Human Salivary Aldehyde Dehydrogenase: Purification, Kinetic Characterization and Effect of Ethanol, Hydrogen Peroxide and Sodium Dodecyl Sulfate on the Activity of the Enzyme

Oral enzymatic detoxification system: Insights obtained from proteome analysis to understand its potential impact on aroma metabolization

The oral cavity is an entry path into the body, enabling the intake of nutrients but also leading to the ingestion of harmful substances. Thus, saliva and oral tissues contain enzyme systems that enable the early neutralization of xenobiotics as soon as they enter the body. Based on recently published oral proteomic data from several research groups, this review identifies and compiles the primary detoxification enzymes (also known as xenobiotic-metabolizing enzymes) present in saliva and the oral epithelium. The functions and the metabolic activity of these enzymes are presented. Then, the activity of these enzymes in saliva, which is an extracellular fluid, is discussed with regard to the salivary parameters. The next part of the review presents research evidencing oral metabolization of aroma compounds and the putative involved enzymes. The last part discusses the potential role of these enzymatic reactions on the perception of aroma compounds in light of recent pieces of evidence of in vivo oral metabolization of aroma compounds affecting their release in mouth and their perception. Thus, this review highlights different enzymes appearing as relevant to explain aroma metabolism in the oral cavity. It also points out that further works are needed to unravel the effect of the oral enzymatic detoxification system on the perception of food flavor in the context of the consumption of complex food matrices, while considering the impact of food oral processing. Thus, it constitutes a basis to explore these biochemical mechanisms and their impact on flavor perception.

A comparative study based on activity, conformation and computational analysis on the inhibition of human salivary aldehyde dehydrogenase by phthalate plasticizers: Implications in assessing the safety of packaged food items

Phthalate plasticizers are commonly used in various consumer-end products. Human salivary aldehyde dehydrogenase (hsALDH) is a detoxifying enzyme which defends us from the toxic aldehydes. Here, the effect of phthalates [Di-2-ethylhexyl phthalate (DEHP), Diethyl phthalate (DEP) and Dibutyl phthalate (DBP)] on hsALDH has been investigated. These plasticizers inhibited hsALDH, and the IC₅₀ values were 0.48 ± 0.04, 283.20 ± 0.09 and 285.00 ± 0.14 μM for DEHP, DEP and DBP, respectively. DEHP was the most potent inhibitor among the three plasticizers. They exhibited mixed-type linear inhibition with inclination towards competitive-non-competitive inhibition. They induced both tertiary and secondary structural changes in the enzyme. Quenching of intrinsic hsALDH fluorescence in a constant manner was observed with a binding constant (K_b) of 8.91 × 10⁶, 2.80 × 10⁴, and 1.31 × 10⁵ M^-1, for DEHP, DEP and DBP, respectively. Computational analysis showed that these plasticizers bind stably in the proximity of hsALDH catalytic site, reciprocating via non-covalent interactions with some of the amino acids which are evolutionary conserved. Therefore, exposure to these plasticizers inhibits hsALDH which increases the risk of aldehyde induced toxicity, adversely affecting oral health. The study has implications in assessing the safety of packaged food items which utilize phthalates.

Current View on the Mechanisms of Alcohol-Mediated Toxicity

Alcohol is a psychoactive substance that is widely used and, unfortunately, often abused. In addition to acute effects such as intoxication, it may cause many chronic pathological conditions. Some of the effects are very well described and explained, but there are still gaps in the explanation of empirically co-founded dysfunction in many alcohol-related conditions. This work focuses on reviewing actual knowledge about the toxic effects of ethanol and its degradation products.

Correlation between the Activity of Aldehyde Dehydrogenase and Oxidative Stress Markers in the Saliva of Diabetic Patients

BACKGROUND

Reactive aldehydes are involved in diseases associated with oxidative stress, including diabetes. Human salivary aldehyde dehydrogenase (hsALDH) presumably protects us from many toxic ingredient/contaminant aldehydes present in food.

OBJECTIVE

This study aimed to probe the activity of hsALDH in patients with diabetes and than to correlate it with various oxidative stress markers in the saliva.

METHODS

The saliva samples were collected from total 161 diabetic patients from Rajiv Gandhi Centre for Diabetes, Jawaharlal Nehru Medical College (JNMC), AMU, Aligarh, (India). HsALDH activity and markers of oxidative stress [8-hydroxydeoxyguanosine (8-OHDG), malondialdehyde (MDA) and advanced glycation end products (AGEs)] were measured in the saliva samples.

RESULTS

Patients with early stage of diabetes had higher activity of hsALDH when compared with the control group. As the history of diabetes increases, the activity of the enzyme decreases and also higher oxidative stress markers (8-OHDG, MDA and AGEs) are detected in the saliva samples. Negative significant correlation between hsALDH activity and oxidative stress markers were observed (p <0.0001).

