A cDNA clone of prostaglandin F synthase (PGFS) was isolated from human lung by using cDNA of bovine fung-type PGFS as a probe and its protein expressed in Escherichia coli was purified to apparent homogeneity, The human PGFS catalyzed the reduction of prostaglandin (PG) D-2, PGH(2) and phenanthrenequinone (PQ), and the oxidation of 9 alpha,11 beta-PGF(2) to PGD(2), The k(cat)/K-m values for PGD(2) and 9 alpha,11 beta-PGF(2) were 21000 and 1800 min(-1) mM(-1), respectively, indicating that the catalytic efficiency for PGD(2) and 9 alpha,11 beta-PGF(2) was the highest among the various substrates, except for PQ, The PGFS activity in the cytosol of human lung was completely absorbed with antihuman PGFS antiserum. Moreover, mRNA of PGFS was expressed in peripheral blood lymphocytes and the expression in lymphocytes was markedly suppressed by the T cell mitogen concanavalin A. These results support the notion that human PGFS plays an important role in the pathogenesis of allergic diseases such as asthma, (C) 1999 Federation of European Biochemical Societies.