Table 1.

The phospholipid composition of the human erythrocyte membrane

Phospholipid	% of whole membrane, ±SD	Outer leaflet, %	Inner leaflet, %
PC	29.3  ±  1.5	44.8	14.0
SM	25.5  ±  1.4	42.1	9.1
LPC	1.0  ±  0.8	2.0	—
CPs	55.8  ±  2.2 (55.6)	88.9 (88.9)	23.1 (22.2)
PE	27.6  ±  1.5 (27.8)	11.1 (11.1)	43.9 (44.4)
PS	14.9  ±  1.7	—	29.6
PI	0.6  ±  0.5	—	1.2
PA	1.1  ±  0.5	—	2.2
APs	16.6  ±  1.8 (16.7)	0.0 (0.0)	32.9 (33.3)

The overall composition was taken from ref. 13. Very similar values have been obtained by many independent investigators (cf. ref. 14). The compositions of the individual leaflets were calculated based on the overall composition and transbilayer distribution data (15–17). The values were corrected assuming 100% recovery (determined recovery was 99.6%). The relative amounts of phospholipid in the outer and inner leaflets were calculated to be 49.7 and 50.3 mol %, respectively. Lysophosphatidylcholine was assumed to localize exclusively to the outer leaflet, because it can be rapidly depleted upon incubation with lipoprotein acceptors (18), and second, the cytoplasm of erythrocytes contains a lysophospholipase (19) and a reacylating enzyme (20), which are expected to degrade or acylate, respectively, any lysolipids entering the inner leaflet. The model-predicted values closest to the experimental ones are shown in parentheses. SD is for a group of 10 individuals.