CONCLUSION

The activity of hsALDH increases in early stages of diabetes most probably to counter the increased oxidative stress associated with diabetes. However, in later stages of diabetes, the activity of the enzyme decreases, possibly due to its inactivation resulting from glycation.

Antimicrobial photodynamic therapy mediated by methylene blue in surfactant vehicle on periodontopathogens

BACKGROUND

Periodontal disease (PD) is a chronic inflammatory disease caused by the presence of microbial biofilm. The aim of this study was to evaluate antimicrobial effect of antimicrobial photodynamic therapy (A-PDT) mediated by methylene blue (MB) in monomer form on A. actinomycetemcomitans and P. gingivalis.

METHODS

A. actinomycetemcomitans ATCC 29523 and P. gingivalis ATCC 33577 were cultured on anaerobic jars at 37 °C for 48 h, and we tested APDT in the presence of 0.25% sodium dodecyl sulfate (SDS) in phosphate-buffered saline (PBS) or in PBS alone. APDT was carried out with 100 μM MB under laser radiation (PhotolaseIII, DMC, Brazil) at ʎ =660 nm and parameters as following (P =100 mW; I =250 mW/cm², and doses of 15, 45 and 75 J/cm²).

RESULTS

Following A-PDT, PBS groups of A. actinomycetemcomitans presented 4 Logs of microbial death after 5 min irradiation. However, there was no bacterial reduction in SDS groups. On the other hand, P. gingivalis was sensitive to APDT in the presence of 0.25% SDS with 2 logs reduction from dark toxicity.

CONCLUSION

The presence of 0.25% SDS can lead to different responses depending on the different microbial species.

Citral Inhibition of Human Salivary Aldehyde Dehydrogenase

Human salivary aldehyde dehydrogenase (hsALDH) protects us from the toxic effect of aldehydes. It has both diagnostic and therapeutic importance. Citral possesses many biological and pharmacological properties. The aim of this work was to investigate the inhibitory effect and the mechanism of inhibition of citral on hsALDH. Citral inhibits the dehydrogenase activity of hsALDH. It decreased the substrate affinity and to a lesser extent, the catalytic efficiency of hsALDH. Citral showed linear mixed-type inhibition with a higher tendency of competitive behavior with little, but significant, non-competitive inhibition. The nucleophilicity of active site Cys residue is not a significant contributing factor in the inhibition process. Citral shows uncompetitive inhibition towards the co-enzyme (NAD⁺). α-helix and β-sheet content of the enzyme were changed in presence of citral. Biophysical studies showed that citral quenches the intrinsic fluorescence of hsALDH in a static manner by forming complex with the enzyme. Molecular docking study showed that both the isomers of citral bind to the catalytic site of hsALDH interacting with few evolutionary preserved amino acid residues through multiple non-covalent interactions. Ligand efficiency metrics values indicate that citral is an efficient ligand for the enzyme in terms of its physicochemical and pharmacokinetic properties.

Enhancement in the Catalytic Activity of Human Salivary Aldehyde Dehydrogenase by Alliin from Garlic: Implications in Aldehyde Toxicity and Oral Health

BACKGROUND

Lower human salivary aldehyde dehydrogenase (hsALDH) activity increases the risk of aldehyde mediated pathogenesis including oral cancer. Alliin, the bioactive compound of garlic, exhibits many beneficial health effects.

OBJECTIVE

To study the effect of alliin on hsALDH activity.

METHODS

Enzyme kinetics was performed to study the effect of alliin on the activity of hsALDH. Different biophysical techniques were employed for structural and binding studies. Docking analysis was done to predict the binding region and the type of binding forces.

RESULTS

Alliin enhanced the dehydrogenase activity of the enzyme. It slightly reduced the Km and significantly enhanced the Vmax value. At 1 µM alliin concentration, the initial reaction rate increased by about two times. Further, it enhanced the hsALDH esterase activity. Biophysical studies indicated a strong complex formation between the enzyme and alliin (binding constant, Kb: 2.35 ± 0.14 x 103 M-1). It changes the secondary structure of hsALDH. Molecular docking study indicated that alliin interacts to the enzyme near the substrate binding region involving some active site residues that are evolutionary conserved. There was a slight increase in the nucleophilicity of active site cysteine in the presence of alliin. Ligand efficiency metrics values indicate that alliin is an efficient ligand for the enzyme.

CONCLUSION

Alliin activates the catalytic activity of the enzyme. Hence, consumption of alliincontaining garlic preparations or alliin supplements and use of alliin in pure form may lower aldehyde related pathogenesis including oral carcinogenesis.

