Activity of ChAT

Brain ChAT has a KD for choline of approximately 1 mmol/1 and for acetyl coenzyme A (CoA) of approximately 10pmol/l. The activity of the isolated enzyme, assayed in the presence of optimal concentrations of cofactors and substrates, appears far greater than the rate at which choline is converted to ACh in vivo. This suggests that the activity of ChAT is repressed in vivo. Surprisingly, inhibitors of ChAT do not decrease ACh synthesis when used in vivo this may reflect a failure to achieve a sufficient local concentration of inhibitor, but also suggests that this step is not rate-limiting in the synthesis of ACh [18-20]. 

Table 2, Effect of thyroid hormone (T4) and NGF on the activities of ChAT, GAD, glutaminase, GS and CNPase, as markers of neural cell types, in the basal forebrain of 10-day-old normal and hypothyroid (Tx) rats ... 

Young rats made hypothyroid at birth are intraventricularly given the minimum amount of NGF that produces a significant increase in basal forebrain ChAT activity in normal animals.7,36 in contrast to normal young, treatment with low doses of NGF has no significant effect on the activity of ChAT in thyroid deficient rats (Table 2).7,32 However, when hypothyroid... 

The reaction of choline with mitochondrial bound acetylcoenzyme A is catalysed by the cytoplasmic enzyme choline acetyltransferase (ChAT) (see Fig. 6.1). ChAT itelf is synthesised in the rough endoplasmic reticulum of the cell body and transported to the axon terminal. Although the precise location of the synthesis of ACh is uncertain most of that formed is stored in vesicles. It appears that while ChAT is not saturated with either acetyl-CoA or choline its synthesising activity is limited by the actual availability of choline, i.e. its uptake into the nerve terminal. No inhibitors of ChAT itself have been developed but the rate of synthesis of ACh can, however, be inhibited by drugs like hemicholinium or triethylcholine, which compete for choline uptake into the nerve. 

Acetylcholine synthesis and neurotransmission requires normal functioning of two active transport mechanisms. Choline acetyltransferase (ChAT) is the enzyme responsible for ACh synthesis from the precursor molecules acetyl coenzyme A and choline. ChAT is the neurochemical phenotype used to define cholinergic neurons although ChAT is present in cell bodies, it is concentrated in cholinergic terminals. The ability of ChAT to produce ACh is critically dependent on an adequate level of choline. Cholinergic neurons possess a high-affinity choline uptake mechanism referred to as the choline transporter (ChT in Fig. 5.1). The choline transporter can be blocked by the molecule hemicholinium-3. Blockade of the choline transporter by hemicholinium-3 decreases ACh release,... 

Yamamura and Snyder (195) found that Interruption of the septal hippocampal tract, and elimination thereby of cholinergic afferents to the hippocampus, reduced by about 70X the activity of choline acetylase In homogenates of the hippocampus, but did not alter the binding of [ H]3-qulnuclldli l benzllate by the particles of such homogenates. They proposed, as a possible explanation, chat... 

Shltov (215) found chat benactyzlne and two other anticholinergic compounds Increased the action of magnesium sulfate on the EEC of the rabble In proportion to the activity of the compounds In activating cholinesterase. 

Fig. 84. Illustrations of choline-acetyltransferase (ChAT)-like immunoreactivity in the rabbit cerebellum. A. Sagittal view of the rabbit cerebellum delineating the lobules according to Larsell (Larsell, 1970). Mean measurements of ChAT activity are indicated by numbers in parentheses. B. Magnified view of the ventral vermis. The vermis contains areas of ChAT-positive mossy fiber terminals (indicated by dots). These areas in lobules 1 and 9d are illustrated in C and D, respectively. E. View of the right paraflocculus of the rabbit. ChAT-like immunoreactivity and ChAT activity was highest in the ventral paraflocculus, particularly lobule 2. The numbers in parentheses are mean measurements of ChAT activity, expressed as mmol of Ach synthe-sized/hr. g tissue at 37°C, for each cerebellar lobule in six rabbits. Barmack et al. (1992a).

Cerebellar Golgi cells display strong AChE activity (Shute and Lewis, 1965 Brown and Palay, 1972 and others), but there has been no evidence of ChAT-immunoreactive Golgi cells in the rat (Ojima et al., 1989 Barmack et al., 1992a Jaarsma et al., 1995c). 

Cultures exposed at mo developmental stages for 10 days daily-in serum-free medium to chJorpyrifos, parathion, or their oxon derivatives (1 pJVf) on culture days 5-15 (immature) or 25-35 of culture (differentiated). Cells harvested immediately or after 11-day recovery. Activities of AChE, choline acetyl transferase (ChAT), glutamic acid dehydrogenase (GAD), glutamine synthetase, and CMP measured. OP effects on AChE activity compared to physostigminc. 

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