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Homocholine and Short‐Chain N ‐Alkyl Choline Analogues as Substrates for Torpedo Choline Acetyltransferase
Author(s) -
Luqmani Y. A.,
Richardson P. J.
Publication year - 1982
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1982.tb08638.x
Subject(s) - choline , choline acetyltransferase , torpedo , substrate (aquarium) , chemistry , enzyme , acetylation , choline chloride , sodium , membrane , biochemistry , chromatography , salt (chemistry) , stereochemistry , acetylcholine , biology , organic chemistry , endocrinology , ecology , acetylcholine receptor , receptor , gene
The kinetic parameters, K m and V max , for the acetylation of choline and several close analogues were determined by using (a) purified choline acetyltransferase and (b) a hypotonically lysed synaptosomal extract prepared from the electric organ of Torpedo marmorata. Whereas the K m for choline was similar in both cases (0.51 and 0.42 m m ), the crude enzyme showed a three‐ to fivefold greater affinity for its analogues than the purified enzyme, the activity decreasing rapidly with increased N ‐alkyl substitution. Homocholine was a poor substrate, but was clearly acetylated by both preparations. The effect of salt on analogue acetylation by the crude enzyme was studied by increasing NaCl concentration from zero to 150 m m . There was an increase in both K m and V max for all substrates; choline, N,N,N ‐dimethylmonoethylaminoethanol, ‐monomethyldiethylaminoethanol and ‐dimethylmonobutylaminoethanol showed the greatest changes, whilst N,N,N ‐triethylaminoethanol and ‐dimethylmonopropylaminoethanol and homocholine were much less affected. However, in all cases, the kinetic parameter V max / K m remained unchanged. The maximal velocities of the different substrates varied more under conditions of high than of low salt. Sodium chloride up to 300 m m had no effect on the amount of enzyme which was bound to membranes in the synaptosomal extract. It is concluded that choline acetyltransferase has a high degree of substrate specificity, which is slightly altered by purification. The effects of salt cannot be explained as a consequence of nonspecific ionic association with membranes.