Calcium supplementation during lactation blunts erythrocyte lead levels and delta-aminolevulinic acid dehydratase zinc-reactivation in women non-exposed to lead and with marginal calcium intakes

Impact of Occupational Cement Dust Exposure on Hematological Health Parameters: A Cross-Sectional Study

Introduction Contact with the dust of cement consisting of toxic components brings about inflammatory damage (often irreversible) to the body of a human being. The circulatory system exhibits sensitivity to inflammatory changes in the body, and one of the earliest changes may be observed in the blood parameters like mean corpuscular hemoglobin (MCH) and mean corpuscular hemoglobin concentration (MCHC). MCHC and MCH are possibly easily accessible and affordable parameters that can detect harmful changes in the body before any irreversible damage occurs. Objectives This research aimed to seek the changes in MCHC and MCH upon occupational contact with the toxic dust of cement. Methods The execution of this research was done in the Department of Physiology, Dhaka Medical College, Bangladesh, and a cement plant in Munshiganj, Bangladesh. This research was carried out between September 2017 and August 2018. Individuals (20 to 50 years old, 92 male adults) participated and were grouped into the group with occupational cement dust impact (46 subjects) and the group without occupational dust of cement impact (46 subjects). Data was collected in a pre-designed questionnaire. An independent sample t-test was conducted to analyze statistical and demographic data like body mass index and blood pressure. A multivariate regression model was applied to note the impact of cement dust on the group working in this dusty environment. Again, a multivariate regression model was employed to observe whether the duration of exposure to this dust affected MCHC and MCH. The significance level was demarcated at p < 0.05 Stata-15 (StataCorp LLC, College Station, TX, US) for statistical analysis, and GraphPad Prism v8.3.2 (Insight Venture Management, LLC, New York, NY, US) was employed to present the data graphically when required. Results There was a reduction in MCHC by 0.58 g/dL and MCH levels by 0.68 pg in the cement dust-exposed subjects when compared to controls, but not significant (95% CI: -0.93, 2.10; p  = 0.448 and 95% CI: -0.37, 1.73; p = 0.203, respectively). However, MCHC was reduced significantly by 0.51 g/dL (p = 0.011) with the duration of exposure to the dust. Conclusion The study showed that MCHC was significantly reduced with the duration of exposure to cement dust in cement plant workers. Such alterations may hamper heme synthesis, hemolysis, and inflammatory changes in the body.

Red Cell Distribution Width and Mean Corpuscular Volume Alterations: Detecting Inflammation Early in Occupational Cement Dust Exposure

Introduction Cement dust emitted during cement manufacture consists of toxic components. Occupational cement dust exposure may cause inflammation in the human body, which may be detected early by observing changes in blood parameters such as red blood cell distribution width (RDW) and mean corpuscular volume (MCV). Objectives The study aims to observe the effect of occupational cement dust exposure on RDW and MCV. Methods This study was performed in the Department of Physiology of Dhaka Medical College, Dhaka, Bangladesh, and a factory in Munshiganj, Bangladesh, from September 2017 to August 2018. Ninety-two participants between 20 and 50 years were included (46 subjects were occupationally exposed to cement dust, and 46 were not exposed to cement dust). A pre-designed questionnaire was used for data collection. An independent sample t-test was used to analyze basic information, such as blood pressure and BMI. The multivariate regression model was used to analyze the effect of cement dust exposure on the study group. The impact of cement dust exposure duration was analyzed using the multivariate regression model. The level of significance was p < 0.05. The statistical analysis was performed using STATA-15 (StataCorp, College Station, TX), and the graphical presentation used GraphPad Prism v8.3.2. Results The cement dust-exposed participants had a significantly higher value of MCV by 1.19 fi (95% CI = 0.02, 4.84; p = 0.049) and a 5.92% increase in RDW (95% CI = 5.29, 6.55; p < 0.001) than that of the control group. Conclusion The study reveals that exposure to cement dust causes significant changes in RDW and MCV. These changes may indicate hemolysis due to inflammation.

