Comparative enzymatic acetylation of carnitine and choline by human placenta syncytiotrophoblast membrane vesicles

Microvillous membrane vesicle preparations from the maternal surface of human placental syncytiotrophoblast were examined for the presence of carnitine and choline acetyltransferase activity. Radiometric assay for acetylcholine employed butyronitrile-tetraphenylboron extraction of the quaternary ions. Acetylcarnitine was assayed by anion exchange chromatography. The data reveal that carnitine is the primary substrate for the vesicle acetyltransferase enzyme(s), whereas choline appears to be a minor substrate. For acetylcarnitine synthesis, the Km is 0.749 mM carnitine and Vmax is 641 pmol X mg protein-1 X minute-1, respectively; for acetylcholine synthesis, the Km is 0.5 mM choline and Vmax is 53 pmol X mg protein-1 X minute-1, respectively. Approximately ten times more acetylated product was formed with carnitine than with choline. The carnitine-mediated reaction obeyed Michaelis-Menten kinetics, whereas the choline reaction exhibited anomalous behavior. Vesicle preparations were stable for 21 days at -80 degrees C. Preliminary studies on hypotonically lysed vesicles demonstrate that the acetyltransferase is particulate and is bound to the membrane of the vesicle. These findings demonstrate that carnitine acetyltransferase activity is in the plasmalemma membrane of the syncytiotrophoblast and suggest a role for this enzyme, analogous to the mitochondrial fatty acid shuttle system, in the maternofetal translocation of fatty acyl residues.