IDENTIFICATION AND CHARACTERIZATION OF A NOVEL HUMAN MICROSOMAL GLUTATHIONE-S-TRANSFERASE WITH LEUKOTRIENE C-4 SYNTHASE ACTIVITY AND SIGNIFICANT SEQUENCE IDENTITY TO 5-LIPOXYGENASE-ACTIVATING PROTEIN AND LEUKOTRIENE C-4 SYNTHASE

5-Lipoxygenase-activating protein (FLAP) and leukotriene C-4 (LTC(4)) synthase, two proteins involved in leukotriene biosynthesis, have been demonstrated to be 31% identical at the amino acid level. We have rec ently identified and characterized a novel member of the FLAP/LTC(4) s ynthase gene family termed microsomal glutathione S-transferase II (mi crosomal GST-II). The open reading frame encodes a 16.6-kDa protein wi th a calculated pi of 10.4, Microsomal GST-II has 33% amino acid ident ity to FLAP, 44% amino acid identity to LTC(4) synthase, and 11% amino acid identity to the previously characterized human microsomal GST (m icrosomal GST-I). Microsomal GST-II also has a similar hydrophobicity pattern to FLAP, LTC(4) synthase, and microsomal GST-I. Fluorescent in situ hybridization mapped microsomal GST-II to chromosomal localizati on 4q28-31. Microsomal GST-II has a wide tissue distribution (at the m RNA level) and was specifically expressed in human liver, spleen, skel etal muscle, heart, adrenals, pancreas, prostate, testis, fetal liver, and fetal spleen. In contrast, microsomal GST-II mRNA expression was very low (when present) in lung, brain, placenta, and bone marrow. Thi s differs from FLAP mRNA, which was detected in lung, various organs o f the immune system, and peripheral blood leukocytes, and LTC(4) synth ase mRNA, which could not be detected in any tissues by Northern blot analysis. Microsomal GST-II and LTC(4) synthase were expressed in a ba culovirus insect cell system, and microsomes from Sf9 cells containing microsomal GST-II or LTC(4) synthase were both found to catalyze the production of LTC(4) from LTA(4) and reduced glutathione. Microsomal G ST-II also catalyzed the formation of another product, displaying a co njugated triene UV absorption spectra with a maximum at 283 nm, sugges ting less catalytic stereospecificity compared with LTC(4) synthase. A lso, the apparent K-m for LTA(4) was higher for microsomal GST-II (41 mu M) than LTC(4) synthase (7 mu M). In addition, unlike LTC(4) syntha se, microsomal GST-II was able to catalyze the conjugation of 1-chloro -2,4-dinitrobenzene with reduced glutathione. Therefore, it is propose d that this novel membrane protein is a member of the microsomal gluta thione S-transferase family, also including LTC(4) synthase, with sign ificant sequence identities to both LTC(4) synthase and FLAP.