Cytosolic phospholipase A2α (cPLA2α) hydrolyzes arachidonic acid from cellular membrane phospholipids, thereby providing enzymatic substrates for the synthesis of eicosanoids, such as prostaglandins and leukotrienes. Considerable understanding of cPLA2α function has been derived from investigations of the enzyme and from cPLA2α-null mice, but knowledge of discrete roles for this enzyme in humans is limited. We investigated a patient hypothesized to have an inherited prostanoid biosynthesis deficiency due to his multiple, complicated small intestinal ulcers despite no use of cyclooxygenase inhibitors. Levels of thromboxane B2 and 12-hydroxyeicosatetraenoic acid produced by platelets and leukotriene B4 released from calcium ionophore–activated blood were markedly reduced, indicating defective enzymatic release of the arachidonic acid substrate for the corresponding cyclooxygenase and lipoxygenases. Platelet aggregation and degranulation induced by adenosine diphosphate or collagen were diminished but were normal in response to arachidonic acid. Two heterozygous single base pair mutations and a known SNP were found in the coding regions of the patient’s cPLA2α genes (p.[Ser111Pro]+[Arg485His; Lys651Arg]). The total PLA2 activity in sonicated platelets was diminished, and the urinary metabolites of prostacyclin, prostaglandin E2, prostaglandin D2, and thromboxane A2 were also reduced. These findings characterize what we believe is a novel inherited deficiency of cPLA2.
David H. Adler, Joy D. Cogan, John A. Phillips III, Nathalie Schnetz-Boutaud, Ginger L. Milne, Tina Iverson, Jeffrey A. Stein, David A. Brenner, Jason D. Morrow, Olivier Boutaud, John A. Oates
Effects of proline substitution at residue 111 of cPLA2α.
Comparison of the wild-type cPLA2α structure (left) with a model of the p.[S111P] mutation (right). Backbone carbon atoms are shown in gray, oxygen atoms and hydroxy groups in red, nitrogen atoms in blue, and side-chain carbon atoms of the 111 position in green. Hydrogen-bonding interactions are shown in dashed black lines and sterically unfavorable close distances are shown in solid red lines. A hydrogen-bonding interaction has an ideal distance of 2.7–3.0 υ between oxygen and nitrogen-hydrogen–bond donors and acceptors and an ideal distance of 3.1–3.2 υ for sulfur to nitrogen-hydrogen–bond donors and acceptors. Nonbonded distances smaller than 2.5 υ between nitrogen and oxygen-hydrogen–bonding donors and acceptors are sterically and energetically unfavorable and are generally not observed in nature. Distances longer than 3.5 υ do not contribute favorable energy toward folding stabilization. Modeling of proline in position 111 reveals that 1 ideal hydrogen-bonding interaction of 2.7 υ between the p.[S111] side-chain hydroxyl moiety and the backbone carbonyl group of p.[T108] has been replaced by 3 sterically unfavorable interactions (<2.5 υ). Two of these unfavorable interactions lie between the hydrophobic proline Cγ and Cδ atoms and the p.[T108] carbonyl, while 1 unfavorable interaction is between the proline Cδ and the amide nitrogen of p.[S111]. The p.[S111P] mutation is predicted to cause unfavorable interactions (red lines) in the absence of a structural rearrangement of the protein. The predicted decrease in stability of the adjacent β-strands likely affects the entire Ca2+-binding domain.