Alcohol Dehydrogenase System Research Articles

Dietary ethanol and lipid synthesis in Drosophila melanogaster.

When cultured on a defined diet, ethanol was an efficient substrate for lipid synthesis in wild-type Drosophila melanogaster larvae. At certain dietary levels both ethanol and sucrose could displace the other as a lipid substrate. In wild-type larvae more than 90% of the flux from ethanol to lipid was metabolized via the alcohol dehydrogenase (ADH) system. The ADH and aldehyde dehydrogenase activities of ADH were modulated in tandem by dietary ethanol, suggesting that ADH provided substrate for lipogenesis by degrading ethanol to acetaldehyde and then to acetic acid. The tissue activity of catalase was suppressed by dietary ethanol, implying that catalase was not a major factor in ethanol metabolism in larvae. The activities of lipogenic enzymes, sn-glycerol-3-phosphate dehydrogenase, fatty acid synthetase (FAS), and ADH, together with the triacylglycerol (TG) content of wild-type larvae increased in proportion to the dietary ethanol concentration to 4.5% (v/v). Dietary ethanol inhibited FAS and repressed the accumulation of TG in ADH-deficient larvae, suggesting that the levels of these factors may be subject to a complex feedback control.

Clofibrate, clinofibrate投与ラット肝におけるエタノール代謝系への影響

Peroxisomal catalase and hepatic oxygen consumption were monitored non-invasively in perfused rat liver.1) The effects of CPIB on ethanol oxidative metabolism were studied in perfused rat livers. In some of the experiments, S-8527 was compared with CPIB.2) When the rats were fed for two weeks either with CPIB (0.5%) or S-8527 (0.1%), CPIB induced hepatomegaly, but otherwise revealed the similar hypolipidemic effects to S-8527.3) The rate of oxygen consumption in CPIB-treated liver was higher than that of the control group.4) The role of alcohol dehydrogenase system in ethanol metabolism was considered to be of less significance in CPIB-treated liver.5) The catalase heme content was increased approximately up to nine-fold in CPIB-treated liver. This result indicated that H2O2 generation rate in CPIB-treated liver was significantly increased. Catalase H2O2 (compound I) was also monitored by organ surface spectrophotometry.6) Chronic treatment with CPIB caused hepatic peroxisomal and microsomal induction. S-8527 (0.1%), however, affected neither O2 consumption nor H2O2 generation rate.These findings suggest that the hypolipidemic mechanisms of CPIB are not exactly the same as those of S-8527.

A mutational analysis of the alcohol dehydrogenase system in barley

Evidence that the alcohol dehydrogenase enzymes in barley are specified by two genes, Adh1 and Adh2 whose products (ADH1 and ADH2) associate by dimerization to give three isozyme sets, Set I (ADH1 . ADH1 homodimer), Set II (ADH1. ADH2 heterodimer) and Set III (ADH2. ADH2 homodimer) is presented. Procedures for the induction and recovery of null activity mutants at the Adh1 locus are described. One such mutant, Adh1-M9, when homozygous results in the absence of Set I and Set II isozymes and in a decrease in the rate of induction of alcohol dehydrogenase activity caused by flooding.

Ethanol metabolism in guinea pig: Ethanol oxidation and its effect on [formula omitted] ratios, oxygen consumption, and ketogenesis in isolated hepatocytes of fed and fasted animals

