Plasma ChE

Acetylcholinesterase (EC 3.1.1.7) (AChE) Acetylcholine acetylhydrolase True ChE ChE I ChE Acet-ylthiocholinesterase Acetylcholine hydrolase Acetyl (3-methylcholinesterase Erythrocyte ChE Butyrylcholinesterase (EC 3.1.1.8) (BChE or BuChE) ChE Pseudocholinesterase Plasma ChE Acylcholine acylhydrolase Non-specific ChE ChEII Benzoylcholinesterase Propionylcholinesterase... 

As an inhibitor of plasma (pseudo) cholinesterase, DFP produced only minimal reversal of scopolamine-induced incapacitation. Note the very low plasma ChE levels, with only minor decreases in RBC ChE (Fig. 71). DFP does improve near vision when applied to the eye (David Flarper, unpublished data) suggesting that paralysis of the muscles of visual accommodation is probably peripheral in origin. Persistence of pupillary enlargement (in the face of systemic treatment with physostigmine) may be due to physiological or pK factors, causing limited access to the iris. 

It should be recalled here that the alcoholic hydroxyl of serine does not possess a dissociation constant within the pH range, accessible to enzymic reactions. Therefore, this amino acid cannot influence the pH-activity curve. On the other hand, it is well known that DFP inhibition is initially reversible and becomes only slowly irreversible. This has been demonstrated for true ChE from electric eel by Nachmansohn and associates (46) and for plasma ChE by Mackworth and Webb (47). Similarly, a stepwise reaction with inhibitors, containing the diethyl phosphoryl moiety, has been made probable by Hobbiger (34)- Therefore, it appears possible that phosphates are first attacked by the imidazol moiety of the esteratic site, in conformity with the catalytic influence of free imidazol on phosphate hydrolysis (48). This step is followed by transfer to serine. The final product is a trialkyl phosphate XV, which is not split by imidazol (scheme F). 

Fig. 11. Inhibitory constants of tetra-alkylammonium salts, R4N, as function of n, the number of carbon atoms in the four substituents together (according to Table IV and V). O------O Eel esterase. ------ Human plasma ChE.

Different relationships are observed with plasma ChE. Here the ions... 

In one study (2), plasma and RBC ChE activities were followed from May to September in two mixer-loaders (ML) who used a Swampmate closed-transfer system (Cherlor Manufacturing Co., Salinas, California), and in three mixer-loader applicators (MLA) who used a Model SS 12-4 closed-transfer system (Soil Serv, Salinas, California). The results shown in Fig. 1 indicate that the activities of plasma and RBC ChE were depressed during the application season, but returned to normal by the middle of September. The MLA showed less plasma and RBC ChE depression. It is interesting to note that the plasma ChE showed the "rebound effect," recording levels way above baseline. 

Table VI., Analysis of RBC and Plasma ChE Values of Exposed Workers and Controls...

Plasma CHE was determined by a radiometric assay (9) using acetylcholine and Btf 284c51. Plasma creatine kinase fCK) was determined by a spectrophotometric method (10). 

DEE levels ranged from negligible to a total of more than 100 ug/cm (Table I). Plasma CHE decreased with increasing single exposures of DEF [except for the birds riding on the rig (RIG)] to approximately 50 the CHE of off-site birds. Both ROW and ADJ birds showed decreased CHE levels with repeated exposures. CHE activity took 2-3 weeks to recover. 

Figure 5. Plasma CHE in scaleless chickens after field applications of DEF (CHE in nmol/min/mL DEF in In ng/cm2). Single-day exposures of groups are in Table 1, omitting birds on the sprayer (RIG). Modified from Ref. 8.

The dose-response relationship between DEF fallout and plasma CHE levels was remarkably consistent, extending over 5 orders of magnitude. The lack of a large decrease of CHE in the birds that rode on the applicator may have been due to impeding the fallout of DEP in the cage by a thick rubberized screen with which we lined the cage to protect the birds (unnecessarily as it turned out) from jostling. 

ORNL noted that plasma-ChE values in male rats provided the least variable indicator of the lowest-observed-adverse-effect level (LOAEL) and NOAEL for GA and that there was evidence (based on mean plasma-ChE values) of a dose-response relationship. Therefore, ORNL used that data to determine the LOAEL and NOAEL for ChE inhibition by GA. ORNL considered 56.25 g/kg per day to be the LOAEL because of the significant reduction in plasma-ChE concentrations observed in male rats at this dose (relative to controls and baseline values). Because of the lack of consistent change in plasma- and RBC-AChE values (relative to controls and baseline values), ORNL considered the low dose of 28.13 A[Pg.43]

The NOAELadj (115 pg/kg per day) used by ORNL for derivation of the RfD for GA was based on the dose that did not cause a significant depression in plasma-ChE activity in rats (Bucci et al. 1992). The subcommittee notes that ChE inhibition is typically considered a biomarker of expo... 

