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The Journal of Lipid Research, Vol. 40, 940-947, May 1999
Copyright © 1999 by Lipid Research, Inc.

Original Article

Albumin modifies the metabolism of hydroxyeicosatetraenoic acids via 12-lipoxygenase in human platelets

Correspondence to: Par Westlund

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 µM) 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 10⁵ for 15 -HETE, 1.4 x 10⁵ for 12-HETE, and 0.9 x 10⁵ for 5 -HETE respectively.

These results suggest an important role of albumin for the regulation of the availability of substrates for platelet 12-lipoxygenase.—Dadaian, M., and P. Westlund. Albumin modifies the metabolism of hydroxyeicosatetraenoic acids via 12-lipoxygenase in human platelets. J. Lipid Res. 1999. 40: 940–947.

Supplementary key words: 12-hydroxyeicosatetraenoic acid, 5-hydroxyeicosatetraenoic acid, 15-hydroxyeicosatetraenoic acid, 5,15-dihydroxyeicosatetraenoic acid, lipoxins

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