain the glyconeogenic pathway. They incorporate pyruvate-14C and citrate-14C into platelet glycogen and contain an apparent fructose-1,6-diphosphatase. The apparent fructose-1,6-diphosphatase was activated by monophosphate (AMP) and (ADP), inhibited by (ATP), and shown to be rate limiting for glyconeogenesis at physiologic concentration of .

Find the latest version: https://jci.me/106212/pdf Glycogenesis and Glyconeogenesis in Human Platelets INCORPORATION OF GLUCOSE, PYRUVATE, AND CITRATE INTO PLATELET GLYCOGEN; GLYCOGEN SYNTHETASE AND FRUCTOSE-1,6-DIPHOSPHATASE ACTIVITY SIMON KARPATKiN, AmTHuR CHARMATZ, and RIcmAuw M. LANGER From the Department of Medicine, New York University Medical Center, New York 10016

A B S T R A C T Washed human platelets are capable of stores, equivalent to those of skeletal muscle (1, 3). depositing 1-4 as well as probable 1-6 glucosyl link- Glycogenolysis in human platelets represents approxi- ages onto preexistent glycogen primer. They are also mately 50% of glycolytic flux through the Embden- capable of degrading (glycogenolysis) newly synthe- Meyerhof pathway in the presence of physiologic con- sized 1-4 as well as probable 1-6 glucosyl linkages. A centrations of glucose (1, 3). Human platelets contain higher rate of glycogen synthesis was found in plate- a glycogen degradative enzyme, phosphorylase, which let suspensions containing lower concentrations of plate- is similar in some respects to that of skeletal muscle lets. This was shown to result from decreased glycogen (4) but different in other respects (5). degradation and consequent increased residual glycogen This investigation was designed to determine whether primer in low platelet suspensions. The increased gly- glycogen synthesis takes place in human platelets and cogen content of low platelet suspensions was not a re- whether the "gluconeogenic" pathway was present, i.e., sult of platelet washing, removal of platelets from plasma, conversion of pyruvate and citrate to glucose or glyco- or release of platelet metabolites into the media. The gen. Isotopic labeling techniques were employed to mea- enzyme glycogen synthetase was found to be present sure incorporation of precursor into glycogen. The ex- at a rate of 5.2 /Amoles of (UDP) tent of glycogen labeling was determined by techniques glucose incorporated into glycogen per gram platelets which employed the isolation of previously labeled plate- per hour at 37'C. The Km for UDP glucose was 6.6 let glycogen during intervals of glycogen synthesis and mmoles/liter. At optimum concentration of glucose glycogen degradation. The isolated platelet glycogen was 6-phosphate, the Km was reduced 4.6 fold and V.a. was then analyzed for 1-4 glucosyl linkages by specific enzy- increased 4.3-fold. matic degradation. Human platelets contain the glyconeogenic pathway. They incorporate pyruvate-1'C and citrate-"C into plate- METHODS let glycogen and contain an apparent fructose-1,6-di- Fresh human platelets were obtained, washed, processed, . The apparent fructose-1,6-diphosphatase and incubated at 370C as described previously (1, 3, 6). was activated by adenosine monophosphate (AMP) and All materials were obtained from vendors as described adenosine diphosphate (ADP), inhibited by adenosine previously (1, 3, 6). Uniformly labeled glucose-1'C, 261 mc/ and shown to be rate limiting for mmole, citrate-14C labeled in the 1-5 position, 1.84 mc/ triphosphate (ATP), mmole, and uniformly labeled uridine diphosphoglucose-1'C glyconeogenesis at physiologic concentration of adenine (UDPG), 204 mc/mmole were obtained from Nuclear- nucleotide. Chicago Corporation, Des Plaines, Ill. Uniformly labeled pyruvate-14C, 10.5 mc/mmole was obtained from International Chemical & Nuclear Corp., Burbank, Calif. 