Carbonate inhibition of leukotriene D4-dipeptidase in human serum

Human serum contains an inhibitor of leukotriene D4 (LTD4) dipeptidase which was separated from the enzyme by ultrafiltration (Amicon, YM-10). Removal of the inhibitor resulted in a 3- to 5-fold increase in total LTD4-dipeptidase activity in the material retained by the filter. Inhibitor activity (which was assayed with a partially purified LTD4-dipeptidase) was recovered in the filtrate. Ultrafiltration of serum using YM-3, YM-1, and YC-05 membranes suggested an inhibitor molecular weight of less than 500. Elution of inhibitor activity from a Bio Gel P2 gel filtration column was identical to the elution pattern of pure carbonate. The inhibitor was heat stable (95 degrees C, 30 min), stable in 0.1 N NaOH, but rapidly inactivated by 0.1 N HCl at both 4 degrees C and 30 degrees C. Partially purified LTD4-dipeptidase was inhibited by carbonate and phosphate but not by nitrate, sulfate, or chloride. Based on these observations it was concluded that the inhibitor of LTD4-dipeptidase in human serum either was carbonate or required carbonate. The relative concentrations of LTC4, LTD4, and LTE4 appear to be important parameters in determining the duration and intensity of LT mediated reactions. The relative concentration of carbonate in serum or extracellular fluids might, therefore, be a factor in modulating localized LT mediated responses.