Hepatotoxicity and ALAD Activity Profile for Prediction of NOAEL of Metal Welding Fumes in Albino Rats

Metal fume pollutants of urban Kano, a city of over 10 million people, and widespread metal works have increased exposure with related health effects. Few data on metal fume toxicity and atmospheric levels have been documented in Nigeria and Kano in particular. Hence, the work was aimed at evaluating the metal fume toxicity to laboratory rat species for setting the permissible limit of exposure in urban Kano. The investigation involved the collection of metal welding fumes and subsequent laboratory analysis. Experimental animals were then exposed intratracheally to varying doses of the fumes which were equivalent to normal metal workers' daily routine of 2, 4, and 8 h for 3, 5, 10, and 20 years. Following euthanization, whole blood samples were collected and functions of liver and delta-aminolevunilic acid dehydratase were evaluated in the serum. Exposure to the fumes has caused significant mortality that was observed to be dose-dependent and statistically different (p < 0.05); moreover, the fumes had synergistically affected the functions of liver. In addition, the fumes had increased (statistically) the activity delta-aminolevinilic acid dehydratase. This has indicated that exposure to metal welding fumes being multi-elemental is toxic and had produced mortality at exposure to higher doses of metal welding fumes. It was therefore established from the study that no-observed-adverse-effect level (NOAEL) for metal welding fumes is 25.73 mg with LD₅₀ of 270 mg which corresponds to the metal worker's 4-h shifts daily for 5 years under existing working conditions. It was recommended that regular monitoring should be put in place to limit exposure and extent of engagement in metal works beyond NOAEL levels.

The association of cadmium and lead exposures with red cell distribution width

Elevated red blood cell distribution width (RDW), traditionally an indicator of anemia, has now been recognized as a risk marker for cardiovascular disease incidence and mortality. Experimental and acute exposure studies suggest that cadmium and lead individually affect red blood cell production; however, associations between environmental exposures and RDW have not been explored. We evaluated relationships of environmental cadmium and lead exposures to RDW. We used data from 24,607 participants aged ≥20 years in the National Health and Nutrition Examination Survey (2003-2016) with information on blood concentrations of cadmium and lead, RDW and socio-demographic factors. In models adjusted for age, sex, race/ethnicity, education, poverty income ratio, BMI, alcohol consumption, smoking status and serum cotinine, RDW was increasingly elevated across progressively higher quartiles of blood cadmium concentration. A doubling of cadmium concentration was associated with 0.16 higher RDW (95% CI: 0.14, 0.18) and a doubling of lead concentration with 0.04 higher RDW (95% CI: 0.01, 0.06). Also, higher cadmium and lead concentrations were associated with increased odds of high RDW (RDW>14.8%). The associations were more pronounced in women and those with low-to-normal mean corpuscular volume (MCV) and held even after controlling for iron, folate or vitamin B12 deficiencies. In analysis including both metals, cadmium remained associated with RDW, whereas the corresponding association for lead was substantially attenuated. In this general population sample, blood cadmium and lead exposures were positively associated with RDW. The associations may indicate hemolytic or erythropoietic mechanisms by which exposure increases mortality risk.

Association between blood erythrocyte lead concentrations and hemoglobin levels in preschool children

Despite decades of intensive research, lead (Pb) toxicity still remains one of the most frequently investigated subjects in environmental health. Whole blood lead (BPb) is usually used to evaluate Pb exposure for both screening and clinical diagnosis. However, it is generally recognized that BPb is not a sensitive biomarker for Pb exposure in hematological studies. Considering hematocrit (HCT) variation in different situations, HCT-adjusted BPb or erythrocyte Pb (EPb) may be more relevant when evaluating the hematotoxicity of blood Pb. Data collected from 855 preschool children, 3-7 years of age, allowed us to examine the relationship between EPb and hemoglobin (Hb) levels. Multivariate linear regression was performed to determine the significance of EPb as predictor of Hb after covariate adjustment; then, mean differences of Hb levels between quartiles of EPb and BPb (1st quartile as reference) were determined using ANOVA followed by Student's t test. The dose-response curve between EPb and HCT was plotted using locally weighted scatterplot smoothing (LOWESS) method. A doubling of EPb was associated with a 2.44 g/L decrease in Hb level. Compared to the 1st quartile group of EPb, the 3rd and 4th quartile groups showed significant decreases in Hb levels (3.01 and 3.97 g/L, respectively). Compared to the 1st quartile group of BPb, the 2nd quartile group showed a decrease in Hb levels (0.63 g/L), while the 3rd and 4th quartile groups showed increases in Hb levels (0.78 and 1.45 g/L, respectively). Increased EPb levels are significantly associated with decreased Hb levels in preschool children. HCT must be taken into consideration in investigating the hematological effects of Pb. Compared to BPb, EPb or HCT-adjusted BPb appear as a more effective biomarker to interpret the hematotoxicity of lead. Furthermore, blood erythrocytes are not only a repository of Pb but also a primary target of its toxicity.

