The distribution of bound acetylcholine and choline acetylase in rat and pigeon brain

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Abstract
Homogenates of rat and pigeon brain in 0. 25 M sucrose solution have been fractionated by differential centrifuging and the distribution of bound acetylcholine and choline acetylase in the fractions has been determined. The mitochondrial fractions from rat and pigeon brain contained respectively 70% and 54% of the total bound acetylcholine of the whole homogenate (on a dry-weight basis these figures correspond to 18 and 33 [mu]/g). Choline acetylase activity was detected in all fractions of the homogenate except the supernatant fractions. Activities of the enzymes (per g dry weight) in the mitochondrial and microsomal fractions were higher than the activity of the respective homogenates. For rat- and pigeon-brain mitochondrial fractions the respective rates were 27 [mu]g and 76 [mu]/g dry weight/hour at 38[degree], equivalent to 3 and 7 times those in the whole homogenates. On incubating the brain-mitochondrial suspension with choline no synthesis of acetylcholine occurred but between 64% (pigeon brain) and 46% (rat brain) of the bound acetylcholine was released into the surrounding medium. On incubation of pigeon-brain particles with choline plus acetate a synthesis of acetylcholine took place and the initial level of bound acetylcholine was maintained. The rate of acetylcholine synthesis, the capacity of the particles for binding acetylcholine and the rate of release of the bound ester were all greater in pigeon brain than in rat brain.