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esorption, this effect interferes with the embryogenesis, so that hatching is impossible andthe changes in female reproductive system of N. bullata are degenerative.An incubation of N. bullata ovaries with labeled pentapeptides 2a-2c shows differentresults. Contrary to the quick degradation of the peptide 2a, its analogs 2b and 2c weredegraded much slower. Just 2 hours after incubation, the pentapeptide 2b and itsdegradation product Tyr-D-Asp-Pro have been still present in ovaries as detected byradio-HPLC. Similarly, the pentapeptide 2c degrades slowly to Asp-Pro-D-Ala-Pro andPro-D-Ala-Pro. With these analogs, the total radioactivity accumulation in ovaries wassignificantly decreased in comparison with the parent peptide 2a (Figure 1). An imbalanceddecrease, however, might be ascribed to position of D-amino acid residue toward to Tyr 1residue and to changed conformation of D-amino acid containing analogs.To rationalize the results of degradation and accumulation studies with labeledpentapeptides 2a-2c, we utilized NMR spectroscopy for molecular modeling and estimationof conformational changes in corresponding peptides. NOEs measured were used asdistance restrains and applied on previously unrestrained optimized structures. The energyminimization (AMBER; for all NOE restraints distance limit 4A) provided the structuresshown in Figure 2. Peptide 1e adopts the most extended form that differs from 1a mainlyby orientation of Pro 5 residue. The most compact seems to be a molecule of 1c that couldbe stabilized with the H-bond between OH group of Tyr 1 and C=O of Pro 3 (see dotted line).The metabolic stabilization of the molecules with D-amino acids is linked to changedconformation, which might perturb peptides interaction with enzyme and prolong theirhalf-life. The analogs with partially restricted structures, due to the presence of D-aminoacids, are flexible enough to maintain and even potentiate oostatic effect in N. bullata.Thus, the presence of D-amino acids in oostatic peptides results in more oocytes influencedduring the period of their development, which enhances the regulatory potency ofcorresponding analogs.1a 1c 1eFig. 2. The calculated energy minimized structures of 1a, 1c and 1e (NOE distancerestraints applied).AcknowledgmentsSupported by the Research Project Z4 055 0506 of the Institute of Organic Chemistry andBiochemistry CAS, v.v.i. and by the Czech Science Foundation (Grant No. 203/06/1272)References1. Borovsky, D., Carlson, D.A., Griffin, P.R., Shabanowitz, J., Hunt D.F. FASEB J. 4, 3015-3020(1990).2. Hlaváček, J., Tykva, R., Bennettová, B., Barth, T. Bioorg. Chem. 26, 131-140 (1998).3. Tykva, R., Hlaváček, J., Vlasáková, V., Černý, B., Borovičková, L., Bennettová, B., Holík, J.,Slaninová, J. J. Chrom. B. 848, 258-263 (2007).4. Hlaváček, J., Černý, B., Bennettová, B., Holík, J., Tykva, R. Amino Acids 33, 489-497 (2007).5. King, R.C., Ovarian Development in Drosophila melanogaster, 33-54. Academic Press, New York,1970.591