The effect of vitamin C on the erythrocyte antioxidant enzymes in intoxicated-lead rat offsprings

OBJECTIVE

Lead exposure or lead poisoning is known to cause a large spectrum of physiological, biochemical, and behavioural disorders in animals. This study was aimed at assessing the effect of vitamin C on the erythrocyte superoxide dismutase, glutathione peroxidase and the glutathione reductase activities in intoxicated- lead rat offsprings.

METHODS

This study was performed on the pups from female Wistar albino rats. The rats were divided into 4 groups and the treatments were administered through drinking water. Group1 (control group) consumed distilled water. Group 2 (lead group) consumed a solution of lead acetate (300mg/L). Group3 (lead + vitamin C) consumed a solution of lead (300mg/L) which was supplemented with vitamin C (2g/L). Group4 (vitamin group) consumed a solution of vitamin C (2g/L). The enzyme activities were determined in all the 4 groups.

RESULTS

The administration of lead showed a decrease in the enzyme activities. The superoxide dismutase activity was increased after the administration of lead in combination with vitamin C. The lead treated rats showed significantly lower body weights at birth and at weaning. The vitamin C treatment showed a significant increase in the body weight. The haemoglobin levels were significantly decreased in the lead-treated rats. The addition of vitamin C to the lead treatment and vitamin C alone could elevate the haemoglobin levels significantly.

CONCLUSION

The results of this study showed that lead alterates the erythrocyte antioxidant enzyme activities. There was an increase in the superoxide dismutase activity following the treatment with vitamin C. This study suggests that the treatment with vitamin C during lactation has a therapeutic effect in the treatment of lead intoxication. The administration of vitamin C prevents haemoglobin reduction in the erythrocytes.

Effects of delta-aminolevulinic acid dehydratase polymorphisms on susceptibility to lead in Han subjects from southwestern China

This study is to determine the distribution of the delta-aminolevulinic acid dehydratase (ALAD) polymorphism among Han subjects of the Chinese population and to study whether the polymorphism in the ALAD gene modifies the toxicity of lead in lead-exposed workers. For this purpose we conducted a cross-sectional study on 156 Chinese workers who were exposed to lead in lead-acid battery and electric-flex manufacturing plants. The authors found that the allele frequencies of ALAD1 and ALAD2 were 0.9679 and 0.0321, respectively. Workers with the ALAD 1-1 genotype were associated with higher blood lead levels than those with the ALAD 1-2 genotype. Blood and urine lead levels were much higher in storage battery workers than in cable workers. The self-conscious symptom survey showed that the incidences of debilitation, amnesia and dreaminess were much higher in those had more than five years of tenure or contact with lead on the job within the ALAD 1-1 genotype subgroup. Laboratory examinations showed that serum iron and zinc levels in workers' with the ALAD 1-2 genotype were higher than those with the ALAD 1-1 genotype, especially in storage-battery workers. Correlation analysis indicated that the blood lead level negatively correlated with serum calcium, iron and zinc level. The data of this study suggest that the ALAD gene polymorphism and serum ion levels may modify the kinetics of lead in blood. Therefore, the authors recommend that an adequate intake of dietary calcium, iron, and zinc or the calcium, iron, and zinc supplementation should be prescribed to Chinese lead exposed workers.

Natural antioxidants protect against lead-induced damage during pregnancy and lactation in rat's pups

The aim of this study was to add further evidence to the biochemical changes produced in lead-exposed pups and to investigate the potential role of natural antioxidants against the lead-induced damage. Pregnant Wistar rats received treatments with drinking water, divided into four groups, as follows: (1) distilled water; (2) lead (300mg/L); (3) lead+Zn (20mg/L)+vitamins A (50,000U/L), C (2g/L), E (500mg/L) and B(6) (500mg/L); and (4) vitamins+Zn solution. We found a significant decrease in haemoglobin and haematocrit values as well and an increase in haemolysis among lead-exposed pups. Vitamins and zinc supplementation were effective in restoring delta-aminolevulinic acid dehydratase, inhibited by lead in erythrocytes, but did not reach control values. Lead exposure increased the production of thiobarbituric acid reactive substances (TBARS) and catalase activity in kidneys and liver that were reduced by the co-administration of vitamins and zinc. Our findings suggest that administration of antioxidants during gestation and lactation could prevent some of the negative effects of lead.

