FIG. 2. Translocation of 5-lipoxygenase. PL, phospholipids. of 5-lipoxygenase ATP and calcium-binding sites, if they indeed exist at all, remains to be elucidated. 5-Lipoxygenase, anionic at neutral pH, exhibits rather hydrophobic properties during chromatographic procedures, yet is overall hydrophilic based upon its predicted structure. One central portion of the protein encoded by exon 7 (amino acids 278-326) corresponds exactly to the most hydrophobic region and could represent an important structural domain or region associated with the catalytic site and 20: 4 binding (33, 36). 5-Lipoxygenases display extensive homologies to other mammalian and plant lipoxygenase proteins (34, 37). The human enzyme is nearly identical to the rat enzyme (93% identity) and shares 40% homology with the known plant lipoxygenases from soybean (38-40) and pea seeds (41). The latter homology is significantly higher in the carboxyl half of the proteins (60-65%). There is one remarkably conserved sequence where 12 or 13 amino acids are identical in all sequences (36). An additional sequence of 40 amino acids, which contains conserved histidine residues and acidic and basic residues, has been proposed as a likely region for binding of the nonheme iron atom in the soybean isozymes (38). This area also contains the sequence homologous to the interface binding domain mentioned above. 15-Lipoxygenase, whose sequence has recently been deduced (37), displays 39% sequence identity with human 5-lipoxygenase and 61% similarity. It now appears that the amino-terminal portions of the various lipoxygenases are quite divergent in structure and the middle and carboxyl-terminal regions are quite related. It remains to be determined, though, how the sequence similarities and differences affect the specificity of molecular oxygen insertion during catalysis.

5-Lipoxygenase mRNA has been detected in human leukocytes, lung, and placenta, and in several rat tissues (33, 35). Expression of 5-lipoxygenase has been normally thought to reside in cells of myeloid lineage. Whether tissue parenchymal cells or tissue-infiltrated macrophage/mast cells contain 5-lipoxygenase mRNA is a question that will soon be answered by in situ hybridization and immunohistochemical studies. The cDNA for 5-lipoxygenase has recently been expressed in two different systems:(i) mammalian osteosarcoma cells (42); and (ii) a baculovirus/insect cell system (43). In the former system, a human 5-lipoxygenase cDNA has been cloned into the mammalian expression vector pR135 and transfected into the osteosarcoma cell line 143.98. 2. Under control of the cytomegalovirus immediate early promoter, hygromycin-resistant colonies were screened for 5-lipoxygenase gene expression. Clones were isolated which expressed 5-lipoxygenase activity, and the enzyme activity was recovered in the 10,000 X g supernatants. The recombinant protein in terms of its subcellular localization, molecular weight, antigenicity, and requirement for stimulatory factors appeared to be identical to human peripheral blood leukocyte 5-lipoxygenase.