A nicotinamide-adenine dinucleotide assay utilizing liver alcohol dehydrogenase

Regeneration of NAD(+) cofactor by photosensitized electron transfer in an immobilized alcohol dehydrogenase system

The irradiation with visible light of a photosensitizer dye like methylene blue was used to regenerate by electron transfer the oxidized form of a pyridine nucleotide coenzyme (NAD(+)). The process has been studied on a common enzymatic reaction: ethanol oxidation by alcohol-NAD(+) oxidoreductase immobilized on polyacrylamide gel or porous glass balls. In the experimental conditions used, the initial NAD(+) recycling rates were 2.33 x 10(4) cycles/h (polyacrylamide) and 3 x 10(4) cycles/h (glass balls). A total number of 49.5 x 10(4) cycles was obtained for 13 runs of 2 h. The enzyme immobilization strongly increased its stability: after 28 days at 20 degrees C, the residual activity was 25% of the initial value.

The NAD+ /NADH redox state in astrocytes: independent control of the NAD+ and NADH content

The intracellular redox state is established by several redox pairs, such as NAD(+) /NADH and NADP(+) /NADPH and glutathione. This redox state is a crucial determinant of cellular metabolism and function. Astrocytes are an important cell population contributing to brain metabolism and brain energy supply, so a careful control of these redox pairs is essential for proper brain function. Despite this, little is known about control of the NAD(+) and NADH content within the brain or in astrocytes. Therefore, we here analyzed the NAD(+) and NADH content of mouse tissue and cultured cortical astrocytes. The NAD(+) /NADH ratio increased in most tissues during development from newborn to adult mice. The basal redox ratio of cultured astrocytes was about 3.8 and similar to the redox ratio of the cortex of newborn mice. Although the NADH content of these cells was highly sensitive to the concentration of energy substrates and to modulation of energy metabolism, the NAD(+) content was surprisingly constant under these conditions. In contrast, application of nicotine amide or nicotinamide mononucleotide, which are precursors for NAD(+) biosynthesis, slowly increased NAD(+) content while leaving NADH levels unaffected. Finally, inhibiting the NAD(+) -degrading enzyme poly-(ADP-ribose)-polymerase increased NAD(+) content slightly without affecting NADH levels, whereas inhibition of sirtuins had no effect. These results indicate that, in addition to converting NAD(+) to NADH and vice versa during redox reactions, the content of both partners of this redox pair is additionally controlled by other mechanisms.

Acetaldehyde metabolism by liver mitochondrial ALDH from UChA and UChB rats: effect of inhibitors

We have observed that blood acetaldehyde (AcH) levels after an ethanol dose were significantly higher in disulfiram-pre-treated UChA (low ethanol consumer) than in UChB (high ethanol consumer) rats. In order to explore these results further, we studied the effect of disulfiram (300 mg/kg i.p.) and chlorpropamide (80) mg/kg i.p.) pre-treatment on blood AcH levels after oral ethanol (60 mmol/kg) and on AcH metabolism by liver mitochondrial aldehyde(s) dehydrogenase(s) from UChA and UChB rats. AcH metabolism by liver mitochondrial aldehyde dehydrogenase (ALDH) was studied by following AcH disappearance rate and the formation of NADH at 340 nm in the incubation medium. The results showed that chlorpropamide, like disulfiram, produced a higher blood AcH level consistent with a greater inhibition of the low-Km mitochondrial ALDH in the UChA rats than in the UChB rats. These drugs did not inhibit the high Km mitochondrial ALDH. Kinetic studies of mitochondrial ALDH show that low-Km mitochondrial ALDH from UChB rats exhibits a higher affinity for NAD than UChA rats. This observation could explain the different inhibition of ALDH by both drugs, assuming that the inhibitors reduce NAD availability, the rate limiting step in the mitochondrial ALDH oxidation.

