Transverse distribution and movement of lysophosphatidylcholine in sarcoplasmic reticulum membranes as determined by 13C NMR and lysophospholipase.

1. The transverse distribution of 1-palmitoyl-sn-glycero-3-phospho-N-[Me-13C]-choline in vitro incorporated in sarcoplasmic reticulum has been measured by means of 13C NMR and DyCl3 as an impermeable shift reagent. 2. Lysophosphatidylcholine added to the membranes equilibrates within 30 min at 20 degrees C between outer and inner membrane leaflet so that 42% is located in the inner leaflet. 3. Lysophosphatidylcholine diffuses back from the inner leaflet to the outer upon lysophospholipase action on the outer lysophosphatidylcholine pool.