8-Amylase, INTRODUCTION phosphorylase, amylo-1,6-glucosidase, phosphohexose isomerase, and fructose-1,6-diphosphate were obtained from Human platelets have a predominantly aerobic glycolytic Sigma Chemical Co., St. Louis, Mo. Fructose-1,6-diphos- (1, 2). They contain considerable glycogen phatase (1.1 U/mg) was obtained from Worthington Bio- chemical Corp., Freehold, N. J. Dr. Karpatkin is a Career Scientist of the Health Research Adenosine triphosphate (ATP), glucose, and lactate were Council of the City of New York (I-459). assayed enzymatically from neutralized perchloric acid ex- Received for publication 2 July 1969 and in revised form tracts, as described previously (1, 3). Fructose-1,6-diphos- 18 August 1969. phate was assayed by coupling the fructose-1,6-diphospha- 140 The Journal of Clinical Investigation Volume 49 1970 tase reaction to hexose isomerase and glucose 6-phosphate ,8-Amylase digestion of platelet glycogen. This procedure dehydrogenase and measuring the change in absorbance of was a minor modification of the method of Steinetz (8). triphosphopyridine nucleotide (NADP) to its reduced form, Briefly, 100 ,ug of f8-amylase was incubated with 1 mg of NAPDH, at 340 mg. The final concentration of reagents platelet glycogen in 1 ml of NaAc buffer, 16 mmoles/liter, employed were 0.46 mm NADP, 12.5 mm MgC12, 2 /g/ml pH 4.8 for 14 hr at 250C. These incubation conditions were glucose 6-phosphate dehydrogenase, 8 ,ug/ml hexose iso- found to be optimum. In control samples, ft-amylase was re- merase, 40 ,ug/ml fructose-1,6-diphosphatase, 20 mm mer- placed by a comparable volume of buffer. The glycogen captoethanol (MSH), in 50 mm Tris buffer, pH 7.5, and samples were then precipitated and redissolved. The dif- 200 ul/ml of neutralized perchlorate tissue extract. The ference in radioactivity and glycogen content between control reaction was started with addition of hexose isomerase. and enzyme digested samples represented the 1-4 linked tier Isotope experiments. Platelets were incubated in the pres- of branched glycogen beyond the 1-6 glucosyl linkage. The ehce of sufficient 1'C isotope to provide 1 X 106 cpm/ml in- limit dextrin obtained could be hydrolyzed to less than 5% cubation fluid. Various combinations of labeled and unlabeled of original radioactivity, as well as glycogen content, by in- glucose, pyruvate, or citrate were added to the incubation cubation with phosphorylase and amylo-1,6-glucosidase for media. After appropriate incubation, glycogen was pre- 20 hr at 250C. pared from platelets by extraction with 33%o KOH at 100°C Glycogen synthetase was measured by a radioisotopic pro- followed by two precipitations. Glycogen was as- cedure which was essentially a modification of the proce- sayed with anthrone reagent as described previously (3, 6). dure of Villar-Palasi and Larner (9). The platelet pellet Glycogen molarity is expressed as glucose units. The labeled was frozen in liquid nitrogen, ground in sand to a fine glycogen was dissolved in 1 ml of distilled water and an powder, and extracted in three times its volume of 50 mM aliquot pipetted into a glass counting vial containing Bray's Tris buffer, pH 8.2 containing 5 mM ethylenediaminetetraace- solution (7). Radioactivity was determined in a Beckman tate(EDTA) and 5 mm MSH. The optimal concentration LS100 scintillation spectrometer with isoset adjusted to give of MSH found to produce maximal enzyme activity was 5 96% counting efficiency. The radioactivity in the glycogen mmoles/liter. The extract was centrifuged at 4000 g at 40C samples was 60 times background after incubating plate- and the supernatant employed for glycogen synthetase de- lets with glucose-"C, 20 times background with pyruvate-14C, termination. The preparation was stable at 4°C for at least and 6-7 times background in citrate-"C experiments. The 1 wk. A 0.1 ml aliquot was added to 0.4 ml of enzyme incu- counting error was less than 3%. Control experiments re- bation solution at 37°C for 15 min containing (final con- vealed that addition of unlabeled platelet glycogen samples centration): UDPG-14C, 1 x 10. cpm/ml (varying concen- did not reduce the counting efficiency of a standard quantity tration of unlabeled UDPG), 0.5 g/100 ml glycogen, 5 mM of radioactivity. The concentration of tissue sample employed MSH, 5 mm EDTA, 50 mm Tris buffer, pH 8.2, and vary- for radioactivity measurements was well within the equal ing concentration of glucose-6-phosphate. The incubation was linear range of sample concentration addition vs. radio- terminated by the addition of 33% KOH, the glycogen activity obtained. extracted, and radioactivity assayed as described above.

