On the nature of the 5-lipoxygenase reaction in human leukocytes: characterization of a membrane-associated stimulatory factor.

When 10,000 X g supernatants of human leukocyte homogenates were subjected to centrifugation at 100,000 X g for 75 min, the activity of 5-lipoxygenase decreased by 30-60%, even though no enzyme was detectable in the resuspended 100,000 X g pellet. Recombination of the 100,000 X g supernatant and pellet resulted in a restoration of the lost enzymatic activity, indicating the presence of a 5-lipoxygenase stimulatory factor in the microsomal membrane preparation. Dialysis of human leukocyte supernatants resulted in an apparent decrease in 5-lipoxygenase activity, but only in samples that contained the membrane-associated stimulatory factor, suggesting that the factor required a small molecular weight component for optimal function. The 5-lipoxygenase stimulatory activity was highly unstable to washing of the 100,000 X g pellet or to incubation (16-20 hr) at 4 degrees C. In contrast, the activity was remarkably stable to heat (100 degrees C for 40 min). The responses of the 12- and 15-lipoxygenases in human leukocyte homogenates to the membrane-associated factor and to dialysis were notably different from that of the 5-lipoxygenase. These results demonstrate, therefore, that the 5-lipoxygenase is unique among the human lipoxygenases, not only in its requirement for Ca2+ and ATP but also in its regulation by a membrane-associated stimulatory factor. The mechanism of action of this regulatory factor is of obvious interest for the understanding of the control of leukotriene and lipoxin biosynthesis.