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The Metabolism and Antiketogenic Effects of Sorbitol. Sorbitol Dehydrogenase

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The Metabolism and Antiketogenic Effects of Sorbitol. Sorbitol Dehydrogenase 257 The Metabolism and Antiketogenic Effects of Sorbitol. Sorbitol Dehydrogenase By R. L. BLAKLEY (Research Fellow of the Medical Research Council of New Zealand) Department of Biochemistry, Medical School, University of Otago, Dunedin, New Zealand (Received 23 November 1950) Using the perfused liver of fasted or phlorrhizinized Tissue 8lices were cut by the conventional freehand dogs, Embden & Griesbach (1914) discovered that method. Approx. 15 mg. dry weight of slices per Warburg sorbitol was converted into fermentable reducing flask were used. sugars from which the only identifiable osazone Homogenates were prepared with the all-glass Potter- Elvehjem apparatus. The organs were removed immediately prepared was phenylglucosazone. On the basis of after the animals had been killed by a blow on the head or, changes in optical rotation and in the Seliwanoff in the case of rats, by rupturing the spinal cord, and chilled reaction during perfusion, these authors considered in ice water. After homogenizing, the material was kept at that the primary product of sorbitol oxidation was 0° until used. fructose which was subsequently transformed into Manometric methods. The standard methods of Warburg glucose. Additional evidence consistent with this were used for the measurement of 02 consumption by slices, view was provided by Anschel (1930), who observed homogenates and other preparations. Slices were suspended a large increase in the excretion offructose following in phosphate saline, pH 7-4 (Krebs, 1933). Substrates dis- administration of sorbitol to a case of fructosuria. solved in a small volume of distilled water were added to the main compartment of Warburg vessels; in controls with The work of Embden & Griesbach prompted the substrate omitted an equal volume of distilled water was suggestion that sorbitol might prove a useful carbo- added. Acid substrates were added as neutral Na salts; hydrate source in diabetes mellitus (Thannhauser & amines were added as neutralized hydrochlorides. Flasks Meyer, 1929); but attempts to assess the utilization were equilibrated in the thermostat for 7-10 min. before of sorbitol in diabetics by measurement of respira- manometric readings were commenced. tory quotients and blood-sugar levels led to much controversy. The evidence has been reviewed by Analytical methods Carr & Krantz (1945). The effect of feeding sorbitol Sorbitol was determined by the periodate method of upon the deposition of glycogen in rat liver has also Rappoport, Reifer & Weinmann (1937). been the subject ofconflicting reports. Nevertheless, Reducing sugar was determined according to Somogyi most workers agree that sorbitol can act as a glycogen (1945a) after deproteinizing with Ba(OH)2-ZnSOg (Somogyi, precursor (Carr & Forman, 1939; Johnston & Deuel, 1945b). When experiments were performed in the presence of 1943; Blatherwick, Bradshaw, Ewing, Larson & methylene blue, the dye was removed from the deproteinized Sawyer, 1940). Some insight into the enzymic filtrate by treatment with a minimum amount of Merck's mechanism of these metabolic reactions was gained medicinal charcoal. Reducing sugar was determined in the by Breusch (1942, 1943), who demonstrated sorbitol solution (Somogyi, 1945a) after removal of the charcoal by filtration. dehydrogenase activity in brei prepared from the Ketose was determined by Cole's modification of Roe's livers ofstarved cats. The criteria employed were the method (see Bacon & Bell, 1948). When necessary the solu- reduction of oxaloacetate and of methylene blue. tion was cleared of methylene blue as above. The colour The work described in this paper was undertaken intensity was estimated by measuring optical density at with the object of obtaining more precise definition 483 mI,. In estimations offructose or sorbose the colour was of the initial steps in sorbitol metabolism. Such in- always compared with that developed in a standard solution formation is particularly desirable in view of the of the hexose in question, because sorbose gives approxi- antiketogenic property of sorbitol, described by mately half the colour of an equal concentration offructose. Edson (1936), who found that sorbitol is more Glucose was determined manometrically using glucose oxidase (Keilin & Hartree, 1948) in the presence of catalase effective than glucose or fructose in suppressing (solution ofcrystallized enzyme), but without the addition of spontaneous ketogenesis in the liver slices of starved ethanol which raised the blank 02 uptake to an unduly large rats. and variable value. EXPERIMENTAL Glucose oxidase was prepared from Aspergillus niger (National Collection of Type Cultures, no. 594), a culture of Rats of the Wistar strain, aged 8-12 months, were used. which was kindly supplied by Dr J. S. D. Bacon. The mould They were fed a stock diet ofpollard, bran, maize meal, meat was grown as described by Mann (1944), harvested on the meal and bone flour. In starvation experiments food was 4th day of incubation, and a dry powder prepared from the completely withheld for 22-36 hr. but free access to water mycelium by homogenizing in a Waring blender with cold allowed. acetone. The acetone-dried powder was extracted with Biochem. 