Acetylation of Choline and Homocholine by Membrane‐Bound Choline‐O‐Acetyltransferase in Mouse Forebrain Nerve Endings

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Abstract
The choline analog homocholine is not acetylated in vitro by choline‐O‐acetyltransferase (ChAT, EC 2.3.1.6), which is solubilized by 100 mM‐sodium phosphate buffer washes of a crude vesicular fraction of mouse forebrain. However, both homocholine and choline are acetylated by a form of ChAT which is nonionically associated with a subcellular fraction of mouse forebrain containing membrane‐associated organelles and occluded acetylcho‐line (P4). Acetylation of homocholine by membrane‐associated ChAT is saturable. 4‐(1‐Naphthylvinyl)pyridine (NVP) inhibits the acetylation of both choline (60%) and homocholine (40%) by membrane‐associated ChAT but reduces the acetylation of choline alone by soluble ChAT (76%). Choline and homocholine serve as competitive alternative substrates for the same membrane‐associated ChAT, whereas homocholine acts only as a competitive inhibitor of choline acetylation by soluble ChAT. Acetylhomocholine competitively inhibits the acetylation of choline by both soluble and membrane‐associated ChAT more dramatically than does the natural end product, acetylcholine.