1.3- 1.2 - 1.1-

Z 0.9- o U0.8 (i 0.7- , 0.6- U 0 SUSPENSION 0 0.5- 0.4-

0.1

2 HOURS FIGURE 1 Effect of platelet concentration on incorporation of glucose-14C into platelet glycogen. Washed human platelets were incubated with 5 mm glu- cose, 1 X 10 cpm/ml, at 37°C at two platelet suspension concentrations: 0.5%O, 0; and 5%, 0, x. Data are expressed as ,umoles/g wet weight, and umoles/10 gmoles glycogen, - -- -. Each time point represents 4-6 experiments. SEM was 10%o or less. Glycogenesis and Glyconeogenesis in Human Platelets 141 TABLE I Effect of Platelet Concentration on Glycogen Depletion, Lactate Production, and A TP Depletion*

Glycogen Platelet depletion Lactate ATP level suspension at 1 hr production at 1 hr ,gmoles/g pmoles/g per hr pmoles/g IVU 0.14- LOW PLATELET SUSPENSION 0.5% 12.5 ±0.80 (5)T 24.4 ±4.2§ (5) 2.01 (3) 1 5.0% 18.6 4±0.91 (7) 35.0 ±5.8 (5) 1.51 (3) z 0.13- 0 0.12- 0 * Data given for washed human platelets incubated at 370C 0.11- for 1 hr in the presence of 5 mm glucose. v) 0.09/ t Number of experiments. § SEM differences are significant at P < 0.01 level. 0.08- 0.07- RESULTS 0.06- Glycogen synthesis with glucose as precursor 0.05- HIGH PLATELET SUSPENSION 0.04- Effect of platelet concentration. The rate of incor- 0.03- poration of D-glucose-14C into platelet glycogen was mea- 0.02 - sured at a physiologic platelet concentration of 0.5% 0.01 ----.- . (or 380,000/mm3) and at a more concentrated platelet X___---°---o our 0.5 5 10 concentration of 5%. To surprise, the less concen- mM GLUCOSE trated platelet suspension incorporated considerably more glucose into glycogen at 1/2, 1, and particularly 2 hr, FIGURE 2 Effect of glucose concentration on incorporation of glucose-'4C into platelet glycogen. Washed human platelets Fig. 1. Similar results were obtained when data were were incubated with 0.5, 5, and 10 mm glucose, 1 X 106 expressed per milliliter platelets, or when different cpm/ml at 370C for 1 hr at two platelet suspension concen- concentrations of extracellular glucose were employed, trations: 0.5%, 0; and 5%, 0. Data are expressed as Fig. 2. Platelets incubated at physiologic concentration ,umoles/g wet weight, -, and umoles/10 /emoles glycogen, also exhibited slower rates of glycogen depletion and lac- . Each time point represents four experiments. SEM was 10%o or less. tate production than the more concentrated suspensions used previously (1, 3, 6), Table I. Platelets incubated at The reaction was linear with time and enzyme extract con- high concentration did not aggregate macroscopically or centration. Control experiments revealed negligible adsorption microscopically and behaved identically when incubated of UDPG-'4C to glycogen in the glycogen isolation proce- in plasma (20 mm glucose), Table II. dure. All counts were at least 100 times background and counting error was less than 0.5%o. An enzyme with fructose-1,6-diphosphatase activity was pre- TABLE I I pared from a frozen platelet pellet treated as above for gly- Effect of Platelet Concentration in Plasma and Human Ringer cogen synthetase but extracted in three times its volume Solution on Glycogen Depletion and Lactate Pro- of 50 mm Tris buffer, pH 7.5, and 20 mm MSH. The ex- duction in the Presence of 20 mM Glucose* tract was similarly centrifuged and enzyme activity of the su- pernatant measured. Further purification of this enzyme was Plasma Human Ringer obtained by extraction in 40 mg/ml charcoal (Norite A), followed by centrifugation at 105,000 g for 60 min at 40C. 0.58%1 4.37% 0.58% 4.37% The supernatant was treated with crystalline (NH4)2 SO0 at 4VC, which served to precipitate the enzyme at the 24-32 Lactate g/100 ml fraction. This resulted in a 20-fold purification production, and 10%o yield from the 4000 g platelet extract supernatant. ,umole/g/hr 21.5 31.3 18.8 35.4 The specific activity of this preparation at pH 7.5, 30'C was 1 /smole fructose 6-phosphate/min per mg . This Glycogen preparation was stable for several hours at 0C, losing depletion variable degrees of activity after storage overnight at 4GC. 1 The final assay cuvette concentration contained 50-100 ,L/ at hr, ml platelet extract, 0.46 mm NADP, 0.1-2 mm fructose-1,6- ,umole/g 6.7 11.6 8.2 13.1 diphosphate, 4 Aeg/ml glucose 6-phosphate dehydrogenase, 16 jig/ml hexose isomerase, and 20 mm MSH, in 50 mM * Platelets were incubated at 370C for 1 hr in their original Tris buffer, pH 7.5 (10). For orthophosphate release ex- plasma and in a human Ringer solution adjusted to the glucose periments, an assay was employed which was described previ- concentration of the plasma. ously (3). § Platelet suspension. 142 S. Karpatkin, A. Charmatz, and R. M. Langer 400