1951, 49 17 258 R. L. BLAKLEY I95I 0-03M-potassium phosphate buffer, pH 6, the yellow filtrate Coenzyme I. A crude preparation (12 % purity) was made treated with 7 vol. of cold acetone and the mixture im- from baker's yeast by the method of Williamson & Green mediately centrifuged in order to separate the viscous pre- (1940). Some of this material was treated with norit cipitate. The vacuum-dried precipitate was stable at 20. according to Clark, Dounce & Stotz (1949), and the purity When required, it was dissolved in 0-2M-potassium phos- raised to 25%. Cozymase (purity 34 and 60%) obtained phate buffer, pH 6, and dialysed against 0-01M-potassium from Schwartz Laboratories, Inc., New York, was used in phosphate buffer, pH 6. This solution had high glucose some experiments. oxidase activity, but did not oxidize galactose, fructose, Reduced coenzyme I (CoiH2) was prepared by the method of sorbose, mannose or sorbitol. Ohlmeyer (1938). Acetoacetic acid was determined by the aniline citrate method (Edson, 1935) except when determined simul- Metabolic units taneously with ,B-hydroxybutyric acid when the modified - = PI' 02 consumed/mg. dry wt. of tissue/hr. Van Slyke technique (Edson, 1935) was employed. QK.to=/4el CO2 equivalent to fi-ketonic acid formed/mg. Lactic acid was determined by the method of Friedemann dry wt. of tissue/hr.; 1 ,umol. fl-ketonic acid _ 22-4 p.l. CO8. & Graeser (1933). QHydroxy=p4- CO2 equivalent to ,-hydroxybutyric acid Nitrogen was determined by the titrimetric micro- formed/mg. dry wt. of tissue/hr.; 1 umol. P-hydroxybutyric Kjeldahl procedure of Hiller, Plazin & Van Slyke (1948). acid _ 22-4 p.l. CO2. Coenzyme I (CoI) was estimated by the method of LePage (1947), using the value El o/= 8500 at 340 mp. In enzymic RESULTS reactions the reduced coenzyme was estimated from the Oxidation and antiketogenic effect of increase in optical density at 340 mp. above the value of the sorbitol in liver 8lices blank at zero time. Hexokinase activity was estimated according to Colowick Sorbitol is rapidly oxidized by rat-liver slices & Kalckar (1943), aldolase activity according to Taylor, whether the animals have been well fed or starved. Green & Cori (1948). This is indicated by the increase in oxygen con- pH Measurements were made with a glass electrode. sumption observed when sorbitol is present in the Optical density was measured in a Beckman quartz spectro- suspension medium (Table 1). In agreement with photometer, DU model. Edson (1936) it was consistently found that OO1M- Material1 sorbitol caused a marked decrease (25-50%) in acetoacetate formed endogenously by liver slices Sorbitol. A sample of D-sorbitol monohydrate obtained from fasted rats. Since this observation does not from the Pfanstiehl Chemical Co. was used throughout this exclude the possibility ofacetoacetate being reduced work. [oa]s ° = - 1.73±0.050 in water (c, 9 1). Acetoacetate was prepared as described by Ljunggren to fi-hydroxybutyrate in the presence of sorbitol, (1924). both ketone bodies were determined in presence and Cytochrome c was prepared by the method of Keilin & absence of sorbitol. The results show that formation Hartree (1937) and assayed according to Potter (1945). of ,-hydroxybutyrate was decreased to about the Table 1. Oxidation and antiketogenesis in rat-liver 8liCe8 (Slices incubated in conical Warburg vessels containing 3-00 ml. of phosphate saline, pH 7-4. Inseals contained 0-20 ml. 2N-NaOH. Time, 2 hr. Gas, 02. Temp. 38°.) Exp. Substrate added no. State of animal (0-01 M) - Qo2 QKeto 1 Well fed Nil 11-9 1-02 Pentane-1-carboxylate 16-1 3-78 Pentane-1-carboxylate + sorbitol 17-0 4-22 2 Fasted 36 hr. Nil 11-0 1-93 Sorbitol 15-8 0-83 Glucose 11-7 1-73 Fructose 13-9 1-29 Sorbose 9-8 1-04 L-Arabinose 11-7 1-62 D-Arabinose 11-8 1-41 3 Fasted 36 hr. Nil 13-8 3-82 Glucose-l-phosphate (K salt) 13-5 3-33 4 Fasted 24 hr. Nil 10-4 2-33 Sorbitol 12-7 0-89 Malonate (0-01 M) 10-3 2-68 Sorbitol + malonate (0-01 M) 9-6 1-47 5 Fasted 24 hr. Nil 11-8 3-44 Sorbitol 15-4 1-89 In Exp. 5, QHY,OXOY was 0-96 in absence of added substrate, 0-50 in presence of sorbitol. V01. 49 METABOLISM OF . SORBITOL 259 same extent as acetoacetate production in the cycle inhibitors, such as malonate, on antiketo- presence of O-OlM-sorbitol (Table 1, Exp. 5). Thus genesis due to sorbitol possesses some significance. sorbitol decreased the total ketone bodies formed The effect of malonate was tested by incubating spontaneously in liver slices of starving rats. liver slices from fasted rats with malonate in Although sorbitol reduces the spontaneous keto- presence and absence ofsorbitol.
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