Platelet-activating factor acetylhydrolase activity in maternal, fetal, and newborn rabbit plasma during pregnancy and lactation.

The inactivation of the biologically active ether-containing glycerophospholipid platelet-activating factor (PAF) is catalyzed by the enzyme PAF acetylhydrolase (1-alkyl-2-acetyl-sn-glycero-3-phosphocholine acetylhydrolase, EC 3.1.1.48). The specific activity of acetylhydrolase has been assayed in maternal rabbit plasma prior to and throughout pregnancy and after parturition. The specific activity of acetylhydrolase was 131 +/- 8 nmol.min-1.ml-1 of plasma (mean +/- SEM) in the nonpregnant rabbit. Similar specific activities were found throughout the first half of pregnancy; however, on day 15 the acetylhydrolase activity began to decrease and reached a minimum around day 27. Within 24-48 hr following delivery, the specific activity of the enzyme increased to the levels found in the nonpregnant animals. The specific activity of acetylhydrolase in fetuses (gestational ages of 21-30 days) and neonates increased from 22 nmol.min-1.ml-1 of plasma (21-day-old) to 328 nmol.min-1.ml-1 of plasma (35-day-old rabbits). The decrease in enzymatic activity of the pregnant rabbit plasma cannot be accounted for by the presence of an inhibitor as determined by plasma-mixing experiments with nonpregnant and pregnant (27-day) plasma. The properties of this enzyme in the rabbit were consistent with those reported by others, i.e., substrate specificity, inactivation by heat and various inhibitors, and Ca2+ independency. The activity of PAF acetylhydrolase was associated primarily with the high density lipoprotein fraction in rabbit plasma. The decrease of this enzymatic activity during pregnancy may serve to remove the protective effect on myometrium to allow an increased amount of PAF to come in contact with this tissue.