mammalian cells are unable to synthesize choline "de novo" and therefore must rely on exogenous sources from the diet. the uptake of choline is accomplished predominantly by the high-affinity, sodium dependent choline transporter (cht) and requires atp as an energy source. on the other hand, choline may enter the cell through the activation of low-affinity, sodium-independent organic cation transport proteins (octs) and/or carnitine/organic cation transporters (octns), and do not require atp. lastly, choline may enter the cell through intermediate-affinity transporters, which include the choline transporter-like protein 1 (ctl1).