Ethanol metabolism was studied in isolated hepatocytes of fed and fasted guinea pigs. Alcohol dehydrogenase (EC 1.1.1.1) activities of fed or fasted liver cells were 2.04 and 1.88 μmol/g cells/min, respectively. Under a variety of in vitro conditions, alcohol dehydrogenase operates in fed hepatocytes at 34–74% and in fasted liver cells at 23–61% of its maximum velocity, respectively. Hepatocytes of fed animals, incubated in Krebs-Ringer bicarbonate buffer, oxidized ethanol at an average rate of 0.69 μmol/g wet weight cells/min, whereas cells of 48-h fasted animals consumed only 0.44 μmol/g/min under identical conditions. Various substrates and metabolites of intermediary metabolism significantly enhanced ethanol oxidation in fed liver cells. Maximum stimulatory effects were achieved with alanine (+138%) and pyruvate (+102%), followed in decreasing order by propionate, lactate, fructose, dihydroxyacetone, and galactose. In contrast to substrate couples such as lactate/pyruvate and glycerol/dihydroxyacetone, sorbitol with or without fructose significantly inhibited ethanol oxidation. The addition of hydrogen shuttle components such as malate, aspartate, or glutamate to fasted hepatocytes resulted in significantly higher stimulation of ethanol uptake than in fed hepatocytes. Also, the degree of inhibition of shuttle activity by n-butylmalonate was more pronounced in fasted liver cells (77% inhibition) than in fed cells (59% inhibition). These data as well as oxygen kinetic studies in intact guinea pig hepatocytes utilizing uncouplers (carbonyl cyanide- p-trifluoromethoxyphenylhydrazone, dinitrophenol), electron-transport inhibitors (rotenone, antimycin), and malate-aspartate shuttle inhibitors (aminooxyacetate, n-butylmalonate) strongly suggested that the malate-aspartate shuttle is the predominant hydrogen transport system during ethanol oxidation in guinea pig liver. Comparison of the alcohol dehydrogenase-inhibitors 4-methylpyrazole and pyrazole on ethanol oxidation demonstrated that the alcohol dehydrogenase system is quantitatively the most important alcohol-metabolizing pathway in guinea pig liver. Supporting this conclusion, it was found that the H 2O 2-forming substrate glycolate slightly increased ethanol oxidation in liver cells of control animals (+26%), but prior inhibition of catalase by 3-amino-1,2,4-triazole resulted in a significant increase (+25%) instead of a decrease in alcohol oxidation. This finding does not support a quantitatively important role of peroxidatic oxidation of ethanol by catalase in liver. Cytosolic NAD NADH ratios were greatly shifted toward reduction during ethanol oxidation. These reductive shifts were even more pronounced when cells were incubated in the presence of fatty acids (octanoate, oleate) plus ethanol. Inhibitor studies with 4-methylpyrazole demonstrated that the decrease of the cytosolic NAD NADH ratio during fatty acid oxidation was due to an inhibition of hydrogen transport from cytosol to mitochondria and not the result of transfer of hydrogen, generated by fatty acid oxidation, from mitochondria to cytosol. Lactate plus pyruvate formation was slightly inhibited by ethanol in fed hepatocytes but greatly accelerated in fasted cells; this latter effect was mostly the result of increased lactate formation. Such regulation may represent a hepatic mechanism of alcoholic lactic acidosis as observed in human alcoholics. The ethanol-induced decrease of the mitochondrial NAD NADH ratio was prevented by addition of 4-methylpyrazole. Endogenous ketogenesis was greatly increased (+80%) by ethanol in fed liver cells. This effect of ethanol was blunted in the presence of glucose. Propionate, by competing with fatty acid oxidation, was strongly antiketogenic. This effect was alleviated by ethanol. In 48-h fasted hepatocytes, endogenous ketogenesis was enhanced by 84%. Although ethanol did not further stimulate endogenous ketogenesis under these conditions, alcohol significantly increased ketogenesis in the presence of octanoate or oleate. This stimulatory effect of ethanol was almost completely prevented by 4-methylpyrazole. These findings demonstrate that the syndrome of alcoholic ketoacidosis may be due, at least partially, to the additional stimulation of ketogenesis by or from ethanol during fatty acid oxidation in the fasting state.

Primary deuterium and tritium isotope effects upon V/K in the liver alcohol dehydrogenase reaction with ethanol.

The primary isotope effect upon V/K when ethanol stereospecifically labeled with deuterium or tritium is oxidized by liver alcohol dehydrogenase has been measured between pH 6 and 9. The deuterium isotope effect was obtained with high reproducibility by the use of two different radioactive tracers, viz. 14C and 3H, to follow the rate of acetaldehyde formation from deuterium-labeled ethanol and normal ethanol, respectively. Synthesis of the necessary labeled compounds is described in this and earlier work referred to. V/K isotope effects for both tritium and deuterium have been measured with three different coenzymes, NAD+, thio-NAD+, and acetyl-NAD+. With NAD+ at pH 7, D(V/K) was 3.0 and T(V/K) was 6.5. With increasing pH, these values decreased to 1.5 and 2.5 at pH 9. The intrinsic isotope effect evaluated by the method of Northrop [Northrop, D.B. (1977) in Isotope Effects on Enzyme-Catalyzed Reactions (Cleland, W. W., O'Leary, M, H., & Northrop, D. B., Eds.) pp 112-152, University Park Press, Baltimore] varies little with pH. It amounts to about 10 with NAD+ and about 5 with the coenzyme analogues. Commitment functions and their dependence upon pH calculated in this connection appear to be in agreement with known kinetic parameters of liver alcohol dehydrogenase. This assay method was also applied in vivo in the rat. Being a noninvasive method because only trace amounts of isotopes are needed, it may yield information about alternative routes of ethanol oxidation in vivo. In naive rats at low concentrations of ethanol, it confirms the discrete role of the non alcohol dehydrogenase systems.