Acetaldehyde metabolism by brain mitochondria from UChA and UChB rats

The acetaldehyde (AcH) oxidizing capacity of total brain homogenates from the genetically high-ethanol consumer (UChB) appeared to be greater than that of the low-ethanol consumer (UChA) rats. To gain further information about this strain difference, the activity of aldehyde dehydrogenase (AIDH) in different subcellular fractions of whole brain homogenates from naive UChA and UChB rat strains of both sexes has been studied by measuring the rate of AcH disappearance and by following the reduction of NAD to NADH. The results demonstrated that the higher capacity of brain homogenates from UChB rats to oxidize AcH when compared to UChA ones was because the UChB mitochondrial low Km AIDH exhibits a much greater affinity for NAD than that of the UChA rats, as evidenced by four-to fivefold differences in the Km values for NAD. But the dehydrogenases from both strains exhibited a similar maximum rate at saturating NAD concentrations. Because intact brain mitochondria isolated from UChB rats oxidized AcH at a higher rate than did mitochondria from UChA rats only in state 4, but not in state 3, this strain difference in AIDH activity might be restricted in vivo to NAD disposition.

Internal supply of coenzyme to an amperometric glucose biosensor based on a chemically modified electrode

A biosensor for glucose using glucose dehydrogenase immobilized on a chemically modified graphite electrode was supplied with coenzyme, nicotinamide adenine dinucleotide (NAD+), through pores in the material. A graphite rod was hollowed out, leaving 0.3 mm at the end contacting the solution, filled with 10 mM NAD+ and pressurized. The response factor was 40% of that obtained when 2 mM NAD+ was mixed with the sample solution in a flow system. The coenzyme consumption was 11 microliters h-1 representing a 500-fold saving compared to supply through the bulk solution. The biosensor had a linear calibration curve from the detection limit, 1 microM, to 2 mM glucose and a repeatability of 0.3%. The graphite electrode was modified by adsorption of a bis-(benzophenoxazinyl)-terephthaloyl derivative in order to be able to oxidize NADH at 0 mV versus Ag/AgCl, 0.1 M KCl.

Regeneration of nicotinamide cofactors for use in organic synthesis

The high cost of nicotinamide cofactors requires that they be regenerated in situ when used in preparative enzymatic synthesis. Numerous strategies have been tested for in situ regeneration of reduced and oxidized cofactors. Regeneration of reduced cofactors is relatively straightforward; regeneration of oxidized cofactors is more difficult. This review summarizes methods for preparation of the cofactors, factors influencing their stability and lifetime in solution, methods for their in situ regeneration, and process considerations relevant to their use in synthesis.

Spin ECHO proton NMR studies of the metabolism of malate and fumarate in human erythrocytes. Dependence on free NAD levels

The NAD-dependent conversion of malate to lactate in human erythrocytes was studied by spin echo proton NMR. A pathway involving the decarboxylation of oxaloacetate catalysed by haemoglobin is proposed to account for the observed reaction. NADP-dependent reaction was negligible. The rate of the reaction was measured in intact erythrocytes under controlled conditions. This rate correlates with that obtained with lysates at 30 microM free NAD and that obtained with purified human erythrocyte enzymes at about 15 microM NAD. The total extractable NAD in the intact cells was 70-90 microM. Experiments with cells containing elevated NAD levels could be explained by a significant fraction of the NAD being weakly bound (Kd about 1 mM) to haemoglobin.

Amplification by enzymatic cycling

Enzymatic cycling provides a methodology for virtually unlimited amplification of analytical sensitivity. The most widely applicable cycling systems are those for NAD and NADP, since these can be used to increase the sensitivity of methods for a host of other substances. However, cycling systems for ATP plus ADP, GTP + GDP, glutathione and coenzyme A have also proven to be very useful. A total of 19 cycling procedures are described in greater or lesser detail. Some of these are capable of amplification rates in excess of 20,000 per hour in a single cycling step (20,000 x 20,000 with two one hour cycling steps). Advantages, disadvantages, limitations and other practical considerations are stressed, as well as the means for coupling the cycling systems to assays for other substances.

