We have developed mice deficient in membrane-bound dipeptidase (MBD, EC 3.4.13.19), the enzyme believed to be responsible for the conversion of leukotriene D 4 (LTD 4 ) to leukotriene E 4 (LTE 4 ). The MBD mutation generated by us was demonstrated to be a null mutation by Northern blot analysis and the absence of ␤-lactamase activity in lung, kidney, small intestine, and heart. MBD gene deletion had no effect on viability or fertility. The mutant mice retain partial ability to convert LTD 4 to LTE 4 , ranging from 80-90% of the wild-type values in small intestine and liver to 16% in kidney and 40% in lung, heart, and pancreas. MBD is also believed to function consecutively after ␥-glutamyl transpeptidase to cleave cystinyl-bis-glycine (cys-bis-gly) generated from glutathione cleavage. Our data indicate that kidney homogenates from MBD-deficient mice retain ϳ40% of their ability to cleave cys-bis-gly, consistent with only modest elevations (3-5-fold) of cys-bis-gly in urine from MBDdeficient mice. These observations demonstrate that the conversion of LTD 4 to LTE 4 and the degradation of cys-bis-gly are catalyzed by at least two alternative pathways (one of which is MBD) that complement each other to varying extents in different tissues.

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