Photochemical redox transformations of dimers of NAD + and N′-methylnicotinamide

Electrochemically generated dimers of N′-methylnicotinamide and NAD + both undergo photooxidation on irradiation at 254 nm in aqueous medium to yield the respective parent monomers. For the NAD dimer the quantum yield for photodissociation was about 0.01, whether irradiated at 254 nm or at wavelengths to the red of 320 nm. Irradiation at the latter wavelengths, where the NAD + monomer itself does not absorb and is not photosensitive, led to quantitative regeneration of coenzyme activity in the alcohol dehydrogenase system. The photodissociation reaction exhibited no oxygen effect. The photochemically generated dimer of N′-methylnicotinamide was also photooxidized to the parent monomer by irradiation at 254 nm at pH 9.5. The foregoing process of electrochemical (or photochemical) reduction and photochemical oxidation, comprising a closed cycle of electron and proton transport, is similar to that previously observed for a number of pyrimidine analogs. Furthermore, the NAD dimer is a substrate of snake venom nucleotide pyrophosphatase and is hydrolyzed to release NMN dimer which, on irradiation at 254 nm, also undergoes photooxidation to the parent NMN + monomer. A mechanism for the photooxidation reaction is formulated and relevant biological implications of the foregoing are presented.

Effect of ethylene glycol toxicity on hepatic carbohydrate metabolism in rats

The effects of ethylene glycol (EG) ingestion on glucose tolerance, hepatic glycogen metabolism, anerobic glycolysis, pentose phosphate pathway, and Embden-Meyerhof-tricarboxylic acid (EM-TCA) cycle pathway were investigated in male albino rats. EG was administered po at a daily dose of 2 ml/kg for 6 days. The analyses were done on Day 7. An impaired glucose metabolism was seen from diminished glucose tolerance. Hepatic glycogen concentrations were low associated with decreased glycogen synthetase and increased phosphorylase activities. [U- 14C]Glucose incorporation into liver glycogen both in vivo and in vitro was decreased. Studies on the incorporation of 14C of [1- 14C]- and [6- 14C]glucose into respiratory CO 2 showed augmentation of the pentose phosphate pathway, while no significant alteration was observed in glucose oxidation through the EM-TCA cycle pathway. Anerobic glycolysis by liver slices was decreased. The increased NADPH made available by the enhanced pentose phosphate pathway activity following EG treatment may suggest possible oxidation of EG through the microsomal ethanol oxidizing system (MEOS) which is NADPH dependent, as well as through the alcohol dehydrogenase system.

Effect of pH on the process of ternary-complex interconversion in the liver-alcohol-dehydrogenase reaction.

1. Kinetic relationships referring to multiple-turnover conditions have been derived for the slowest exponential transient appearing in two-substrate enzyme reactions proceeding by an ordered ternary-complex mechanism. The validity of these and previously derived theoretical relationships for this mechanism has been tested by application to the liver alcohol dehydrogenase reaction. 2. All essential features of the transient-state kinetics of alcohol oxidation by NAD+ in the liver alcohol dehydrogenase system can be qualitatively and quantitatively explained in view of the compulsory-order mechanism in the proposed scheme. There is no kinetic evidence for any half-of-the-sites reactivity of the enzyme. A consistent set of rate constants is reported for the enzymic oxidation of benzyl alcohol at pH 8.75. 3. Transient-state rate parameters for benzyl alcohol/benzaldehyde catalysis by liver alcohol dehydrogenase have been determined at different pH. The interpretation of such rate parameters is critically discussed with reference to their informative value for the purpose of determination of rate constants (k and k') for the process of ternary-complex interconversion in the proposed scheme. It is concluded that the apparent rate constant (k') for hydride transfer from benzyl alcohol to NAD+ is dependent on a proton dissociation step with a pKa of 6.4, whereas the rate constant (k) for hydride transfer from NADH to benzaldehyde exhibits no corresponding dependence on proton association. 4. The asymmetric pH dependence of the forward and reverse rate of ternary-complex interconversion during liver alcohol dehydrogenase catalysis appears to reflect an obligatory step of alcohol/alcoholate ion equilibration occurring at the ternary-complex level. It is suggested that the observed pKa 6.4 dependence of the transient rate of alcohol oxidation can be attributed to a coupled acid-base system involving minimally the enzyme-bound alcohol and the protein residues Ser-48 and His-51.