Purification and characterization of a novel glutamate dehydrogenase from Geotrichum candidum with higher alcohol and amino acid activity

Crude enzyme from Geotrichum candidum S12 exhibited high activity towards hexanol at pH 4.0, distinguishing it from currently known enzymes. To identify the dominant enzyme contributing to this activity, the crude enzyme extract was separated into different fractions by ammonium sulfate precipitation, MonoQ anion-exchange chromatography, and Sephacryl S-200 gel filtration chromatography. Afraction with high activity towards hexanol at pH 4.0 was obtained, exhibiting 38-fold improved purity and a specific activity of 3802.7 U/mg. After electrophoretic analysis, the fraction showed a molecular weight of 223 kDa by Native-PAGE and 51.4 kDa by SDS-PAGE. The purified fraction was identified as a glutamate dehydrogenase (GDH) by peptide mass fingerprinting data. This fraction showed high activity towards glutamate, α-ketoglutarate, hexanol, and isoamyl alcohol with a Km value of 41.74, 4.01, 20.37, and 19.37 mM, respectively, but with no activity towards methanol, ethanol, 1-propanol, and isobutanol. As a comparison, the GDH from yeast had no activity towards hexanol and other alcohols. Kinetic analysis revealed that glutamate and hexanol served as competitive inhibitors to each other for the purified GDH. The GDH showed the highest activity towards hexanol at pH 4.0 and 30 °C, and was the most stable at pH 2.2-7.0 and ≤40 °C. The presence of ADP, Fe²⁺, K⁺, and Zn²⁺ increased the enzymatic activity towards hexanol and EDTA, Pb²⁺, Mn²⁺, ATP, and DTT decreased the activity. These novel characteristics expand the reported properties of GDH and suggest the newly characterized GDH has unique potential for practical application.

Thymoquinone binds and activates human salivary aldehyde dehydrogenase: Potential therapy for the mitigation of aldehyde toxicity and maintenance of oral health

Human salivary aldehyde dehydrogenase (hsALDH) is a very important anti-oxidant enzyme present in the saliva. It is involved in the detoxification of toxic aldehydes and maintenance of oral health. Reduced level of hsALDH activity is a risk factor for oral cancer development. Thymoquinone (TQ) has many pharmacological activities and health benefits. This study aimed to examine the activation of hsALDH by TQ. The effect of TQ on the activity and kinetics of hsALDH was studied. The binding of TQ with the enzyme was examined by different biophysical methods and molecular docking analysis. TQ enhanced the dehydrogenase activity of crude and purified hsALDH by 3.2 and 2.9 fold, respectively. The K_m of the purified enzyme decreased and the V_max increased. The esterase activity also increased by 1.2 fold. No significant change in the nucleophilicity of the catalytic cysteine residue was observed. TQ forms a strong complex with hsALDH without altering the secondary structures of the enzyme. It fits in the active site of ALDH3A1 close to Cys 243 and the other highly conserved amino acid residues which lead to enhancement of substrate binding affinity and catalytic efficiency of the enzyme. TQ is expected to give better protection from toxic aldehydes in the oral cavity and to reduce the risk of oral cancer development through the activation of hsALDH. Therefore, the addition of TQ in the diet and other oral formulations is expected to be beneficial for health.

Activation of Human Salivary Aldehyde Dehydrogenase by Sulforaphane: Mechanism and Significance

Cruciferous vegetables contain the bio-active compound sulforaphane (SF) which has been reported to protect individuals against various diseases by a number of mechanisms, including activation of the phase II detoxification enzymes. In this study, we show that the extracts of five cruciferous vegetables that we commonly consume and SF activate human salivary aldehyde dehydrogenase (hsALDH), which is a very important detoxifying enzyme in the mouth. Maximum activation was observed at 1 μg/ml of cabbage extract with 2.6 fold increase in the activity. There was a ~1.9 fold increase in the activity of hsALDH at SF concentration of ≥ 100 nM. The concentration of SF at half the maximum response (EC₅₀ value) was determined to be 52 ± 2 nM. There was an increase in the V_max and a decrease in the K_m of the enzyme in the presence of SF. Hence, SF interacts with the enzyme and increases its affinity for the substrate. UV absorbance, fluorescence and CD studies revealed that SF binds to hsALDH and does not disrupt its native structure. SF binds with the enzyme with a binding constant of 1.23 x 10⁷ M^-1. There is one binding site on hsALDH for SF, and the thermodynamic parameters indicate the formation of a spontaneous strong complex between the two. Molecular docking analysis depicted that SF fits into the active site of ALDH3A1, and facilitates the catalytic mechanism of the enzyme. SF being an antioxidant, is very likely to protect the catalytic Cys 243 residue from oxidation, which leads to the increase in the catalytic efficiency and hence the activation of the enzyme. Further, hsALDH which is virtually inactive towards acetaldehyde exhibited significant activity towards it in the presence of SF. It is therefore very likely that consumption of large quantities of cruciferous vegetables or SF supplements, through their activating effect on hsALDH can protect individuals who are alcohol intolerant against acetaldehyde toxicity and also lower the risk of oral cancer development.