The D and L forms of the enzyme HIV-1 protease have been prepared by total chemical synthesis. The two proteins had identical covalent structures. However, the folded protein-enzyme enantiomers showed reciprocal chiral specificity on peptide substrates. That is, each enzyme enantiomer cut only the corresponding substrate enantiomer. Reciprocal chiral specificity was also evident in the effect of enantiomeric inhibitors. These data imply that the folded forms of the chemically synthesized D- and L-enzyme molecules are mirror images of one another in all elements of the three-dimensional structure. Enantiomeric proteins are expected to display reciprocal chiral specificity in all aspects of their biochemical interactions.

The common mechanistic origin of the phenomena of segmen: insolubility in solution-phase and sequence-related incomplete aminoacylations in solid-phase peptide synthesis is used as the basis of a model for difficult sequences in which partial @-sheet hydrogen bonding by the pendant peptide chains of the peptidoresin is the dominant causative principle. A predictive method based on optimized Chou and Fasman type coil conformational parameters is proposed for the identification of such difficult sequences and is tested by its successful application to 101 previously performed solid-phase peptide syntheses (986 aminoacylation reactions). The use of optimized chemical tactics, secondary structure disrupting reagents and tertiary amide linkages between amino acid residues is discussed with a view to improving the solid-phase synthesis of peptides containing difficult sequences.In the more than 25 years of its application the Merrifield solid-phase principle2 has been responsible for the production of a great number and variety of synthetic peptides including polypeptides such as the 124-residue ribonuclease A3 and the 140residue interleukin-3.4 In spite of the improved understanding of the chemical basis of the procedure, and the development of optimized appro ache^,^^ a class of peptide sequences has emerged that resists facile synthesis, the so-called "difficult sequence^".^^^^* The difficulty arises out of the need for near-quantitative aminoacylation reactions at each cycle of a stepwise solid-phase synthesis to prevent the Occurrence of peptide side products lacking one or more internal amino acids, but with properties similar to the target Difficult sequences are characterized by reproducible stretches of repetitive incomplete aminoacylations and are more prevalent in some peptides than others (Table I).This phenomenon has been described as the most serious potential problem in stepwise solid-phase peptide s y n t h e s i~.~ Results and DiscussionSequence-Related Incomplete Aminoacylations. The rate of formation of any given peptide bond in solid-phase peptide synthesis may be affected by the nature of the support, the dispersing solvent, the acylating reagent, and the structure of the protected peptide chain up to that point." The first three of these factors are amenable to optimization of the chemical tactics employed. Clark-Lewis, I. In Synrhetic Peprides in Biology and Medicine; Alitalo, K.. Partanen, P., Vaheri, A., Eds.; Elsevier Science P u b lishers (IO) (a) Atherton. E.; Woolley, V.; Sheppard, R. C. J . Chem. Soc., Chem. Commun. 1980, 970-1. (b) Atherton, E.; Slaeppard, R. C. In Peptides: Structure and Function. Proceedings of the 9th American Peptide Symposium; Deber, C. M., Hruby, V. J., Kopple, K. D.. Eds.; Pierce Chemical Co.: Rockford, IL, 1985; pp 41 5-8. ( I I ) Cameron, L. R.; Holder, J. L.; Meldal. M.; Sheppard. R. C. J . Chem. SOC., Perkin Trans. I 1988, 2895-901. 0002-7863/90/ 15 12-6039$02.50/0 (12) Hudson, D. J. Org. Chem. 1988, 53, 617-24. (13) Kent, S . B. H.; Hood, L. E.; Beilan, H.; Me...

No abstract

The human gonadotropin-releasing hormone (GnRH) precursor comprises the GnRH sequence followed by an extension of 59 amino acids. Basic amino acid residues in the carboxyl terminal extension may represent sites of processing to biologically active peptides. A synthetic peptide comprising the first 13 amino acids (H X Asp-Ala-Glu-Asn-Leu-Ile-Asp-Ser-Phe-Gln-Glu-Ile-Val X OH) of the 59-amino acid peptide was found to stimulate the release of gonadotropic hormones from human and baboon anterior pituitary cells in culture. The peptide did not affect thyrotropin or prolactin secretion. A GnRH antagonist did not inhibit gonadotropin stimulation by the peptide, and the peptide did not compete with GnRH for GnRH pituitary receptors, indicating that the action of the peptide is independent of the GnRH receptor. The GnRH precursor contains two distinct peptide sequences capable of stimulating gonadotropin release from human and baboon pituitary cells.

The ability of synthetic vertebrate luteinising hormone releasing hormones (LHRHs) and their long-acting analogues to maintain elevated plasma luteinising hormone (LH) concentrations and to stimulate ovarian growth was investigated in incubating bantam hens. Chicken LHRH-II (pGlu1-His2-Trp3-Ser4-His5-Gly6-Trp7-Tyr8-Pro9-G ly10-NH2) was more effective than chicken LHRH-I (pGlu1-His2-Trp3-Ser4-Tyr5-Gly6-Leu7-Gln8-Pro9-Gly10-N H2) or porcine LHRH (pGlu1-His2-Trp3-Ser4-Tyr5-Gly6-Leu7-Arg8-Pro9-Gly10-N H2) in stimulating the release of LH. Long-acting analogues of chicken LHRHs (chLHRHs) were created by substituting D-amino acids in position 6. An intravenous injection (10 micrograms/bird) of D-Arg6-chLHRH-II or of a long-acting mammalian analogue of LHRH (buserelin) resulted in a sustained release of LH for up to 8 h. Less sustained releases of LH were observed after the same doses of D-Ala6-chLHRH-I or of D-Trp6-chLHRH-I. Repeated subcutaneous injections of D-Arg6-chLHRH-II or buserelin at 7 to 9 h intervals for 9 d resulted in loss of pituitary gland responsiveness to these analogues. For this reason, the treatment failed to maintain elevated plasma LH concentrations and did not stimulate the growth of the ovary or oviduct.