Metabolism of dynorphin A1-13 in human CSF

In-vitro incubation of human cerebrospinal fluid (CSF) obtained from patients ranging from 22-78 years with 10 microM of dynorphin A1-13 (Dyn A1-13) resulted in several cleavage products. Dyn A1-12 and A2-13 were identified as the major CSF metabolites by matrix-assisted laser desorption mass spectrometry (LD-MS). Further metabolites were Dyn A1-6, A2-12 and A4-12. LD-MS further suggested the formation of Dyn A1-8, A1-7, A1-10, A7-10, A3-12, A7-12, A3-13, A7-13 and A8-13. The metabolic half-life of Dyn A1-13 at 37 degrees C was approximately 2.5 h (range 1.75-8.5 h), compared to less than one minute in plasma. The half-life of Dyn A1-13 decreased markedly with age or age-associated processes (n = 20, r2 = 0.498). Noncompartmental kinetic analysis in the absence or presence of enzyme inhibitors (leucinethiol 10 microM, captopril 100 microM and GEMSA 20 microM) suggested that Dyn A1-13 is mainly metabolized by carboxypeptidase to A1-12 (51%) and by aminopeptidases to A2-13 (35%). The generation of A1-6 (13%) was only detected under enzyme inhibition. The extent of conversion into the main metabolites did not follow an age-associated trend, thus over-all enzyme levels but no specific enzymatic systems are elevated with age.