300/ z LU. 0 0

Uj

0 OUTER TIER 200 -1-4 LINKAGE

100- / CORE / °- ~~LIMIT DEXRIN

30 60 120 MINUTES~~~INNER FIGURE 3 Examination of labeled platelet glycogen limit dextrin and outer tier (1-4 linkages) during glycogen synthesis. Washed platelets were labeled with glucose-1'C to form labeled glycogen by incubating a 5% suspension in 5 mm glucose, 1 x 106 cpm/ml at 370 for varying time intervals. The platelet glycogen was then isolated and treated with p8-amylase to remove available 1-4 linkages, *-0, (see Methods). The remaining core, represented limit dextrin, 0-0, was assayed for glycogen content as well as radioactivity. Total platelet glycogen and radioactivity were obtained from control samples similarly handled but not treated with fi-amylase. Data are expressed as cpm/hmole glycogen. Each time point represents 4-7 experiments, except the last time point which represents the average of two experiments. SEM was 10%o or less. In view of the glycogen synthesis data of Figs. 1 and for varying periods of time, the labeled glycogen iso- 2, it seemed likely that higher glycogen synthesis took lated, and the degree of labeling of outer glycogen place in association with higher platelet glycogen levels. branch tier (1-4 linkages) compared with that of the This in turn strongly suggested that platelet gly- inner core (limit dextrin) obtained after fi-amylase di- cogen was acting as a primer for further glucosyl unit gestion. (This enzyme digests the outer tier, 1-4 link- deposition on outer tiers of preexistent glycogen branches. ages.) The specific activity of the outer 14 linkage tier Examination of platelet glycogen prelabeled with increased up to 2 hr, (Fig. 3). The specific activity of glucose-'C' during glycogen synthesis. Platelets were the inner core increased exponentially as would be ex- incubated in human Ringer's solution with glucose-UC pected if new glucosyl units were deposited on already existent glycogen branches, some of which were then 1 Because of the requirement of prohibitive quantities of converted to 1-6 linkages (followed by 1-4 linkages). radioisotopes for the incubation of significant numbers of During glycogen degradation. Platelets were incu- platelets at physiologic platelet concentrations, all further bated for 1 hr at 370C in the presence of glucose--4C in experiments were performed with platelet suspensions of with The 5%. As will be noted, the information desired did not re- order to label platelet glycogen isotope. plate- quire the use of physiologic platelet concentrations. lets were then collected, washed in human Ringer, and Glycogenesis and Glyconeogenesis in Human Platelets 143 then reincubated in human Ringer's solution in the ab- (b'moles/0.025 g wet weight platelets per 15 min). sence of glucose-14C for varying time intervals. The la- Glucose 6-phosphate at nearly optimum concentration beled glycogen was isolated and treated with and with- increased Vm.. 4.3-fold to 0.143 and lowered the ap- out f-amylase as above. As can be noted from Fig. 4, parent average Km to 1.43 mmoles/liter or 4.6-fold. the per cent of recently synthesized outer chain (1-4 This indicated that both forms of glycogen synthetase linkages) tiers, with respect to total synthesized glyco- (i.e. independent glucose 6-phosphate form, I, and de- gen, decreased with increasing interval of incubation pendent glucose 6-phosphate form, D) are present in of prelabeled platelets. Of interest was the increase in human platelets. At physiologic levels of glucose 6-phos- specific activity of prelabeled platelets (counts per min- phate in platelets, (1) 0.05 mmole/liter, glycogen syn- ute per micromole glycogen), with increasing incubation thetase activity is capable of operating at 30% of theo- time. The specific activity of total labeled glycogen as retical V.ax. This represents 7.0 /moles glycogen syn- well as limit dextrin-labeled glycogen increased with thesized/g per hr. time, the latter at 2 hr. These increasing considerably Glycogenesis with pyruvate and citrate as pre- data indicated that more unlabeled glycogen was being degraded than labeled glycogen (newly synthesized gly- cursor cogen), suggesting a nonsequential or heterogeneous Both pyruvate and citrate were incorporated into mode of glycogen depletion. platelet glycogen, Table III. Free glucose could not be detected in the total incubation platelet suspension with Glycogen synthetase a method capable of detecting 1.5 X 10' mole/liter. Fig. 5 depicts kinetic data for platelet glycogen syn- After 1 hr incubation with pyruvate or citrate at 37GC, thetase. The Km in the absence of glucose 6-phosphate optimum pyruvate incorporation into glycogen was at 20 was 6.6 mmoles/liter. Theoretical V.a. was 0.033 mmoles/liter and optimum citrate incorporation into

