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Platelet-activating factor (PAF) is a product of the embryo and the endometrium in early pregnancy. The actions of PAF may be regulated by its degradation and this is largely achieved by the enzyme PAF acetylhydrolase (PAF:ah; EC 3.1.1.47). The present study characterized the PAF:ah in the endometrium and uterine fluid of mice during early pregnancy. The enzyme activity from uterine endometrium and luminal fluids had the same biochemical characteristics as the plasma form of the enzyme. The three sources of enzyme activity (i) had an apparent native molecular mass greater than 106 Da, but this was reduced after detergent treatment and purification to 60–65 kDa; (ii) bound to cholesterol hemisuccinate agarose matrix; and (iii) were found in the high density lipoprotein-enriched fraction after density gradient ultracentrifugation. In castrate females, oestradiol-17β (E2) caused a dose-dependent increase in the activity of the enzyme in endometrium and luminal fluid. Progesterone (P4) inhibited the E2-induced increase in PAF:ah in uterine tissue. Treatment with E2 alone caused an increase in endometrial PAF:ah activity within 24 h, which declined within 48 h. In luminal fluid, the same treatment caused increased activity within 24 h, peaking after 48 h of treatment and then declining. In E2-treated castrate females, mRNA for an intracellular (but not plasma) form of PAF:ah was detected, yet the intracellular form was not detected biochemically. The results suggest that most of the enzyme activity was not produced locally, but probably resulted from the influx of the plasma form of the enzyme. Extra keywords: enzyme, oestradiol 17-β, progesterone.

Summary. A Brahman sire (Select Brahman) whose steer progeny had both high marbling scores and high intramuscular fat content has been identified. The effect was independent of dam breed (5 different dam breeds were analysed), was present in both grain-finished and pasture-finished progeny, and was independent of hormone growth promotant treatment. From pasture, over twice as many progeny from the Select sire (62.5%) had high marbling scores as progeny from 10 other Brahman sires (25.9%). The effect was not as marked in grain-finished steers but the Select sire still produced one-third more high marbling progeny (84.6%) than the other sires (63.6%). The Select sire’s progeny were equal to or better than progeny from Tuli sires in producing high marbling scores. The intramuscular fat content (3.5 ± 0.3 g/100 g) of the progeny from the Select sire was greater than double that of progeny from other Brahman sires (1.9 ± 0.2 g/100 g; P<0.0001) and higher than progeny from Tuli sires (2.3 ± 0.1 g/100 g; P<0.0001). The increased marbling and intramuscular fat content were not associated with increased subcutaneous fat deposition, decreased muscle deposition, lower growth rate or smaller mature size since there were no differences in age, hot standard carcass weight, rump fat depth and rib eye muscle area between the Select sire’s progeny and those of other Brahman sires. The identification of a high marbling trait in a Bos indicus sire is of considerable significance to the northern Australian beef industry because it enables markets requiring marbled carcasses to be targeted while maintaining the advantages of tropical adaptation.