Nicotinamide coenzyme regeneration. The rates of some 1,4-dihydropyridine, pyridinium salt, and flavin mononucleotide hydrogen-transfer reactions

The rates of H-transfer between various 1,4-dihydropyridines and pyridinium salts (including NADH and NAD+), and from 1,4-dihydropyridines to FMN, have been measured. The reactions are found to be sufficiently slow for H-transfer to be rate-determining to a significant extent when such Systems are applied for nicotinamide coenzyme recycling purposes. The rates of H-transfer parallel the magnitudes of the donor–acceptor redox potential differences (ΔE0′); ΔE0′ values may therefore be used as qualitative guides in formulating and selecting redox couples of NAD/H recycling value. On the basis of deuterium isotope effects, it is concluded that formation of a complex prior to H-transfer is not rate determining for 1,4-dihydropyridine–NAD+ reactions. This behavior is in contrast to that of other model alcohol dehydrogenase Systems.

Kinetic Transients in the Reduction of Aldehydes Catalysed by Liver Alcohol Dehydrogenase

The transient-state kinetics of enzymic reduction of acetaldehyde and benzaldehyde by NADH, catalyzed by horse liver alcohol dehydrogenase, have been examined under single-turnover conditions, obtained by carrying out reactions either with limiting amounts of enzyme in the presence of 20 mM pyrazole or with limiting amounts of substrate. Analysis of the variation with substrate, coenzyme, and enzyme concentrations of amplitudes and time constants for the exponential transients observed at 328 nm and 300 nm shows that the kinetics of enzymic aldehyde reduction are qualitatively and quantitatively consistent with the relationships derived in the preceding paper for an ordered ternary-complex mechanism involving identical and independent catalytic sites. It is concluded that there is no evidence whatsoever for the kinetic significance of a half-of-the-sites reactivity or any other kind of subunit interaction in the liver alcohol dehydrogenase system. The biphasic transients observed at 328 nm for the reduction of aromatic aldehydes such as benzaldehyde are a normal kinetic characteristic of the ordered ternary-complex mechanism, being attributable to accumulation of the ternary enzyme-NAD-product complex when product dissociation from this complex is slow in comparison to its formation by ternary-complex interconversion.

8-(6-Aminohexyl)-amino-adenine nucleotide derivatives for affinity chromatography

8-(6-Aminohexyl)-amino-AMP and 8-(6-aminohexyl)-amino-NAD(H) are synthesized from AMP and NAD +, respectively, in a two-step procedure via the 8-Br-adenine nucleotides. The AMP derivatives and the NAD + derivatives possess some biological activity as substrates in the adenylate kinase and as coenzymes in the alcohol and lactate dehydrogenase systems, respectively. The synthesized derivatives may be coupled to Sepharose by cyanogen bromide activation. The covalently attached ligands show strong affinities for various dehydrogenases. Examples of the selective purification of several dehydrogenases using the new affinity columns are presented.

Effect of long-term administration of different alcoholic beverages to rats on liver lipid metabolism and bile acid conjugation.

The effect of various alcoholic beverages (ethanol, brandy, whisky, gin, and red wine) on hepatic triglyceride, phospholipid and cholesterol concentration, incorporation rate of glycerol into hepatic triglycerides and phospholipids and bile acid conjugation rate was studied in rats fed alcohol as 50% of the caloric intake for 8–9 months. Two groups of rats served as controls: one fed glucose instead of alcohol and another fed a standard laboratory diet. A special series of rats fed the same amount of ethanol for 10 days was also investigated. In the long-term study no unambiguous changes of hepatic lipid concentration, incorporation rate of glycerol into hepatic lipids, and bile acid conjugation rate were found in the alcohol-fed rats as compared with the glucose-fed rats. In the 10-day study the ethanol rats showed a 60% increase of hepatic triglyceride concentration and of the incorporation rate of glycerol into hepatic triglycerides as well as a 30% increase of the rate of bile acid conjugation compared with glucose fed controls. The results suggest that other pathways for ethanol metabolization than the NADH2-producing alcohol dehydrogenase system become of increased importance with prolonged ethanol administration. The various alcoholic beverages showed almost no differences in their effects on lipid metabolism or bile acid conjugation rate.

