Abstract
Human leukotriene C 4 synthase specific activity in the human monocytic leukemia cell line THP-1 (0.302 ± 0.062 nmol LTC 4 formed · min −1 mg −1) was 7.6-fold higher than in U937 cells (0.040 ± 0.017 nmol LTC 4 formed · min −1 · mg −1) and comparable to dimethylsulfoxide-differentiated U937 cells (0.399 ± 0.084 nmol LTC 4 formed · min −1 · mg −1). Using the photoaffinity probe, azido[ 125I]-LTC 4, a single polypeptide with a molecular mass of 18 kDa was specifically labelled in THP-1 microsomal membranes. The rank order of potencies for competition of azido[ 125I]-LTC 4 photolabelling of the 18 kDa protein by glutathione, leukotrienes and their analogs was found to be LTC 2 > (azido[ 127I]-LTC 4 ≈ LTC 4) > (LTD 4 ≈ LTE 4) > (LTA 4 ≈ LTB 4) > S-hexyl glutathione > glutathione, corresponded with the rank order of potencies for inhibition of LTC 4 synthase activity but not inhibition of microsomal glutathione S-transferase activity. The 18 kDa protein specifically labelled by azido[ 125I]-LTC 4 had high specificity for LTC 4 and closely related leukotrienes and was separable from microsomal glutathione S-transferase. We conclude that azido[ 125I]-LTC 4 specifically photolabels LTC 4 synthase which is an 18 kDa polypeptide or contains an 18 kDa subunit.
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