11 400' I I h:1) u

z V 300 -

I- mu

9 200 a.I 0IL. u 0 I-: 100. z0 0 50s

90 120 MINUTES FIGURE 4 Examination of labeled glycogen limit dextrin and outer tier (1-4 linkages) during glycogen degradation. Washed platelets were pre- labeled for 1 hr by incubation of a 5% suspension in 5 mm glucose, 1 X 100 cpm/ml at 370C. These platelets were then washed, resuspended, and reincubated for varying time intervals. The labeled glycogen was isolated at varying time intervals and analyzed for limit dextrin and outer tier (1-4 linkages). Data are expressed as cpm/Atmole glycogen for total glycogen, X--X, and limit dextrin; O O. Per cent length' of outer tier (1-4 linkages) is also given, A- - - -A. Each time point represents four experiments. SEM was 10% or less. 144 S. Karpatkin, A. Charmatz, and R. M. Langer 1/ V

400-

300-

200-

100-

1 /UDPG FIGURE 5 Lineweaver-Burke plot for platelet extract glycogen synthetase activity. See methods for details of UDPG-14C incorpo- ration into glycogen. Data are given at varying glucose 6-phos- phate concentration /Lmoles/ml: 0, 0; 0.05, Q; 0.1, *; 1, E; 2, *; 5, A. The Km for glycogen synthetase activity at the above mentioned glucose 6-phosphate concentration was 6.6,.7.3, 1.44, 1.43, and 1.43, respectively. Vma. was 0.033, 0.044, 0.046, 0.063, 0.10, and 0.143, respectively.

Glycogenesis and Glyconeogenesis in Human Platelets 145 TABLE III TABLE IV Incorporation of Glucose, Pyruvate, and Citrate Effect of AMP, ADP, and A TP on Fructose-i, into Platelet Glycogen* 6-Diphosphatase Activity*

