Localization and activity of membrane dipeptidase in bovine ciliary epithelium

Purpose: To investigate the localization and the activity of membrane dipeptidase (MDP) in the bovine eye.

Methods: A monoclonal antibody (mAb), 49C mAb, raised against bovine ciliary process was used to examine the localization of MDP. Conversion of leukotriene (LT)D4 to LTE4 was evaluated by enzyme-linked immunosorbent assay for LTE4. Hydrolytic activity (beta-lactamase activity) was evaluated with a fluorometric assay. To clarify the contribution of MDP to conversion of LTD4 and beta-lactamase activity, we separated MDP from other enzymes by 49C mAb-conjugated gel.

Results: The antigenic molecule of 49C mAb was shown to be MDP by amino acid sequencing. MDP was immunohistochemically detected in the ciliary pigmented and nonpigmented epithelial cells. Conversion of LTD4 to LTE4 in the ciliary process was much greater than that of the neural retina (NR). beta-Lactamase activity in the ciliary process was apparent, but that in the NR or the retinal pigment epithelium was negligible. Approximately 100% of beta-lactamase activity in the ciliary process was catalyzed by the 49C mAb-bound fraction. Conversion of LTD4 was catalyzed by the 49C mAb-bound fraction (55% of total activity) and by the unbound fraction (45% of total activity).

Conclusions: This study produced the first evidence of the presence of MDP in ciliary epithelial cells. The ciliary epithelium converts LTD4 to LTE4 and shows beta-lactamase activity. Conversion of LTD4 is catalyzed by at least two enzymes, and a major part of the conversion is induced by MDP.