Supplementing young women with both zinc and iron protects zinc-related antioxidant indicators previously impaired by iron supplementation

Iron supplementation impairs antioxidant status, whereas zinc is recognized as an antioxidant micronutrient. We investigated the effect of supplementing both zinc and iron on iron, zinc, and antioxidant status in 18 women (22-31 y) studied during 2 sequential 8-wk periods. From wk 1 to 8, only iron (50 mg/d) (Fe period) was supplemented and from wk 9 to 16, zinc (25 mg/d) (Fe+Zn period) was also given but at a different time of the day. Indicators of iron (serum iron, iron-binding capacity, and serum ferritin), zinc (serum and urinary zinc), and antioxidant status [ferric-reducing ability of plasma (FRAP); erythrocyte osmotic fragility (EOF); erythrocyte aminolevulinic acid dehydratase (delta-ALAD) activity, and in vitro zinc-delta-ALAD activation (Zn-delta-ALAD%)] were measured at baseline and after each supplementation period. Fe period modified indicators of iron status as expected (P < 0.05) but did not affect indicators of zinc status. Fe+Zn period did not affect indicators of iron status but increased serum and urinary zinc (P < 0.02). Antioxidant status was impaired after the Fe period, as indicated by decreased FRAP (P < 0.005) and delta-ALAD activity (P < 0.05) and increased EOF (P < 0.01). After the Fe+Zn period, FRAP values tended to increase (P = 0.1), delta-ALAD activity and EOF returned to baseline values, and Zn-delta-ALAD% decreased (P < 0.001) compared with baseline. In conclusion, supplementing young women with both zinc and iron protects zinc-related antioxidant indicators previously impaired by iron supplementation without impairment of iron status.

Mercury and lead during breast-feeding

Hg and Pb are of public health concern due to their toxic effects on vulnerable fetuses, persistence in pregnant and breast-feeding mothers, and widespread occurrence in the environment. To diminish maternal and infant exposure to Hg and Pb, it is necessary to establish guidelines based on an understanding of the environmental occurrence of these metals and the manner in which they reach the developing human organism. In the present review, environmental exposure, acquisition and storage of these metals via maternal–infant interaction are systematically presented. Though Hg and Pb are dispersed throughout the environment, the risk of exposure to infants is primarily influenced by maternal dietary habits, metal speciation and interaction with nutritional status. Hg and Pb possess similar adverse effects on the central nervous system, but they have environmental and metabolic differences that modulate their toxicity and neurobehavioural outcome in infant exposure during fetal development. Hg is mainly found in protein matrices of animal flesh (especially fish and shellfish), whereas Pb is mainly found in osseous structures. The potential of maternal acquisition is higher and lasts longer for Pb than for Hg. Pb stored in bone has a longer half-life than monomethyl-mercury acquired from fish. Both metals appear in breast milk as a fraction of the levels found in maternal blood supplied to the fetus during gestation. Habitual diets consumed by lactating mothers pose no health hazard to breast-fed infants. Instead, cows' milk-based formulas pose a greater risk of infant exposure to neurotoxic substances.

Early (in uterus and infant) exposure to mercury and lead

Mercury and lead are toxic metals widely spread in the environment with bio-accumulative features that raises public health concerns. Both metals are equally dispersed in the human food chain but exposure and risk of toxicity during early human development are modulated by the diet and nutritional status. Understanding how Hg and Pb occur and interact with nutrients is fundamental to establish guidelines for diminishing exposure and the risk of toxicity. The risk of fetal and infant exposure to Hg can be influenced by maternal amalgam filling (inorganic Hg) and fish consumption (monomethyl Hg), whereas the risk of exposure to Pb is complex: maternal absorption depends on nutrient interactions (Ca and P); and maternal body Pb accumulation responds to all factors known to interact with bone and calcium metabolism. Maternal exposure to Hg and Pb is more important during fetal development than during breastfeeding. Moreover, these metals (especially Pb) are frequently higher in infant formulas which do not carry the nutritional and psychological advantages and protection of breastfeeding. Infant's reference dose is lower for Hg than for Pb, but risk of Pb contamination for fetuses and infant (breast- or formula-fed) is higher and lasts longer than Hg. Breastfeeding is essential to complete infant development. Interruption or suppression of breast-feeding with cow's milk-based formulas is not an option to environmental pollution.

