Synthesis and properties of [A19-(p-fluorophenylalanine)] insulin.

The synthesis of [Phe(F)A19]insulin (porcine) is described. First the protected [Phe(F)19]A-chain was assembled by segment condensation of [1-12] and [13-21] using the dicyclohexyldiimide/1-hydroxybenzotriazole procedure. [Phe(F)19]A-chain was purified by ion exchange chromatography after removal of all the protecting groups (Boc, But, OBut and S-Trt) and its conversion into the tetra-S-sulfonated derivative. [Phe(F)A19]insulin was prepared by combination with porcine B-chain and purified by gel filtration and ion-exchange chromatography. The in vitro biological activity of this analogue was 60%. CD spectra in the near and far UV are qualitatively very similar to those of insulin.

Efficient synthesis of N alpha-acetylarginine methylamide.

A simple preparation of Ac-Arg-(p-TosH or HCl)-NHMe is described. The NG-protonated Z-Arg was coupled with methylamine by the mixed anhydride method. Z-Arg-NHMe was purified as a NG-p-toluene sulfonate salt by crystallization from water. After removal of the Z group by catalytic hydrogenation and acetylation Ac-Arg(p-TosH)-NHMe was obtained. Ac-Arg(HCl)-NHMe was prepared by chromatography of the NG-TosH derivative on Dowex 44 (in Cl- form).

Isolation and amino-acid sequence determination of monkey insulin and proinsulin.

Insulin has been isolated and purified from rhesus monkey pancreas by means of acid-ethanol extraction, gel filtration and ion exchange chromatography. The complete amino-acid sequence of the hormone has been determined by amino-acid analysis of the oxidized A- and B-chains, by end group determination, by the identification of the C-terminal residues (AsnA21 and ThrB30) by carboxypeptidase A digestion and by Edman degradation of the S-carboxymethylated A- and B-chains. The 51-residue monkey insulin was shown to be identical to human insulin. From the known insulin and C-peptide sequence the primary sequence of monkey proinsulin has been proposed.

Human proinsulin VI. Synthesis of protected segment 46-70 of prohormone.

The synthesis of the protected pentacosapeptide Trt-Gly-Gly-Pro-Gly-Ala-Gly-Ser-(But)-Leu-Gln-Pro-Leu-Ala-Leu-Glu( OBut)-Gly-Ser (But)-Leu-Gln-Lys(Boc)-Arg-Gly-Ile-Val-Glu-(OBut)-Gln-OH (Pos. 46-70) of human proinsulin is described. This segment was prepared by the mixed anhydride condensation of the trityl-protected peptides (46-64) or (46-59) with the fragments 65-70 and 60-70, respectively. In both the cases the purification was effected by counter current distribution in a yield of 25% and 24%, respectively.

Preparation and properties of citraconylinsulins.

Acylation of insulin with citraconic anhydride was studied at different pH values. The controlled acylation at pH 8.5 and 7 yielded mainly A1-citraconylinsulin (41%) and A1,B1-dicitraconylinsulin (39%), respectively. Acylation with excess reagent at pH 5.6, followed by partial deblocking at pH 5 for 30 min and 17 h, led mainly to A1,B1-dicitraconylinsulin (51%) and B1-citraconylinsulin (40%), respectively. The order of the deblocking rates for the three citraconyl groups at pH 3.5 and 5 was B29 much greater than A1 greater than B1. The biological activity of citraconyl derivatives: A1-citraconyl, B1-citraconyl and A1,B1-dicitraconylinsulin were found to be 15%, 100% and 15% (in vitro fat cell assay) and 46%, 78% and 48% (in mouse convulsion assay), respectively. In a double insulin immunoassay these derivatives had 40%, 75% and 30% immunoreactivity, respectively.

Semisynthesis of human proinsulin, I. Preparation of arginyl-A-chain cyclic bis-disulfide.