Incubation Concentration mM concen- Fructose-I, Rate of tration 6-diphosphate AMP ADP fructose mM 5 10 20 40 5 10 20 40 ATP 6-phosphate 0.10 2.0 0.36 2.0 1.39 2.0 2.52 at 300C mpmoles/g platelets mp&moles/1mole glycogen jumoles/g Glucose 77.0 106.8 5.38 8.01 per hr (6) (3) + 0 0 0 0 0 0 5.2 91.4 2.18 5.52 6.04 + 0 + 0 0 0 0 11.6 Pyruvate 31.2 83.3 + 0 0 0 + 0 0 8.6 (14) + 0 0 0 0 0 + 0 + 0 + 0 + 0 + 0.69 Citrate 5.51 8.71 11.6 31.9 0.34 0.63 0.92 3.18 0 + 0 0 0 0 0 10.8 (4) (9) 0 + 0 + 0 0 0 17.2 0 + 0 0 0 + 0 16.8 * Washed human platelets were incubated as a 5% suspension 0 + 0 + 0 + 0 18.0 Ringer solution containing varying concentrations in human * A platelet extract obtained from a 4000 g homogenate, wherein final tissue of either glucose-14C, pyruvate-14C, or citrate-14C for 1 hr at dilution in the assay cuvette was 40-fold. Data represent the average of two 370C. different platelet extracts. ( +) refers to presence or absence (0) of substrate I Number of experiments is given in brackets. SEM was less or effector concentration. than 10% for all data with four or more experiments. rates of orthophosphate release after incubation at 300C glycogen was at 40 mmoles/liter 2 Table III. At concen- for 30 min at pH 7.5 in 50 mm Veronal buffer or 50 mM trations of 5 mmoles/liter, pyruvate was less effective Tris buffer. The Km for fructose-1,6-diphosphate varied than glucose, and citrate was less effective than pyru- between 0.06 and 0.2 mmoles/liter. AMP caused activa- vate in incorporation into glycogen. At physiologic glu- tion of the enzyme, rather than inhibition which has been cose concentration, 5 mmoles/liter, net glycogen syn- reported in other tissues (10, 11-13), Fig. 6 A, raising thesis from glucose represented 0.08 /Amoles/g per hr Vmax 6-fold. ADP also activated this enzyme, Fig. 6 B, at 370C. raising Vmax 9-fold. ATP was a potent inhibitor of this enzyme, giving zero rates at physiologic concentrations Fructose-1,6-diphosphatase activity of ATP, 1-3 mmoles/liter (3), employing similar fruc- A fructose-1,6-diphosphatase was present in a 4000 g tose-1,6-diphosphate concentrations as those shown in homogenate of platelets giving a rate of 10.8 Amoles of Fig. 6. fructose 6-phosphate/g platelets per hr at 30'C (four In the presence of physiologic concentrations of fruc- experiments with optimal fructose-1,6-diphosphate con- tose-1,6-diphosphate, AMP, ADP, and ATP (3), the centration of 2 mmoles/liter). The rate was linear with rate was 0.69 Amoles/g per hr, Table IV. increasing homogenate concentration over the range studied and was zero in the absence of fructose-1,6-di- DISCUSSION phosphate. At the concentration of fructose-1,6-diphos- Incorporation of glucose-'4C into isolated human plate- phate present in human platelets (0.087 +0.006 tcmole/g, let glycogen has been reported by Scott (14) and by eight experiments), the rate was 5.2 timole/g per hr, this laboratory (6). Our present results both confirm Table IV. and further document these observations. Human plate- had a pH optimum The enzyme was purified 20-fold, lets are capable of synthesizing new 1-4 and probable of approximately 7, and had no Mg++ ion requirement. 1-6 linkages onto preexistent tiers of branched The enzyme was fairly specific for fructose-1,6-diphos- glucosyl phate. The substrates fructose 6-phosphate, glucose glycogen polymers via glycogen synthetase which has 6-phosphate, glucose 1-phosphate, and P- phos- been measured (and presumably amylo 1-4 > 1-6 trans- phate, at concentrations of 2 mmoles/liter, gave zero glucosylase which could be inferred, Fig. 3). The newly synthesized glycogen is also available to the degradative 'Of interest was the observation that citrate incorporation , (which has been into glycogen was sigmoidal, plateauing at 10 mm extra- glycogenolytic phosphorylase cellular citrate, but then increasing at 20 mmoles/liter and described in human platelets, 4, 5, 14), and probable plateauing at 40 mmoles/liter (all data not given). All in- amylo-1,6 glucosidase (which could be predicted) since cubations were maintained at isotonicity by lowering the were synthesized and NaCl concentration. This observation was not further 1-4 as well as probable 1-6 linkages pursued. degraded (Figs. 3 and 4). Of considerable interest was 146 S. Karpatkin, A. Charmatz, and R. M. Langer the observation of a nonsequential or heterogeneous ternative hypothesis would predict random splitting and mode of glycogen depletion compared with glycogen rejoining of glycogen branches. Of further interest was synthesis. This might suggest a heterogeneous distribu- the marked increase in glycogen synthesis of low platelet tion or compartmentation of glycogenolytic enzymes as suspensions after 1 hr of incubation. This observation compared with the glycogen synthesis enzymes. An al- remains unexplained.

i 0.26 A. A- 00.24 r0.22- E 0.20

0 .18 0 O0.16- !R 0.14- uj 0 .12- 0; o .08 ===~~~~~~~~~~~~ 0 0.08- 4 0.04 a~' 0.02

