LTC4 synthase (LTC4S), the pivotal enzyme for the biosynthesis of LTC4 (ref. 10), is an 18-kDa integral nuclear membrane protein that belongs to a superfamily of membrane-associated proteins in << eicosanoid >> and glutathione metabolism that includes 5-lipoxygenase-activating protein, [[ microsomal glutathione S-transferases ]] (MGSTs), and microsomal prostaglandin E synthase 1 (ref. 13).