The semisynthesis of Arg-A-(SS)2 is described by the following chemical and enzymatic procedures: 1) A-(SSO theta 3)4 was acylated with arginine N-carboxyanhydride by the known method. Arg-A-(SSO theta 3)4 was purified, reduced and oxidized to Arg-A(SS)2. When A(SS)2 was acylated under identical conditions a gel-like product was obtained which could be purified after oxidative sulfitolysis. This was then converted to Arg-A-(SS)2 as described above. Both the pathways gave the desired product in 20-25% yield. 2) Boc-Orn(Msc) was quantitatively attached to A-(SS)2 by the mixed anhydride method, the Msc group was then removed and the ensuing N delta-free amino function was amidinated. Traces of unconverted Orn derivative (less than 6%) were still present. 3) Boc-Arg(HBr) was attached to A-(SS)2 by the mixed anhydride method and Arg-A(SS)2 was isolated after removal of the Boc group in 25% yield. 4) Boc-Arg(DHCH) was attached to A-(SS)2 by the mixed anhydride method. DHCH and Boc groups were removed in two steps and Arg-A-(SS)2 was isolated in 29% yield. 5) Boc-Arg was condensed with A-(SS)2 by trypsin-catalyzed synthesis. The removal of Boc group and purification yielded the desired product in 40-42% yield. This procedure was the most efficient and proceeded stereospecifically.

[Protein semisynthesis with the help of mixed anhydrides and enzymes: chemistry and synthesis of insulins]. / Protein-semisynthese mit Hilfe gemischter Anhydride und Enzyme: Ein Beitrag zur Chemie und Synthese des Insulins.

Proteins play a prominent role in nature and their biosynthesis occurs via stepwise combination of amino acids. One can imitate this method in laboratory or synthesize the polypeptide chain by combining smaller preformed fragments (fragment condensation). Reversible protection of reactive groups and solubility problems arising are the most important features in this regard. Semisynthesis, i.e., coupling of amino acids or peptides to natural material may help to overcome these difficulties. The preparation of hybrid preproinsulin by mixed anhydride synthesis and the conversion of pork insulin to human insulin by enzyme-catalyzed peptide synthesis are two examples of the semisynthesis of proteins. In both cases optimal reaction conditions are essential for maximal yield of the product desired. In spite of the rapid improvement of gene technology, chemical peptide synthesis will retain its value for the preparation of biologically and pharmacologically interesting substances.

[Semisyntheses using proinsulin, II. Preparation and application of Nepsilon 29, Nepsilon 59-bis(methylsulfonylethoxycarbonyl)proinsulin (author's transl)]. / Semisynthesen, ausgehend von Proinsulin, II. Darstellung und Anwendung von Nepsilon 29, Nepsilon 59-bis(methylsulfonylethoxycarbonyl)-proinsulin.

Nepsilon 29, Nepsilon 59-Bis(methylsulfonylethoxycarbonyl)proinsulin was prepared from native beef proinsulin for its possible use in the semisynthesis of preproinsulin. The modification was made possible by first protecting the alpha-amino group of proinsulin either with the tert-butoxycarbonyl group, after reaction with bis(tert-butoxycarbonyl)oxide, or with the citraconyl group, after reaction with citraconic anhydride. Whereas Nalpha 1-tert-butoxycarbonylproinsulin was achieved by direct alkoxycarbonylation, Nalpha 1-citraconylproinsulin was obtained from partial deprotection of the quantitatively acylated proinsulin. The Nalpha 1-protected proinsulin derivatives were further quantitatively reacted with N-(methylsulphonylethoxycarbonyloxy)succinimide. The tert-butoxycarbonyl group was next removed by treatment with trifluoroacetic acid, or the citraconyl group removed by mild acid (pH 2.2) treatment. The resulting Nepsilon 29, Nepsilon 59-bis(methylsulphonylethoxycarbonyl)proinsulin was used in model reactions with the mixed anhydride of tert-butoxycarbonylmethionine with the aim of standardizing the coupling conditions to be used in the eventual synthesis of preproinsulin.

Human proinsulin, VII: synthesis of two protected peptides corresponding to the sequences 1--45 and 46--96 of the prohormone.