1 2 3 4 5 6 7 8 9 10 11 12 13 14 I5 16 I/FRUCTOSE -1,6-DIPHOSPHATE, (mM)Fl

4 5 6 7 8 9 10 11 12 13 14 15 16

j:FRUCTOSE -1.6 - DIPHOSPHATE, (mM)-1 FIGuRE 6 (A) Lineweaver-Burke plot for 20-fold purified platelet en- zyme with fructose-1,6-diphosphatase activity. See Methods section for details. Data are given at varying AMP concentrations, ,.moles/ml: 0, 0; 0.1, Q; 0.5, 0; 1, A; 3, A. The Km for fructose-1,6-diphosphatase ac- tivity was 0.178 mmoles/liter. Vm.x.at the above mentioned AMP con- centrations was 15.6, 41.7, 62.5, 66.7, and 100, respectively. (B) Data are given at varying ADP concentrations (same symbols and concentra- tions as for AMP, part A) for the same enzyme preparation. Vmax at these ADP concentrations was 15.6, 47.6, 71.4, 100, and 143, respectively. Glycogenesis and Glyconeogenesis in Human Platelets 147 Leloir and Cardini have demonstrated glycogen syn- of glycogen synthesis than in platelets incubated at physi- thesis in and muscle from UDPG and glycogen syn- ologic concentration (0.5%). Of interest is a recent ob- thetase (15). Glycogen synthetase activity has been re- servation by Murphy' wherein 24-hr-stored platelet ported in pig platelet extracts with the associated glu- concentrates at 22'C were noted to have 2.3-fold lower cose 6-phosphate-dependent enzyme by Vainer and glycogen content than similarly stored platelet-rich Wattiaux (16). The same applies for human platelets. plasma. Whether the decreased platelet glycogen of The two major sites for glyconeogenesis in mammals platelet concentrates is responsible for the decreased are the liver and kidney (17). To our knowledge, this platelet recovery after platelet concentrate transfusions is the first report of the full glyconeogenic pathway remains to be established. in platelets as well as fructose-1,6-diphosphatase ac- The reason for increased glycogen degradation with tivity. This might have been predicted from a previous increased platelet concentration remains unexplained. report from this laboratory (1) wherein the Krebs It is not due to platelet aggregation or removal of plate- cycle activity in human platelets was emphasized and lets from plasma. It is probably not a result of releaspe the first stage of the gluconeogenic pathway was sug- of a platelet metabolite since rates of lactate production gested, i.e., human platelets were capable of converting are linear at high platelet suspensions (1). It is con- citrate to lactate. ceivable that the increased rate of platelet collision, in- The fructose-1,6-diphosphatase activity of human herent in a higher platelet suspension, may in some way platelets appears unique in its modulation by AMP, activate glycogen degradation, lactate production, and ADP, and ATP and lack of requirement for Mg"+ ion. ATP depletion. It is also conceivable that high plate- AMP, in contrast to its inhibitory effect on other fruc- let suspensions become relatively anaerobic and conse- tose-1,6-diphosphatases (10, 11-13), activates the plate- quently activate glycogenolysis. It is thus suggested that let enzyme. Other specific fructose-1,6-diphosphatases all previous metabolic data be interpreted with caution, from Escherichia coli and Euglena gracilis (10) have since platelet concentration is a new variable important in been described which are neither activated nor inhibited the interpretation of these data. by AMP. ADP is also a potent activator, whereas ATP inhibits the enzyme. Inhibition of fructose-1,6-diphos- ACKNOWLEDGMENTS phatase by ATP has also been reported in rat and rabbit We are indebted to Doctors Aaron Kellner, Fred H. Allen, liver (18). Lack of inhibition by AMP and absence of a Jr., and Carlos Ehrich of The New York Blood Center Mg"+ ion requirement makes one cautious in defining for their cooperation in the supply of fresh platelet-rich this enzyme as a rather than plasma. fructose-1,6-diphosphatase This work was supported by a Research Grant-in-Aide a nonspecific phosphatase. However, substrate speci- from the New York Heart Association. ficity was exhibited, and the enzyme does have a Km in the range of platelet fructose-1,6-diphosphate concen- REFERENCES tration. Thus, an enzyme is present in human platelets 1. Karpatkin, S. 1967. Studies on human platelet . wh ch is capable of converting fructose-1,6-diphosphate Effect of glucose, cyanide, , citrate, and agglutina- to fructose 6-phosphate and which can readily explain tion and contraction on platelet glycolysis. J. Clin. Invest. the conversion of pyruvate or citrate to glycogen. 