Process for producing an insulin

Production of insulin or an insulin analog is provided by combination of an A-chain and a B-chain, which comprises bringing the S-sulfonated form of the A-chain, the S-sulfonated form of the B-chain, and a thiol reducing agent together in an aqueous medium under conditions which produce a mixture having (1) a pH of from about 8 to about 12, (2) a total protein concentration of from about 0.1 to about 50 milligrams per milliliter, and (3) an amount of thiol reducing agent which affords a total of from about 0.4 to about 2.5 --SH groups per each --SSO.sub.3.sup.- group present in the total amount of A- and B-chain S-sulfonates, and allowing formation of insulin or an insulin analog to occur by maintaining the mixture at a temperature of from about 0.degree. C. to about 25.degree. C. and in an environment which provides a source of oxygen.

BACKGROUND OF THE INVENTION 
With the advent of the possibility to generate protein products by 
recombinant DNA methods and specifically the production of insulin A-chain 
and insulin B-chain by such techniques [see Goeddel et al., Proc. Nat'l. 
Acad. Sci. USA 76, 106-110 (1979)], the need for an efficient method for 
combining the A- and B-chains to form insulin has greatly increased. 
Typically, the prior art methods for producing insulin by combination of A- 
and B-chains use as starting materials such A- and B-chains in the form of 
their stable S-sulfonates. In general, the A- and B-chain S-sulfonates, 
either separately or together, are reduced to their corresponding --SH 
compounds, customarily using a large excess of thiol reducing agent. The 
products are isolated from the reducing medium and, if not reduced 
together, are brought together in an oxidizing medium, e.g., air, to 
achieve combination of A- and B-chains with production of insulin. 
Examples of this methodology are found in Du et al., Scientia Sinica 10, 
84-104 (1961); Wilson et al., Biochim. Biophys. Acta 62, 483-489 (1962); 
Du et al., Scientia Sinica 14, 229-236 (1965); Kung et al., Scientia 
Sinica 15, 544-561 (1966); Kexue Tongbao (Republic of China) 17, 241-277 
(1966); and Markussen, J. Acta Paediatrica Scandinavica, Suppl. 270, 
121-126 (1977). 
A modification of this approach involves reduction of the A-chain 
S-sulfonate followed by reaction of the reduced A-chain with B-chain 
S-sulfonate in an oxidizing atmosphere. See, e.g., Katsoyannis et al., 
Proc. Nat. Acad. Sci. (U.S.A.) 55, 1554-1561 (1966); Katsoyannis, Science 
154, 1509-1514, (1966); Katsoyannis et al., Biochemistry 6, 2642-2655 
(1967); U.S. Pat. No. 3,420,810; and Jentsch, Journal of Chromatography 
76, 167-174 (1973). 
Another modification involves partial oxidation of the A-chain --SH 
compound to produce disulfide formation between the A-6 and A-11 cysteine 
residues followed by oxidation of the product with B-chain --SH or B-chain 
S-sulfonate. See, e.g., Belgian Pat. No. 676,069 and Zahn et al., Liebigs 
Ann. Chem. 691, 225-231, (1966). 
In each of the above prior art methods, one element is common i.e., the 
production of insulin by two independent, sequential steps, namely, 
reduction of S-sulfonate to --SH followed by oxidation to --S--S--. 
Dixon et al., Nature 188, 721-724 (1960) describe conditions which suggest 
single solution conversion of A- and B-chain S-sulfonates to insulin using 
a thiol reducing agent and air oxidation. The details are quite sketchy, 
and the yield, based only on activity of product recovered, represented 
1-2%. However, Dixon, in Proc. Intern. Congr. Endecrinol. 2nd London 1964, 
1207-1215 (1965), in somewhat further elaboration, suggests, in Table IV 
at page 1211, that the conditions reported in the earlier publication 
involve separate reduction and oxidation steps. 
In distinction to the above prior art methods, it has now been discovered 
that it is possible under defined reaction conditions to achieve 
attractive levels of production of insulins or analogs of insulins from 
S-sulfonated A- and B-chains by conducting both the reduction and 
oxidation reactions in a single-step, single-solution process. It is to 
such a process that this invention is directed. 