The synthesis of two protected large peptides 1--45, and 46--86 covering the entire amino acid sequence of human proinsulin is described. Peptide 1--45, was synthesized from two intermediate fragments 1--23 and 24--45 and purified by countercurrent distribution in dimethylformamide system (K = 0.5). Peptide 46--86 was synthesized from two intermediate fragments 46--70 and 71--86 and purified by countercurrent distribution in two solvent systems, the dimethylformamide system (K = 0.06), and the toluene system (K = 0.16).

Synthesis of protected decapeptide (B21--30) of human insulin.

The synthesis of the C-terminal decapeptide derivative B21--30 of the B-chain of human insulin (benzyloxycarbonyl-gamma-t-butoxyglutamyl-arginyl-glycyl-phenylalanyl-phenylalanyl-O-t-butyltyrosyl-O-t-butylthreonyl-prolyl-Nepsilon-t-butyl-oxycarbonyllysyl-O-t-butylthreonine t-butyl ester, (XIII) by the fragment condensation of the tripeptide B21--23 with the heptapeptide B24--30 is described. Two new routes for both the tripeptide and the heptapeptide were established. A simplified method for the purification of O-t-butylthreonine t-butyl ester was developed.

Kinetics of human connecting peptide in normal and diabetic subjects.

The metabolic clearance rate (MCR) of synthetic human connecting peptide (C-peptide) was measured with a single-dose injection technique in six normal and seven diabetic subjects and with a constant infusion technique in one normal subject. The MCR of C-peptide did not differ in normal subjects (4.4 ml/min per kg; range, 3.7-4.9) and in diabetic subjects (4.7 ml/min per kg; range, 3.7-5.8). Employment of both techniques in one subject gave similar MCR. The average half-life of C-peptide in plasma calculated from the last 1-h period of the single-dose injection studies was longer in the insulin-dependent diabetics (42.5 min; range, 39.4-48.5) than in the normal subjects (33.5 min; range, 24.9-45.3). These results indicate that the beta-cell secretory capacity of normal and insulin-dependent diabetic subjects can be compared by measuring the C-peptide concentration in peripheral venous plasma. The difference in the half-life of C-peptide in plasma between diabetics and normals suggests an altered kinetics of the disappearance of the peptide, while the overall metabolism, as expressed by the MCR, is similar.

Preparation and application of Nalpha-B1,Nepsilon-B29-bis (ter.-butyloxycarbonyl)insulin.

Two methods are described for the preparation of NalphaB1,Nepsilon29-Boc2-insulin from Nalpha A1-trifluoroacetyl-insulin and Nalpha A1-citraconyl-insulin in 80 - 90% and 65% yields, respectively. Removal of the Boc protections afforded the fully active insulin. Application of this derivative was demonstrated by the preparations of des-GlyA1-insulin and [A1-guanidinoacetyl]insulin. The former compount exhibited 2% activity in the in vitro free fat cell assay and the latter 88 +/- 5% while NalphaB1-NepsilonB29-Boc2-insulin showed 45 +/- 3% activity only.

Characterization of seven C-peptide antisera.

The plasma C-peptide immunoreactivity (CPR) in 10 normal subjects varied considerably when measured with different antisera in parallel assays. The CPR level correlated with the blank "CPR" value measured in plasma devoid of C-peptide and to a lesser degree with the sensitivity of the standard curves obtained with the individual antisera. Storage of plasma samples at different temperatures and for different lengths of time before the analyses were carried out resulted in further variation in the CPR results. This was caused by a time- and temperature-dependent fall in CPR, which was more pronounced with some antisera than with others. This sensitivity to storage of plasma did not correlate with the antigenic characteristics of the antisera as determined by their reactivity with 11 specific fragments of the C-peptide molecule. The contribution of human proinsulin to the CPR concentration relative in normal subjects was considered to be negligible even though the relative immunoreactivity of human proinsulin and C-peptide ranged from 11 to 143 per cent among these antisera. These results suggest that differences in C-peptide antisera are a major reason for the variation in the concentration of circulating CPR as measured in different C-peptide immunoassays.

Production of antisera to synthetic benzyloxycarbonyl-C-peptide of human proinsulin.