2,3-Dimercaptopropanol, 2,3-dimercaptopropane-1-sulfonic acid and meso-2,3-dimercaptosuccinic acid increase lead-induced inhibition of δ-aminolevulinate dehydratase in vitro and ex vivo

We investigated the effects of dimercaprol (BAL), meso-2,3-dimercaptosuccinic acid (DMSA) and 2,3-dimercapto-1-propanesulphonic acid (DMPS) on human blood delta-aminolevulinate dehydratase (delta-ALA-D) activity, the most reliable indicator of lead intoxication in humans, in the presence of lead in vitro. Furthermore, we studied the effects of the chelating agents, administered subcutaneously, on delta-ALA-D activity in blood and tissues of mice submitted to sub-acute lead exposure (50 mg/kg for 15 consecutive days, subcutaneously). In vitro results demonstrated that human blood delta-ALA-D activity was significantly inhibited (62%) by lead acetate. Lead acetate (1-1000 microM) pre-incubated with human blood increased the inhibitory potency of this compound on delta-ALA-D when compared to the assay without pre-incubation (89%). Chelating agents caused a marked potentiation of delta-ALA-D inhibition induced by lead, in vitro. One of the most notable observations in the present study was the correspondence between in vitro and ex vivo effects. In fact, BAL and DMPS increase the inhibitory effect of lead on delta-ALA-D activity from mice blood. The complexes formed (lead and chelators) were more inhibitory than lead alone in kidney and liver enzyme activity, ex vivo.

Is lead considered as a risk factor for high blood pressure during menopause period among Saudi women?

This case-control study was designed to examine the association between blood lead levels and high blood pressure in a restricted subpopulation, Saudi women who were 45-93-year old, during or after menopausal period and not occupationally exposed to lead. Blood lead levels were assessed in 100 women with hypertension and 85 control subjects. Lead concentrations were measured in the whole blood using flameless atomic absorption spectrophotometry. Blood pressure measurements were performed according to the World Health Organization recommendations. Results revealed that the mean blood lead levels for hypertensive were 47.52+/-39.26 and 45.59+/-28.55 microg/l for controls. Participants were classified according to the median of blood lead levels in order to compute odds ratios. After controlling a number of potential confounding variables, the multiple logistic regression analysis revealed that women with blood lead levels of > or = 38.6 microg/l were 5.27 times more likely to be hypertensive than those with blood lead levels of < 38.6 microg/l, but of borderline significance (p = 0.06). Although such observation might support the hypothesis that the depletion of lead from bones during menopause increases blood lead levels placing women at increased risk for high blood pressure, there is a need for further studies with larger number of subjects. A number of risk factors, which were suspected to influence blood lead levels, were also investigated. Use of Kohl, duration of its use, osteoporosis disease and intake of calcium supplements were significantly associated with blood lead levels.