SUMMARY OF THE INVENTION 
Therefore, this invention is directed to a process for combining an A-chain 
of an insulin or insulin analog and a B-chain of an insulin or an insulin 
analog to produce an insulin or an insulin analog, which comprises 
bringing the S-sulfonated form of the A-chain, the S-sulfonated form of 
the B-chain, and a thiol reducing agent together in an aqueous medium 
under conditions which produce a mixture having (1) a pH of from about 8 
to about 12, (2) a total protein concentration of from about 0.1 to about 
50 milligrams per milliliter, and (3) an amount of thiol reducing agent 
which affords a total of from about 0.4 to about 2.5 --SH groups per each 
--SSO.sub.3 -- group present in the total amount of A- and B-chain 
S-sulfonates; and allowing formation of insulin or an insulin analog to 
occur by maintaining the mixture at a temperature of from about 0.degree. 
C. to about 25.degree. C. and in an environment which provides a source of 
oxygen. 
DETAILED DESCRIPTION OF THE INVENTION 
As indicated, this invention is directed to an efficient, single-step, 
single-solution process for producing insulin or an analog of insulin from 
its constituent S-sulfonated A- and B-chains. 
By the term "insulin" is meant, of course, any of the naturally occurring 
insulins, such as human, bovine, porcine, sheep, fish, avian, and the 
like, as well as a hybrid formed from a combination of an A-chain of one 
species and a B-chain of another. 
By the term "insulin analog" is meant any of a wide variety of proteins, 
each of which has the basic A-chain and B-chain containing all of the 
half-cystine residues in a sequential alignment consistent with that of 
the natural insulins. The analogs differ from natural insulins by 
substitution, addition, deletion, or modification of one or more amino 
acid residues, but with retention of the disulfide bond arrangement and at 
least a portion of the insulin-like activity. Examples of such analogs are 
[N-formyl-Gly.sup.1 -A]insulin, Desamino-A.sup.1 -insulin, 
[Sarcosine.sup.1 -A]insulin, [L-Alanine.sup.1 -A]insulin, [D-Alanine.sup.1 
-A]insulin, [Isoasparagine.sup.21 -A]insulin, [D-Asparagine.sup.21 
-A]insulin, [Arginine.sup.21 -A]insulin, [Asparagineamide.sup.21 
-A]insulin, [Sarcosine.sup.1 -A, Asparagine.sup.21 A]insulin, 
[Norleucine.sup.2 -A]insulin, [Threonine.sup.5 -A]insulin, [Leucine.sup.5 
-A]insulin, [Phenylalanine.sup.19 -A]insulin, [D-Tyrosine.sup.19 
-A]insulin, [Tyrosine.sup.18 -A, Asparagine.sup.19 A, Arginine.sup.21 
-A]insulin, Des[B.sup.28-30 -tripeptide]insulin, Des[B.sup.27-30 
-tetrapeptide]insulin, Des[B.sup.26-30 -pentapeptide]insulin, 
Des[B.sup.27-30 -tetrapeptide, Tyrosinamide.sup.26 -B]insulin, 
Des[B.sup.26-30 -pentapeptide, Phenylalaninamide.sup.25 -B]insulin, 
Des[B.sup.1-4 -tetrapeptide]insulin, Des[B.sup.1-5 -pentapeptide]insulin, 
[Lysine.sup.22 -B]insulin, [Leucine.sup.9 -B]insulin, [Leucine.sup.10 
-B]insulin, Des[Phenylalanine.sup.1 -B]insulin, and the like. These and 
others are described in the literature; see, for example, Blundell, T., et 
al, Advances in Protein Chemistry, 26, 330-362, Academic Press, N.Y., N.Y. 
(1972); Katsoyannis, P. G., Treatment of Early Diabetes, 319-327, Plenum 
Publishing Corp. (1979); Geiger, R., Chemiker Zeitung, Reprint 100, 
111-129, Dr. A. Huthig, Publisher, Heidelberg, W. Germany (1976); 
Brandenburg, D. et al., Biochem. J. 125, 51-52 (1971). 