Role of the intracellular distribution of hepatic catalase in the peroxidative oxidation of methanol.

Previous to 1950, when Bonnichsenl isolated crystalline alcohol dehydrogenase (ADH) and showed that it did not react with methanol,$ it was believed that the ADH system was responsible for the oxidation of all primary aliphatic alcohols, including methanol (FIGURE 1 ) . This observation redirected attention to the peroxidative system as a means of oxidizing methanol. As early as 1936, Keilin and Hartree3 showed that catalase catalyzed the oxidation of alcohols to their aldehydes when hydrogen peroxide was supplied in low concentrations, as might be provided in living cells through the action of flavin and other peroxide-generating enzymes. They presented arguments to support their view that catalase is not present in the tissues to protect against peroxide intoxication, as was widely contended, but rather to carry out coupled (peroxidative) oxidations. Employing techniques that permitted very rapid spectral determinations, Chance4 identified the intermediate complexes and analyzed the kinetics of the components of the reaction (FIGURE 1 ) . Ideas were exchanged for several years as to whether or not the peroxidative system participated in the in vivo oxidation of methanol and other alcohols. A direct means of examining the question was provided when Heim and coworkers5 showed that the intraperitoneal injection of 3-amino-1,2,4-triazole (AT) caused a reduction in hepatic and renal catalase activities of 90% or more. Mannering and Parkss found that the AT-induced inhibition of hepatic catalase was accompanied by a 70% reduction of the methanol-oxidizing capacity of rat liver homogenates. The addition of crystalline beef liver catalase to these homogenates restored methanol oxidation to normal. While these in vitro studies pointed to a role of catalase in methanol oxidation, they did little to establish its participation in vivo. Because of the complexity of the system, involving as it does the rate of formation of hydrogen peroxide, which in turn depends upon the concentration of substrates available to the peroxide-generating enzymes as well as the availability of hepatic catalase to the hydrogen peroxide produced by these enzymes, it seemed unlikely that in vitro studies would provide much information as to what was occurring in the intact animal. This presentation is devoted largely to a review of the evidence that shows that the intact rat oxidizes methanol largely through the peroxidative system, but that ethanol oxidation proceeds differently, probably almost entirely via the ADH system. The evidence is based on in vitro studies that employed several approaches: 1 ) 14C-methanol and 1 -Wethano1 oxidation were studied

Carbohydrate metabolism of lactic acid cultures. 3. Glycolytic enzymes of Streptococcus lactis and their sensitivity to antibiotics.

Streptococcus lactis UN possessed galactokinase, uridine diphosphate glucose epimerase (galacto-waldenase), hexokinase, aldolase, lactic dehydrogenase, alcohol dehydrogenase, and aldehyde dehydrogenase enzyme systems. The presence of galactokinase, galactowaldenase, and hexokinase indicated that monosaccharides, galactose, and glucose were phosphorylated and converted into glucose-6-phosphate, which then enters the glycolytic cycle. Galactokinase and UDPG epimerase enzymes were found to be adaptive in nature. However, hexokinase, aldolase, and dehydrogenases for lactate, alcohol, and aldehyde were found to be constitutive in nature, as the organism, regardless of the substrate used, always possessed these five enzymes. Both alcohol and aldehyde dehydrogenases were NAD-specific rather than NADP-specific.Penicillin, streptomycin, aureomycin, and terramycin inhibited the activities of these enzyme systems to varying degrees. Also, penicillin and streptomycin inhibited the production of galactokinase and UDPG epimerase by the organism, but had no effect upon the production of hexokinase.

