Murine thymocytes possess specific cell surface‐associated exoaminopeptidase activities: Preferential expression by immature CD4−CD8− subpopulation

Publisher Website
Google Scholar
Abstract
Murine thymocytes are shown to possess at least three well‐defined exo‐N‐aminopeptidase activities on their surface. One of them cleaves the prolyl bond in the synthetic dipeptide nitroanilide Gly‐Pro‐pNA (Km 0.95 mM and Vmax 8 nmol/h at pH 7.4 and 37°C) and is specifically inhibited by phenylmethane sulfonyl fluoride, diprotin A, Gly‐Pro‐Ala and Gly‐Pro‐Gly‐Gly. These data further support identification of this enzyme with a serine exopeptidase dipeptidyl peptidase IV (DPP IV), previously reported to be specific for collagen. The two other forms of N‐exopeptidase activities are detected when Ala‐pNA and Leu‐pNA are used as substrates. Leu‐aminopeptidase activity (Km 1.4 mM, Vmax 15 nmo/h) and Ala‐aminopeptidase activity (Km 4.0 mM, Vmax 20 nmo/h) are inhibited by inhibitors for thiol‐ and trypsin‐like proteinases, i.e. tosyl lysyl chloromethyl ketone, leupeptin and N‐ethylmaleimide. Addition inhibition of Leu‐aminopeptidase activity by pepstatin, a known inhibitor of carboxyl proteases, suggests that aminopeptidase activity detected with Leu‐pNA is different in part from Ala‐aminopeptidase activity. Among the various lymphoid cell populations tested, the three aminopeptidase activities are increased by three‐ to fourfold in the immature CD4−CD8− thymocyte subset as well as in the thymoma BW5147. In contrast, cortisone‐resistant thymocytes, lymph node and spleen cells exhibit levels of activities almost similar to that of unfractionated thymocytes. During ontogeny, the levels of these activities are increased four‐ to sevenfold on fetal thymocytes (from days 14 to 16). Finally, when thymocytes or spleen cells are cultured with a mitogenic concentration of concanavalin A, their proliferative responses are correlated with an enhancement of the aminopeptidase activities (1.3‐ to 5‐fold). From these results, a correlation between the presence of these peptidases on the cell surface of immature and mature lymphoid cells and biological responsivenesses is suggested.