pH-dependent regulation of leukocyte 5-lipoxygenase activity in inflammatory exudates by albumin

In order to study the regulation of cellular 5-lipoxygenase activity under inflammatory conditions, the effects of inflammatory exudates on rat leukocyte 5 lipoxygenase activity were investigated.

Peritoneal leukocytes and inflammatory exudates were collected from glycogen treated rats.

Glycogen (1 g/kg body weight, in a final volume of 3 ml PBS) was injected intraperitoneally into male Wistar rats. After 4 h, the inflammatory exudate was collected.

Rat peritoneal leukocytes were isolated and the cellular 5-lipoxygenase activity was determined by HPLC after cell stimulation with calcium ionophore A23187.

Inflammatory exudates from glycogen treated animals strongly inhibited cellular 5-lipoxygenase activity of ionophore challenged leukocytes. Albumin was identified as the inhibitor in exudates. Inhibition of cellular 5-lipoxygenase activity by albumin was pH-dependent and was strongly increased by the alkaline pH (7.9-8.0) of the exudate. The albumin effect increased in the range of pH 7.4-8.2 where albumin undergoes a conformational change called neutral to base (N-B) transition. S-Carboxymethyl-albumin had a similar activity to that of albumin, which indicated that the free SH-group at Cys-34 of albumin is not necessary for the effect. The albumin dimer showed a significantly higher inhibition than albumin and it suppressed cellular 5-lipoxygenase activity by 98%. Peptic and tryptic fragments of albumin which comprise domains I, II and II, III, respectively, were less active or inactive. Thus, an intact albumin molecule or the dimer are required for efficient inhibition of cellular 5-lipoxygenase activity.

Our data suggest that during inflammation, albumin extravasation and changes in pH-value are involved in the regulation of the inflammatory reaction by suppression of leukotriene release.