Figure 28.6 Arachidonate is the major precursor of eicosanoid hormones. Prostaglandin synthase catalyzes the first step in a pathway leading to prostaglandins, prostacyclins, and thromboxanes. Lipoxygenases catalyze the initial step in a pathway leading to leukotrienes. Abbreviations: PLA2, phospholipase A2; DG, diacylglycerol.