Activities of choline acetyltransferase, acetylcholinesterase, glutamate decarboxylase, 4-aminobutyrate aminotransferase and carnitine acetyltransferase in nervous tissue from some vertebrates and invertebrates

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1. Radiochemical methods are described for the assay of the following enzymes: choline acetyltransferase, AchE‡, glutamate decarboxylase, GABA-aminotransferase and carnitine acetyltransferase. 2. The activities of the above enzymes have been measured in nervous tissues from a range of different animals. The largest variation in enzyme activities in the animals studied occurs with choline acetyltransferase (approx 1000-fold). Variation in the other enzymes is < 10-fold (except for glutamate decarboxylase where it is about 20-fold). In any given species, the activity of AchE is greater than that of choline acetyltransferase (except for cerebral ganglion from the locust) and the activity of GABA-aminotransferase is greater than that of glutamate decarboxylase. 3. Comparison of maximal activities of choline acetyltransferase with the estimated maximal flux through the TCA cycle indicates that, in vertebrate nervous tissue, maximal rates of Ach synthesis may use negligible amounts of acetyl-CoA in comparison to the cycle. However, in the invertebrates studied, the rates of Ach synthesis may, in the absence of other regulatory mechanisms, compete seriously with the cycle for acetyl-CoA. A similar comparison for the use of intermediates for the GABA “shunt” (i.e. comparison of glutamate decarboxylase activities with rates of cycle) indicates that the shunt may not interfere seriously with the rate of the cycle. 4. Activities of carnitine acetyltransferase were demonstrated in all the nervous tissues investigated and in any given species they are greater than the activities of choline acetyltransferase (except for the locust). This is consistent with suggestions that acetylcarnitine may be involved in transport of acetyl units across the mitochondrial membrane in this tissue.