Z protein or fatty-acid-binding protein is abundant in the cytosol of many cell types including liver cells. It is considered to play an important role in intracellular transport and metabolism of long-chain fatty acids and other organic anions. We studied the role of Z protein in the metabolism of prostaglandin El (PGEJ. Binding of tritiated prostaglandin El to this fatty-acid-binding protein (Z protein) purified from rat liver was determined. Competition experiments show that the binding of this autacoid to Z protein is highly specific. It could not be displaced by other prostaglandins (PGA1, PGA2, PGE2, PGB1, PG12, PGD2, PGF2,, and 6-ketoPGF1,). Z protein might be involved in the metabolism of prostaglandins in the cytosol.Prostaglandin El (PGE1) regulates multiple properties of various cell types through the activation of the receptor-linked adenylate cyclase [l -41. However, the physiological action of prostaglandin El in isolated hepatocytes is still controversial [S]. Variation in prostaglandin El effects on isolated hepatocytes may in part be due to rapid uptake and metabolism of prostanoids. Kinetic studies of prostaglandin El metabolism in hepatocytes show rapid uptake of prostaglandin El and its subsequent conversion to a more polar compound 161. Regulation of hepatic metabolism by prostaglandin El is not fully understood since its specific receptor in these cells is not well established [7] in contrast to other cell systems [8 -113. Modulation of hepatic metabolism by prostaglandin El may be the result of the compromising effect of prostaglandin El binding as well as its rapid catabolism by hepatocytes. In many tissues including liver a cytosolic protein, whch is known as Z protein or fatty-acid-binding protein, is implicated in the fatty acid utilization and metabolism [12-151. It has been proposed that Z protein is capable of transporting the water-insoluble fatty acid through cytosol, controlling metabolism of fatty acids by enhancing p oxidation in rat liver and heart [16, 171; p oxidation is also a major metabolic route for prostaglandins in rat liver, lung and kidney [18, 191. No role for Z protein has been established in the metabolism of prostaglandins in different cell types.Here we report that the purified Z protein from the rat liver, which binds fatty acids and other organic anions, also binds prostaglandin El. Prostaglandin El binding to Z protein is rapid, saturable and highly specific. Our studies suggest that Correspondence to A. K.

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