Provided that appropriate assays were used, the subcommittee finds no reason at this time to alter the practice of using RBC-ChE or plasma-ChE inhibition as the critical end point and agrees with ORNL that such inhibition is the best available critical noncancer end point on which to base the calculation of the RfD for GB. 

ChE (relative to controls) at that dose was statistically significant and because the plasma-ChE activity during week 1 was reduced to 39% of baseline in males and 57% of baseline in females. 

The subcommittee believes that the strength of evidence for the Army s interim RfD of 4 x 10 mg/kg per day is moderately good. There is a possibility that the LOAEL (0.0175 mg/kg per day) used to calculate the RfD for GD is not accurate, because lower doses were not tested and variabihty in the plasma-ChE values was considerable. The subcommittee believes that because ChE inhibition is a biomarker of exposure rather than a toxic effect, use of this end point overestimates the oral toxicity of GD. 

The subcommittee considered other possible critical studies for the derivation of the RfD for VX. In a study by Goldman et al. (1988), VX was administered to Sprague-Dawley rats (25 males and 25 females per group) by subcutaneous (s.c.) injection at doses of 0, 0.25, 1.0, and 4.0 pg/kg for 5 days per week for up to 90 days. A dose-dependent decrease in RBC-AChE concentrations was observed in male and female rats compared with controls. Plasma ChE was significantly depressed at day 30 in both sexes administered VX at a dose of 1.0 pg/kg per day, and at days 30, 60, and 90 in both sexes at a dose of 4.0 pg/kg per day. The data from rats exposed for 30 days was reanalyzed by ORNL using analysis of variance and Dunnett s and Scheffe s comparisons. ORNL reported that RBC-AChE activity was significantly lower in both sexes in all dose groups. 

Appendix C. Statistical Analysis of Plasma-ChE Inhibition in Rats... 

In addition to being found in the nervous system, acetylcholinesterase also occurs in the blood where it is bound to the surface of red blood cells (termed RBC-ChE or RBC-AChE). RBC-AChE activity, as well as the activity of a second type of cholinesterase found in blood plasma (butyrylchoUnesterase, or plasma cholinesterase) have been used to monitor exposure to organophosphate compounds (pesticides and nerve agents). Both RBC-AChE and plasma-ChE activity have been used as bioindicators of potential toxic effects. There is some evidence that RBC-AChE is as sensitive as brain ChE to the effects of nerve agents. Grob and Harvey (1958) reported that the in vitro concentrations producing 50% depression of brain-ChE and RBC-AChE activity were the same in the case of GA (1.5 x 10 mol/L),... 

Changes in plasma-ChE activity in dosed and control animals are shown in Table 4. Over the course of the study, plasma-ChE activity levels in dosed and control animals appear to be more stable than RBC-AChE activity. It was reported that plasma-ChE activity was decreased by about 55% in dosed females at week 7, and by 37.5% in dosed males at week 3. Mean plasma-ChE activity in the female controls exhibited a slow increase over the 13-week test period (from 1743 lU/L at week -1 to 2891 lU/L at week 13). A similar response was seen in the two lowest dose groups of females. In males, mean plasma-ChE activity in controls was lower than preexposure levels (401 lU/L at week -1) at all weeks except week 3 (413 lU/L). In the dosed groups of males, mean plasma-ChE levels were lower than pre-exposure values at all sampling times. Statistical analysis of the plasma-ChE activity indicated that mean values were significantly lower than controls in the mid- and high-dose females at weeks -1, 1, 3, and 7 but not at week 13, and in the high-dose males at weeks 3 and 7. 

The plasma-ChE data were re-analyzed by ORNL (using standard deviations) with ANOVA, and... 

Table 4 Plasma-ChE levels in 90-day subchronic rat study using agent GAa Week of treatment...

The use of a rat study for developing an RfD for GA is complicated by the fact that rodents have a much lower RBC-AChE activity level compared to humans (Ellin, 1981). By itself, this could cause rats to be relatively more sensitive than humans to anticholinesterase compounds however, the lower RBC-AChE activity may be offset by the presence of ahesterases in the blood of rats. Aliesterases, which are not found in human blood plasma, are known to bind to and, therefore, reduce the toxicity of GB, and a similar mechanism may operate in the case of GA. Other species differences, such as in the rates of aging of the GA-ChE complex, in the rates of synthesis of plasma-ChE in the liver, and in the levels of AChE in the nervous system (see Ivanov et al., 1993) may also result in difference between species in sensitivity to GA. Data are insufficient to more fuUy evaluate these possibihties. There is httle human acute toxicity data that can be compared with the available rat data however, acute toxicity data for primates in general (see Table 2) suggests that humans are likely to be more sensitive than rats. Therefore, for the purpose of this assessment, the standard EPA method will be followed which assumes that humans can be as much as ten times more sensitive to a chemical than laboratory animals. 

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