1. The effect of age on the plasma level of the lyso-derivative of platelet-activating factor (lyso-PAF) was studied in 72 normal subjects, (32 females, 40 males) aged 12–64 years. Lyso-PAF was acetylated in vitro to PAF which was measured by bioassay using 5-[14C]hydroxytryptamine-labelled rabbit platelets.

2. Under 40 years there were similar direct relations between plasma lyso-PAF and age in both sexes (linear regression; males, P < 0.001; females, P < 0.002), the level approximately doubling from the adolescent level around 100 ng/ml. However, in the later years the levels fell, the fall seeming to commence earlier in females, so that between 40 and 65 years the level was greater in males [169 ± 47(sd) vs 120 ± 30 ng/ml; P < 0.01].

3. The increase in plasma lyso-PAF up to middle-age may be related to the reported increase in prostanoid production with age, since these platelet and vasoactive compounds can have a common origin in membrane phospholipid. This would be consistent with increasing phospholipase A2 activity and decreasing stability of cell membranes with age, but the later fall in lyso-PAF is then unexplained; the lesser values in females than males in the more advanced years could be related to females' generally lesser vascular disease. However, knowledge of the biological roles and metabolism of PAF is still very limited and the real significance of the findings remains to be determined.

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