Bioelectrochemistry

Electron transfer as opposed to hydrogen transfer was demonstrated to be involved in the oxidation-reduction of the flavoprotein enzyme system. A bioelectrochemical investigation of glucose oxidase (EC 1.1.3.4), d -amino acid oxidase (EC 1.4.3.3), and yeast alcohol dehydrogenase (EC 1.1.1.1) systems was conducted in an attempt to utilize the electron-transferring process as a potential anodic reaction in a biochemical fuel cell. Utilizing a bio-fuel cell constructed of plexiglass, platinum-foil electrodes, and an ion-exchange membrane for conduction between the anolyte and catholyte, the flavoprotein enzymes, both glucose oxidase and d -amino acid oxidase systems in conjunction with an O 2 cathode, generated 175–350 mV. In contrast, alcohol dehydrogenase (yeast), a pyridinoprotein enzyme which requires coenzyme I (NAD+), did not produce any electrical voltage. Elemental iron was found to potentiate the flavoprotein enzyme reaction yielding voltages ranging from 625 to 750 mV. The potentiating effect was probably due to a faster turnover rate of FADH to FAD+ coupled with the additional net oxidation potential of iron.

Kinetics and equilibria in the liver alcohol dehydrogenase system.

Kinetics and equilibria in the liver alcohol dehydrogenase system.

Electrolytic Reductions of Diphosphopyridine Nucleotide, Triphosphopyridine Nucleotide, and Cytochrome c at Controlled Potentials

It has been reported by several persons, including one of the authors, that DPNH obtained by the electrolytic reduction at controlled potentials is not fully active in respect to the reaction with alcohol dehydrogenase system. But statements concening the percentage of activity and the conditions which affect the activity were not identical. This article deals with these problems by reducing DPN electrolytically under different conditions followed by enzymatic examinations. The electrolytic reductions Of TPN and cytochrome c were also performed. Though the data presented are rather complicated, the most active DPNH was prepared in tripolyphosphate buffer with platinum electrode at −2.0 volt vs. S.C.E., under ice-cooling. The effects of phosphates and electrolytic potential are discussed.

Electrolytic Reductions of Diphosphopyridine Nucleotide, Triphosphopyridine Nucleotide, and Cytochrome c at Controlled Potentials

It has been reported by several persons, including one of the authors, that DPNH obtained by the electrolytic reduction at controlled potentials is not fully active in respect to the reaction with alcohol dehydrogenase system. But statements concening the percentage of activity and the conditions which affect the activity were not identical. This article deals with these problems by reducing DPN electrolytically under different conditions followed by enzymatic examinations. The electrolytic reductions Of TPN and cytochrome c were also performed. Though the data presented are rather complicated, the most active DPNH was prepared in tripolyphosphate buffer with platinum electrode at −2.0 volt vs. S.C.E., under ice-cooling. The effects of phosphates and electrolytic potential are discussed.

Retinene isomerase.

Rhodopsin is formed by the condensation of opsin with a cis isomer of retinene, called neo-b. The bleaching of rhodopsin releases all-trans retinene which must be isomerized back to neo-b in order for rhodopsin to regenerate. Both retinene isomers are in equilibrium with the corresponding isomers of vitamin A, through the alcohol dehydrogenase system. An enzyme is found in cattle retinas and frog pigment layers which catalyzes the interconversion of all-trans and neo-b retinene. We call it "retinene isomerase." It is soluble in neutral phosphate buffer, and precipitates between 20 and 35 per cent saturation with ammonium sulfate. In the dark, the isomerase converts all-trans and neo-b retinene to an equilibrium mixture of 5 parts neo-b and 95 parts all-trans. With opsin present to trap neo-b, the isomerase catalyzes the synthesis of rhodopsin from all-trans retinene. This reaction, however, is too slow to account for dark adaptation. Retinene is isomerized by light, but too slowly to supply the retina with neo-b. In aqueous solution the pseudoequilibrium mixture contains about 15 per cent neo-b. When all-trans retinene is irradiated in the presence of isomerase, the rate of formation of neo-b is increased about 5 times, and the pseudoequilibrium shifted so that the mixture now contains about 32 per cent neo-b. The isomerase is specific for all-trans and neo-b retinene. It does not act on two other cis isomers of retinene, nor on all-trans or neo-b vitamin A. The role of the isomerase in vision appears to be as follows: in the light, as rhodopsin is bleached to opsin and all-trans retinene, the latter is in part converted to the neo-b isomer and stored in the pigment epithelium as neo-b vitamin A. During dark adaptation, the dominant process is the trapping by opsin of neo-b retinene supplied from stores of neo-b vitamin A, and the slow isomerase-catalyzed "dark" conversion of all-trans to neo-b retinene.

Dissociation Constants in the Yeast Alcohol Dehydrogenase System, Calculated from overall Reaction Velocities.

Dissociation Constants in the Yeast Alcohol Dehydrogenase System, Calculated from overall Reaction Velocities.