IDENTIFICATION AND CHARACTERIZATION OF A NOVEL MICROSOMAL-ENZYME WITHGLUTATHIONE-DEPENDENT TRANSFERASE AND PEROXIDASE-ACTIVITIES

5-Lipoxygenase activating protein (FLAP), leukotriene-C-4 (LTC4) synth ase, and microsomal glutathione S-transferase II (microsomal GST-II) a re all members of a common gene family that may also include microsoma l GST-I; The present work describes the identification and characteriz ation of a novel member of this family termed microsomal glutathione S -transferase III (microsomal GST-III). The open reading frame encodes a 16.5-kDa protein with a calculated pI of 10.2. Microsomal GST-III ha s 36, 27, 22, and 20% amino acid identity to microsomal GST-II, LTC, s ynthase, microsomal GST-I, and FLAP, respectively. Microsomal GST-III also has a similar hydrophobicity pattern to FLAP, LTC, synthase, and microsomal GST-I. Fluorescent in situ hybridization mapped microsomal GST-III to chromosomal localization 1q23. Like microsomal GST-II, micr osomal GST-III has a wide tissue distribution (at the mRNA level) and is predominantly expressed in human heart, skeletal muscle, and adrena l cortex, and it is also found in brain, placenta, liver, and kidney t issues. Expression of microsomal GST-III mRNA was also detected in sev eral glandular tissues such as pancreas, thyroid, testis, and ovary. I n contrast, microsomal GST-III mRNA expression-was very low (if any) i n lung, thymus, and peripheral blood leukocytes. Microsomal GST-III pr otein was expressed in a baculovirus insect cell system, and microsome s from Sf9 cells containing either microsomal GST-II or microsomal GST -III were both found to possess glutathione-dependent peroxidase activ ity as shown by their ability to reduce 5-HPETE to 5-HETE in the prese nce of reduced glutathione. The apparent K-m of 5-HPETE was determined to be approximately 7 mu M for microsomal GST-LI and 21 mu M for micr osomal GST-III. Microsomal GST-III was also found to catalyze the prod uction of LTC4 from LTA(4) and reduced glutathione. Based on these cat alytic activities it is proposed that this novel membrane protein is a member of the microsomal glutathione S-transferase super family, whic h also includes microsomal GST-I, LTC4 synthase, FLAP, and microsomal GST-II.