46: 409. At concentration of adenine nucleotide and 2. Luganova, I. S., I. F. Seits, and V. Teodorovich. 1958. physiologic Metabolism in human thrombocytes. Biochemistry. 23: glucose-6-phosphate, the platelet extract rates of gly- 379. cogen synthetase (at 370C) and apparent fructose-1,6- 3. Karpatkin, S., and R. M. Langer. 1968. Biochemical diphosphatase (at 30'C) are 7 and 0.69 Amoles g wet energetics of simulated platelet-plug formation. Effect weight per hr. These data suggest that fructose-1,6-di- of thrombin, adenosine diphosphate, and on intra- and extracellular adenine nucleotide kinetics. phosphatase activity is rate limiting for glycogen syn- J. Clin. Invest. 47: 2158. thes's. as it is in other tissues (19, 20). Human platelet 4. Karpatkin, S., and R. M. Langer. 1969. Human platelet phosphorylase has a rate at 300C of 11 Amoles glucose phosphorylase. Biochim. Biophys. Acta. 185: 350. 1-phosphate/g per hr3 at physiologic concentration (3) 5. Karpatkin, S., and R. M. Langer. 1969. Activation of of substrates: glycogen and and known inactive phosphorylase dimer and monomer from human orthophosphate platelets with adenosine triphosphate. J. effectors: AMP, ATP, and ADP. This would predict Biol. Chem. 244: 1953. a greater potential for glycogen degradation as com- 6. Karpatkin, S. 1969. Heterogeneity of human platelets. pared with glycogen synthesis. I. Metabolic and kinetic evidence suggestive of young Platelets incubated at high concentration (5%) ex- and old platelets. J. Clin. Invest. 48: 1073. As a 7. Bray, G. A. 1960. A simple efficient liquid scintillator hibit increased glycogen degradation. result, there for counting aqueous solutions in a liquid scintillation is less glycogen primer and consequently, a lower rate counter. Anal. Biochem. 1: 279. 'Karpatkin, S., and R. M. Langer. Unpublished data. 'Murphy, S. Personal communi'ation. 148 S. Karpatkin, A. Charmatz, and R. M. Langer 8. Steinetz, K. 1967. Laboratory diagnosis of glycogen 15. Leloir, L. F., and C. E. Cardini. 1957. Biosynthesis of diseases. Advan. Clin. Chem. 9: 227. glycogen from uridine diphosphate glucose. J. Amer. 9. Villar-Palasi, C., and J. Larner. 1961. Insulin treatment Chem. Soc. 79: 6340. and increased UDPG-glycogen transglucosylase activity 16. Vainer, H., and R. Wattiaux. 1968. Glycogen synthetase in muscle. Arch. Biochem. Biophys. 94: 436. in blood platelets. Nature (London). 217: 951. 10. Pogell, B. M., and K Taketa. 1965. Allosteric inhibition 17. Krebs, H. A., D. A. H. Bennet, P. Gasquet, T. Gascoyne, of rat liver fructose-1,6-diphosphatase by adenosine and T. Yoshida. 1963. Renal . The ef- 5'-monophosphate. J. Biol. Chem. 240: 651. fect of diet on the gluconeogenic capacity of rat kidney 11. Mendecino, J., and F. Vasarhely. 1963. Renal D-fructose cortex slices. Biochem. J. 86: 22. 1,6-diphosphatase. I. Biol. Chem. 238: 3528. 18. Taketa, K., and B. M. Pogell. 1963. Reversible inacti- 12. Rosen, 0. M., S. M. Rosen, and B. L. Horecker. 1965. vation and inhibition of liver fructose-1,6-diphosphatase Purification and properties of a specific fructose-1,6- by adenosine . Biochem. Biophys. Res. Com- diphosphatase from Candida utilis. Arch. Biochem. Bio- mun. 12: 229. phys. 112: 411. 19. Weber, G., R. Singhal, N. Stamm, E. Fisher, and M. 13. Pontremoli, S., E. Grazi, and A. Accorsi. 1966. Fruc- Mentendiek. 1964. Regulation of enzymes involved in tose diphosphatase from rabbit liver. VII. resi- gluconeogenesis. Advan. Enzyme Regul. 2: 1. dues and adenosine monophosphate inhibition. Bio- 20. Krebs, H. A., E. A. Newsholme, R. Speake, T. Gascoyne. chemistry. 5: 3568. and P. Lund. 1964. Some factors regulating the rate of 14. Scott, R. B. 1967. Activation of gluconeogenesis in animal tissues. Advan. Enzyme in blood platelets. Blood. 30: 321. ReguL. 2: 71.

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