Dipeptidyl peptidase IV (DPPIV) enzyme activity on immature T-cell line R1.1 is down-regulated by dynorphin-A (1–17) as a non-substrate inhibitor Jelka Gabrilovac a, * , Marija Abramic ´ b , Branka Uz ˇarevic ´ c , Ana Andreis d , Ljiljana Poljak e a ‘‘Rud - er Bos ˇkovic ´ Institute’’, Division of Molecular Medicine, Bijenic ˇka c. 54, HR-10002, P.0. Box 180, Zagreb, Croatia b ‘‘Rud - er Bos ˇkovic ´ Institute’’, Division of Organic Chemistry and Biochemistry, Zagreb, Croatia c Clinical Hospital Centre Zagreb, Department of Immunology, Zagreb, Croatia d School of Dental Medicine, Department of Physiology, University of Zagreb, Zagreb, Croatia e Medical Faculty, Department of Anatomy, University of Zagreb, Zagreb, Croatia Received 27 June 2002; accepted 8 November 2002 Abstract In this study we examined surface expression of CD26 and the corresponding enzyme activity of dipeptidyl peptidase IV (DPPIV) on the cells of immature murine T-cell line, R1.1. The data obtained have shown that R1.1 cells express high density of surface CD26 as compared to normal thymus cells. This was associated with strong enzyme activity, which, based on substrates and inhibitor specificity, corresponded to DPPIV. The DPPIVenzyme activity of R1.1 cells was 10 times stronger than that found on normal murine thymus cells (V max = 39 Amol/min/10 6 cells, vs 3.7 Amol/min/10 6 cells, respectively). Upon activation with anti-CD3, up-regulation of both membrane CD26, as well as of DPPIV enzyme activity on R1.1 cells were observed. The finding of strong DPPIV on R1.1 cells makes them suitable model for testing putative substrates/inhibitors of the enzyme in its natural microenvironment. Since in addition to strong DPPIV, R1.1 cells also express kappa opioid receptors (KOR) [European Journal of Pharmacology 227 (1992) 257], we tested the effect of dynorphin-A (1 – 17) , an endogenous opioid peptide with KOR selectivity, on DPPIV of R1.1 cells. Dynorphin-A (1 – 17) down-regulated DPPIV in a dose-dependent manner, with the potency similar to that of substance P, a known natural DPPIV substrate [Journal of Pharmacology and Experimental Therapeutics 260 (1992) 1257]. DPPIV down-regulation was resistant to bestatin and thiorphan, the inhibitors of two cell surface peptidases (APN and NEP, respectively) with potential of dynorphin-A (1 – 17) degradation, suggesting that the mechanism underlying the observed effect does not involve degradative products of dynorphin-A (1 – 17) . DPPIV down-regulation was also resistent to KOR antagonist, NBI, suggesting that the mechanism underlying the observed 0024-3205/03/$ - see front matter D 2003 Elsevier Science Inc. All rights reserved. doi:10.1016/S0024-3205(03)00257-1 * Corresponding author. Tel.: +385-1-45-61-082; fax: +385-1-45-61-010. E-mail address: gabril@rudjer.irb.hr (J. Gabrilovac). www.elsevier.com/locate/lifescie Life Sciences 73 (2003) 151 – 166