Antisera to the C-peptide of human proinsulin were obtained by immunizing guinea pigs with synthetic benzyloxycarbonyl-C-peptide conjugated to human albumin with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide. In three series of 10, the animals were injected with C-peptide conjugated to albumin in the molar ratio of 23 : 1, 15 : 1, and 4 : 1, respectively. Antibodies to human C-peptide were present in all the surviving 25 animals. Fifteen of the antisera were suitable for measuring C-peptide concentrations lower than 0.10 pmol/ml. The antisera demonstrated an increasing immunogenicity with increasing molar ratio of C-peptide to albumin in the conjugate. In the fourth series, ten guinea pigs immunized with benzyloxycarbonyl-C-peptide ionically bound to QAE-Sephadex A-25 did not produce detectable antibodies to C-peptide. A qualitative evaluation of the radioimmunoassay by use of the antiserum with the highest titer and sensitivity, "M 1230", revealed a mean intra-assay and inter-assay coefficient of variance of 3.2 and 9.6%, respectively.

The conformational protential of porcine proinsulin C-peptide.

Statistical analysis of protein sequences lends itself to the identification of regions with a definite inclination to adopt specific main-chain conformations. Application of the model of Chou and Fasman[1,2] to porcine proinsulin C-peptide localizes the tendency to form a helix in the segments (38 to 44) and (51 to 58). A tendency to beta turn formation is predicted for the segment (45 to 50). The realization of this conformational potential under native and various other conditions was examined by CD spectroscopy. Synthetic C-peptide as well as the synthetic fragments (33 - 40), (41 - 52), (41 - 61), (46 - 52), (46 - 61), and (53 - 61) were included in the study. These fragments provide breaks in the amino acid sequence in each of the potentially ordered regions. The strong helical tendency in the (51 - 58) segment can be activated in the fragments (41 - 61) and (46 - 61) by 1 per cent sodium dodecylsulfate, although the spectrum is not indicative of a classical alpha-helix. However, the conformation in the (51 to 58) segment should also be non-random in native C-peptide, since cleavage of the (46 - 61) fragment into the subfragments (46 - 52) and (53 - 61) causes considerable spectral effects. Cleavage of the other potentially helical region (38 to 44) between residues 40 and 41, on the other hand, is without spectral consequences. Therefore, this segment is unlikely to be helical in native C-peptide. In the coherent C-peptide, the helix formation which can be induced by sodium dodecylsulfate in the C-terminal part is apparently inhibited by interaction with the N-terminal half of the molecule. This interaction implies that the chain is folded back on itself, which is consistent with a high probability of bets turn formation in the segment (45 to 50). The CD spectra of the fragments (41 - 52) and (46 - 52), in which the beta turn could occur, are characterized by positive ellipticity about 213 nm. The correlation of the beta turn with this type of spectrum as well as its definite location are discussed, but cannot be proved solely on CD spectroscopic grounds.

Studies on polypeptides, VI. Synthesis, circular dichroism and immunological studies of tyrosyl C-peptide of human proinsulin.

The synthesis of tyrosyl human C-peptide, a sequence of 32 amino acids, by the fragment condensation of the N-terminal octapeptide and C-terminal tetracosapeptide is described. The t-butyl protecting groups were removed by trifluoroacetic acid to obtain N-benzyloxycarbonyl-tyrosyl C-peptide. The hydrogenolytic debenzyl-oxycarbonylation of this derivative proceeded to an extent of only 80-90%, and tyrosyl C-peptide was purified by preparative electrophoresis. This purified tyrosyl C-peptide led to an improved sensitivity of the radioimmunoassay. The synthetic tyrosyl C-peptide in an immunoassay using anti human b-component serum reacted slightly differently from the synthetic human C-peptide. After labelling tyrosyl C-peptide with 125I and then purifying the radioactive product, we observed that 80% of the radioactivity could be bound when reacted with an excess of the serum. The circular dichroism spectrum of tyrosyl C-peptide is very similar to that of synthetic human C-peptide. An analysis of the spectrum indicates that 3-7 amino acids are in the beta-structure and the rest in random coil conformation.