Eight Fragments Have Been Assembled in Proper Order
Dr. Pano G. Katsoyannis now has eight insulin fragments assembled in the proper order. All the amino acids except numbers 10 and 11 in the A chain have been linked in groups
of four, five, and 10 amino acid residues. Two smaller frag ments in the A chain have been joined to form a fragment consisting of amino acids numbered 12 to 21
Total Synthesis of Insulin Near Path consists of making polypeptide units, assembling them into two chains, and joining them Dr. Pano G. Katsoyannis of the Uni versity of Pittsburgh has prepared most of the polypeptide fragments needed to assemble the entire mole cule of sheep insulin ( C&EN, June 25, page 3 7 ) . The approach to insulin's total synthesis consists of making the polypeptide units, assembling them in the proper sequence for each of two chains, then joining the chains, he told the 8th National Medicinal Chemistry Symposium, held at the University of Colorado, Boulder. To totally synthesize insulin, 51 amino acid units must be assembled in a particular sequence and stereochem ical arrangement, Dr. Katsoyannis ex plains. Insulin has two polypeptide chains: the A chain with 21 units and an intrachain ring, and the Β chain with 30 units. The two chains are connected by two disulfide links. Joining the two primary chains would not be too difficult, as the chains of natural insulin have already been sep arated and rejoined to form the origi nal compound, he notes; the difficult part would be to synthesize the chains. Dr. Katsoyannis' plan to do this has some rough terrain to cover. Standing in his way are these problems:
peptide units have circumvented many problems thus far, and further devel opments in technique could find ways to avoid the remaining obstacles, he says. Eight Fragments. Dr. Katsoyannis now has eight fragments of insulin assembled in the proper order. All the amino acids except numbers 10 and 11 in the A chain have been linked in groups of four, five, and 10 amino acid residues. He has started to assemble the smaller fragments into larger ones. Two smaller fragments in the A chain have been joined to form a
• Protecting sensitive groups from undesirable reactions. • Separating insoluble, high molec ular weight polypeptides. • Preventing racemization of amino acid residues. However, careful planning and chemistry in construction of small
Dr. Pano G. Katsoyannis More complicated with insulin's intraand intermolecular disulfide linkages
fragment consisting of amino acids numbered 12 to 2 1 . Small polypeptide units are made by repeated addition of acylated amino acids, followed by deacyiation. Sen sitive substituents like the sulfhydryl group must be blocked to avoid unde sirable side reactions. Dr. Katsoyannis uses benzyl groups to protect the mercapto units until the time comes to form the intrachain disulfide link on the A chain, and the two disulfide links that connect the polypeptide chains. Careful planning of the sequence in each fragment makes it possible to avoid some problems of stereoisomer ism, the Pittsburgh chemist says. Making a fragment with carboxvl terminal amino acid residues that are optically inactive ensures that joining it to another unit will not lead to the formation of isomers. Such an example is found in the fragments A-5 to A-9 and B-13 to B-20. Optical isom erism is avoided another way by joining A-16 to A-17 through use of the azide method. The total synthesis of insulin, Dr. Katsoyannis notes, is a logical ex tension of a process that began with the synthesis of the posterior pituitary hormones by Cornell University's Dr. Vincent du Vigneaud, 1955 Nobel Prize winner in chemistry. These hor mones are nonapeptides. More recently, longer polypeptides related to adrenocorticotropic hormone have been synthesized. A common factor necessary for biological activity is a specific amino acid sequence or a specific geometrical arrangement, Dr. Katsoyannis explains. Synthesis of insulin requires more advanced poly peptide chemistry as it is more com plicated with its inter- and intrachain disulfide linkages, he adds. JULY
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