PAF‐acetylhydrolase activity in familial hypercholesterolaemia

Sir, The letter by Dr C. Napoli points out differences between our recent article in the European Journal of Clinical Investigation [1] and their previous study [2] concerning platelet-activating factor acetylhydrolase (PAF-AH) activity in plasma and low-density lipoprotein (LDL) from patients with heterozygous and homozygous familial hypercholesterolaemia (FH). We have reported that the LDL subfractions of heterozygous FH patients carry significantly higher PAF-AH activity resulting in a higher plasma enzyme activity compared with normolipidaemic subjects. In accordance with our findings are the results reported recently by Guerra et al. [3]. Dr Napoli’s study reports that in homozygous FH LDL-associated PAF-AH activity is about three-fold lower than in normolipidaemic control subjects, whereas the enzyme activity in plasma is in the normal range. We would like to point out that the enzyme activity on LDL of their control group is very low compared with our control subjects [1,4] and that reported by other investigators [3,5]. They also suggest that the low PAF-AH activity on LDL of their homozygous FH patients could be due to the prolonged half-life of LDL in plasma because of the complete absence of the LDL receptor, leading to prolonged exposure of LDL to oxygen radicals, which favours the decrease in PAF-AH activity. Based on this hypothesis we would expect that the enzyme activity on LDL of heterozygous FH could be lower than in control subjects (and not higher as our results suggest) because there is also a prolonged half-life of LDL in plasma, although to a lesser extent. Dr Napoli’s letter has also a comment on the LDL preparation procedure. In our study, the LDL subfractions were prepared by a 4-h densitygradient ultracentrifugation procedure using a near-vertical rotor followed by a 10-h wash of the subfractions (at d 1⁄4 1·063 g mL) to remove albumin, which is always necessary when vertical rotors are used. Thus, the time taken for LDL subfraction preparation in our study is not much longer than that of Dr Napoli’s study (12 h). Furthermore, we would expect that if the LDL preparation time had any effect on PAF-AH activity, there would be a decrease in enzyme activity rather than an increase, as found in our study. In conclusion, we suggest that the difference in LDLassociated PAF-AH activity among heterozygous and homozygous FH patients observed in these studies could be due either to differences in the methodology used for PAF-AH determination or to other factors unrelated to LDL preparation or to LDL half-life in plasma. As the majority of the LDL-associated PAF-AH activity is distributed on the small dense LDL subfraction [4,6], we could suggest that a first approach to this issue could be the determination of the distribution of enzyme activity among the LDL subfractions, which are known to differ in their metabolism [7]. In addition, we should explore the possibility that the low PAF-AH activity on LDL of homozygous FH is unrelated to LDL metabolism but rather it is caused by disturbances of the enzyme secretion from macrophages or liver, the main cellular sources of the plasma form of PAF-AH [8,9].