title: Albumin modifies the metabolism of hydroxyeicosatetraenoic acids via 12-lipoxygenase in human platelets.
abstract: 12-Lipoxygenase and cyclooxygenase 1 are the dominating enzymes that metabolize arachidonic acid in human platelets. In addition to the conversion of arachidonic acid to 12(S)-hydroxyeicosatetraenoic acid, 12-lipoxygenase can also utilize 5(S)-hydroxyeicosatetraenoic acid and 15(S)-hydroxyeicosatetraenoic acid to form 5(S), 12(S)-dihydroxyeicosatetraenoic acid and 14(R), 15(S)-dihydroxyeicosatetraenoic acid, respectively. Furthermore, 15(S)-hydroxyeicosatetraenoic acid works as an inhibitor for 12-lipoxygenase. In the present paper we have studied the influence of albumin on the in vitro metabolism of 5 - and 15 -hydroxyeicosatetraenoic acids, and 5,15 -dihydroxyeicosatetraenoic acid by the platelet 12-lipoxygenase. The presence of albumin reduced the formation of 5(S),12(S)- dihydroxyeicosatetraenoic acid from 5(S)-hydroxyeicosatetraenoic acid, however, it had no effect on the 12(S)-hydroxyeicosatetraenoic acid production from endogenous arachidonic acid. In contrast, when 15(S)-hydroxyeicosatetraenoic acid was incubated with activated platelets, the formation of 14(R), 15(S)- dihydroxyeicosatetraenoic acid was stimulated by the presence of albumin. Furthermore, albumin reduced the inhibitory action 15(S)-hydroxyeicosatetraenoic acid had on 12(S)-hydroxyeicosatetraenoic acid formation from endogenous arachidonic acid. However, addition of exogenous arachidonic acid (20 microm) to the incubations inverted the effects of albumin on the conversion of 15(S)-hydroxyeicosatetraenoic acid to 14(R),15(S)- dihydroxyeicosatetraenoic acid and the production of 12(S)-hydroxyeicosatetraenoic acid in these incubations. Based on the Scatchard equation, the estimates of the binding constants to albumin were 1.8 x 10(5) for 15 -HETE, 1.4 x 10(5) for 12-HETE, and 0.9 x 10(5) for 5 -HETE respectively. These results suggest an important role of albumin for the regulation of the availability of substrates for platelet 12-lipoxygenase.
authors.affiliation: Department of Woman and Child Health, Division of Reproductive Endocrinology, Karolinska Hospital, Bldg. L5, S-171 76 Stockholm, Sweden.
authors.forename: M
authors.initials: M
authors.lastname: Dadaian
authors.forename: P
authors.initials: P
authors.lastname: Westlund
mesh_terms: D001092:Arachidonate 12-Lipoxygenase / Q000097:blood*; D001792:Blood Platelets / Q000378:metabolism*; D006801:Humans; D006893:Hydroxyeicosatetraenoic Acids / Q000097:blood*; D066298:In Vitro Techniques; D011485:Protein Binding; D012709:Serum Albumin / Q000378:metabolism*; D013379:Substrate Specificity
chemical_list: D006893:Hydroxyeicosatetraenoic Acids; D012709:Serum Albumin; C022022:5-hydroxy-6,8,11,14-eicosatetraenoic acid; C025984:15-hydroxy-5,8,11,13-eicosatetraenoic acid; C034922:5,15-dihydroxy-6,8,11,13-eicosatetraenoic acid; D001092:Arachidonate 12-Lipoxygenase