Strain differences in the development of acute tolerance to ethanol

C57B1/6 mouse brain serotonin levels were depleted by feeding animals a diet containing no tryptophan. When such mice were injected with ethanol, they were found to lose their righting reflex for significantly longer periods and to have a lower body temperature than control animals. Animals consuming the diet containing no tryptophan metabolized ethanol more slowly than controls. Although daily injections of kynurenine reinstated ethanol metabolism to normal, the duration of loss of righting reflex and the hypothermia induced by ethanol were unaffected by kynurenine pretreatment. Tryptophan (75 mg/kg) administered six hours prior to ethanol injection returned brain serotonin levels to normal in tryptophan-deprived mice. Mice injected with tryptophan were found to respond to ethanol as did the control animals. When brain ethanol levels were determined at the time the animals lost their righting reflex and when animals regained their righting reflex, tryptophan-deprived mice were found to regain the righting reflex at the same brain ethanol levels as those at which such animals lost their righting reflex. Tryptophan administration to tryptophan-deprived mice resulted in their regaining the righting reflex at higher ethanol levels than those at which they lost the reflex. Similar experiments were carried out on C3H/HeJ and DBA/J2 mice. The results indicate that C3H mice developed some acute tolerance while DBA mice failed to develop any acute tolerance. The possibility exists that the strain difference in the degree of sensitivity to ethanol observed in these mice may be due to differing abilities to develop acute tolerance.

Covalent binding of an NAD analogue to liver alcohol dehydrogenase resulting in an enzyme-coenzyme complex not requiring exogenous coenzyme for activity

1. The NAD analogue, N6-[N-(6-aminohexyl)carbamoylmethyl]-NAD, was covalently bound to horse liver alcohol dehydrogenase in a carbodiimide-mediated reaction and in such a way that it was active with the very same enzyme molecule to which it was coupled. 2. The degree of substitution, i.e. the number of NAD analogues per enzyme subunit, could be varied (0.3-1.6). In one preparation 1.6 coenzyme molecules were bound per subunit; the alcohol dehydrogenase activity of this preparation was 40% of the activity obtained after addition of free NAD in excess. 3. It was calculated that every fourth active site of this preparation was provided with a covalently bound functioning coenzyme analogue, and that this analogue had a cycling rate of about 40 000 cycles/h in a coupled substrate assay. 4. The presence of the covalently bound coenzyme made the active sites difficult to inhibit with a competitive inhibitor. For example, 10 mM AMP inhibited the activity of the preparation by 50% whereas a reference system containing native alcohol dehydrogenase was inhibited by 80% in spite of the fact that the reference system contained about 20 000 times as high a concentration of coenzyme.

Preparation of an alcohol-dehydrogenase--NAD(H)--sepharose complex showing no requirement of soluble coenzyme for its activity

1. Horse liver alcohol dehydrogenase and an NADH analogue, N6-[(6-aminohexyl)carbamoylmethyl]-NADH, have been co-immobilized to Sepharose 4B under conditions permitting binary complex formation between the enzyme and the cofactor. 2. The enzyme-coenzyme-matrix preparations were assayed with a coupled oxidoreduction reaction and showed activities, prior to addition of coenzyme, that were up to 40% of that obtained in excess of free coenzyme. 3. A molar ratio of 1:1 between the amount of bound enzyme was sufficient to obtain high activities in the absence of free coenzyme. 4. The highest recycling rate obtained for the immobilized nucleotide was 3400 cycles per hour. 5. Both thermal and storage stability of alcohol dehydrogenase was increased when the enzyme was co-immobilized with the NADH analogue. 6. The efficiency of the immobilized preparations (measured as product formation per minute and per assay volume) was higher (1.4 to 5 times in our assays) than the corresponding systems of free enzyme (in total enzyme units) and nucleotide in an identical assay volume.