Blood lead changes during pregnancy and postpartum with calcium supplementation

Pregnancy and lactation are times of physiologic stress during which bone turnover is accelerated. Previous studies have demonstrated that there is increased mobilization of lead from the maternal skeleton at this time and that calcium supplementation may have a protective effect. Ten immigrants to Australia were provided with either calcium carbonate or a complex calcium supplement (approximately 1 g/day) during pregnancy and for 6 months postpartum. Two immigrant subjects who did not conceive acted as controls. Sampling involved monthly venous blood samples throughout pregnancy and every 2 months postpartum, and quarterly environmental samples and 6-day duplicate diets. The geometric mean blood lead at the time of first sampling was 2.4 microg/dL (range, 1.4-6.5). Increases in blood lead during the third trimester, corrected for hematocrit, compared with the minimum value observed, varied from 10 to 50%, with a geometric mean of 25%. The increases generally occurred at 6-8 months gestation, in contrast with that found for a previous cohort, characterized by very low calcium intakes, where the increases occurred at 3-6 months. Large increases in blood lead concentration were found during the postpartum period compared with those during pregnancy; blood lead concentrations increased by between 30 and 95% (geometric mean 65%; n = 8) from the minimum value observed during late pregnancy. From late pregnancy through postpartum, there were significant increases in the lead isotopic ratios from the minimum value observed during late pregnancy for 3 of 8 subjects (p < 0.01). The observed changes are considered to reflect increases in mobilization of lead from the skeleton despite calcium supplementation. The identical isotopic ratios in maternal and cord blood provide further confirmation of placental transfer of lead. The extra flux released from bone during late pregnancy and postpartum varies from 50 to 380 microg lead (geometric mean, 145 microg lead) compared with 330 microg lead in the previous cohort. For subjects replete in calcium, the delay in increase in blood lead and halving of the extra flux released from bone during late pregnancy and postpartum may provide less lead exposure to the developing fetus and newly born infant. Nevertheless, as shown in several other studies on calcium relationships with bone turnover, calcium supplementation appears to provide limited benefit for lead toxicity during lactation.

Organochalcogens effects on delta-aminolevulinate dehydratase activity from human erythrocytic cells in vitro

Organochalcogens are important intermediates and useful reagents in organic synthesis, which can increase human exposure risk to these chemicals in the workplace. As well, there are a number of reported cases of acute toxicity following organochalcogen ingestion of vitamins and dietary supplements. Since, the erythrocytic delta-ALA-D activity could be an important indicator of toxicity this report investigated the organochalcogens effects on blood delta-ALA-D in vitro. To investigate a possible involvement of cysteinyl groups in the inhibitory actions of diphenyl diselenide, diphenyl ditelluride and Ebselen (4-100 micro M), the effects of thiol reducing agents (0-3 mM) or zinc chloride (0-2 mM) were examined. Diphenyl ditelluride, diphenyl diselenide and Ebselen inhibited in a concentration-dependent manner delta-ALA-D activity from human erythrocytes. Ebselen was lesser delta-ALA-D inhibitor than (PhSe)(2) and (PhTe)(2), whereas the diorganoyldichalcogenides displayed similar inhibitory potency towards delta-ALA-D. Dithiothreitol, a hydrophobic SH-reducing agent, was able to reactivate and to protect inhibited delta-ALA-D. The pre-incubation of blood with the inhibitors changed considerably the reversing potency of thiols. From these findings we suggest that organochalcogens inactivate in vitro human erythrocyte delta-ALA-D by an interaction with the sulfhydryl group essential of the enzyme activity.

2,3-Dimercaptopropane-1-sulfonic acid and meso-2,3-dimercaptosuccinic acid increase mercury- and cadmium-induced inhibition of delta-aminolevulinate dehydratase

Compounds derived from Dimercaprol, such as meso-2,3-dimercaptosuccinic acid (DMSA) and 2,3-dimercaptopropane-1-sulfonic acid (DMPS), are becoming common agents for treating humans exposed to heavy metals. Heavy metals such as Pb(2+), Hg(2+) and Cd(2+) can inhibit delta-aminolevulinate dehydratase (delta-ALA-D) activity. Delta-ALA-D catalyzes the condensation of two delta-aminolevulinic acid (delta-ALA) molecules with the formation of porphobilinogen, a heme precursor. The effects of DMSA and DMPS alone or in combination with Cd(2+), Hg(2+), or Pb(2+) on hepatic delta-ALA-D were examined. DMPS and DMSA caused a dose-dependent inhibition of hepatic delta-ALA-D. In the presence of Hg(2+) or Cd(2+) the inhibitory potency of DMPS increased. Similarly, the inhibitory effects of Hg(2+) and Cd(2+) were markedly increased in the presence of DMSA. In contrast, the inhibitory effect of DMPS was not changed by inclusion of Pb(2+). As observed with DMSA, Zn(2+) did not modified the inhibitory effect of DMPS. Data of the present report support the idea that the complexes formed (metals-DMSA or DMPS) were more inhibitory than the metal (Hg(2+) and Cd(2+)) or the chelating agent alone to the hepatic delta-ALA-D activity, in vitro. The mechanism of hepatic delta-ALA-D inhibition by Hg(2+)-DMPS/DMSA and Cd(2+)-DMPS/DMSA complexes involve the essential thiol groups of the enzyme.