Although the process of this invention is broadly applicable to the 
production of insulins and insulin analogs, it is highly preferred to use 
it in the production of naturally-occurring insulins, in particular, 
human, bovine, or porcine insulins, and most particularly, human insulin. 
In carrying out the process of this invention, the combination of A- and 
B-chain to form insulin or an insulin analog can be achieved over a wide 
range of ratios of one chain relative to the other. The combination, of 
course, is inherently limited by the chain, whether A or B, present in the 
lesser quantity. In any event, although not essential, the customary ratio 
of A-chain to B-chain, on a weight basis, is from about 0.1:1 to about 
10:1. It is highly preferred to carry out the process of this invention 
using a weight ratio of A-chain to B-chain in the range from about 1:1 to 
about 3:1. It has also been discovered, within this preferred range, that 
certain ranges are especially advantageous for production of a particular 
species of insulin. Thus, in the combinatin of A- and B-chain to produce 
bovine insulin, it is preferred that the ratio of A-chain to B-chain be 
within the range of from about 1.4:1.0 to about 1.8:1.0. As to porcine 
insulin the preferred range is from about 1.0:1.0 to about 1.4:1.0. As to 
human insulin, the preferred range is from about 1.8:1.0 to about 2.2:1.0. 
Another parameter which is significant for carrying out the process of this 
invention at an optimal level is the protein concentration in the reaction 
medium. The process can be successfully carried out over a wide range of 
protein concentrations. Generally, however, the protein concentration will 
range from about 0.1 to about 50 mg. per ml. of reaction medium. 
Preferably, the protein concentration will be in the range of from about 2 
to about 20 mg. per ml. Again, it has been discovered, within this latter 
range, that the optimal protein concentration varies depending upon the 
species of insulin which is produced. Therefore, as to porcine insulin, it 
is preferred that the protein concentration range be from about 8 to about 
16 mg. per ml., whereas, in the production of human or bovine insulin, the 
preferred range is from about 3 to about 8 mg. per ml. 
The process of this invention is carried out in an aqueous medium. The pH 
of the medium measured at room temperature generally will range from about 
8 to about 12. Preferably, it will be from about 9.5 to about 11.0 and 
optimally will be maintained within the range of from about 10.4 to about 
10.6. The pH of the medium may be maintained in the desired range by 
addition of a suitable buffering agent. Typical such buffering agents are, 
for example, glycine, glycylglycine, carbonate, 
tris(hydroxymethyl)aminomethane, N,N-bis(2-hydroxyethyl)glycine, 
pyrophosphate, N-tris(hydroxymethyl)methyl-3-aminopropanesulfonic acid, 
and other like agents which affect pH control within the aforedescribed 
range. The common and preferred buffering agent is glycine. 
The concentration of buffering agent generally ranges from about 0.001 M to 
about 2 M. Preferably, the concentration is from about 0.01 M to about 1 
M, and, most preferably, from about 0.01 M to about 0.1 M. 
The A- and B-chains are brought together in the appropriate aqueous medium 
in the presence of a thiol reducing agent. By "thiol reducing agent" is 
meant a compound that contains at least one --SH group and has the 
capacity to effect reduction of the S-sulfonate groups of the A- and 
B-chains. Although any agent having these characteristics can be employed, 
a much preferred thiol reducing agent is one which, in its oxidized form, 
has been cyclized to a highly stable compound. The thiol reducing agent is 
present in an amount which affords a total of from about 0.4 to about 2.5 
--SH groups per each --SSO.sub.3.sup.- group present in the total amount 
of A- and B-chain, and, preferably, from about 0.9 to about 1.1 --SH 
groups per each SSO.sub.3.sup.- group. 
Examples of typical thiol reducing agents are dithiothreitol (DTT), 
dithioerythritol (DTE), cysteine, 2-mercaptoethanol, methyl thioglycolate, 
3-mercapto-1,2-propanediol, 3-mercaptopropionic acid, thioglycolic acid, 
and other such thiol compounds. Preferred thiol reducing agents are 
dithiothreitol and dithioerythritol. Another preferred thiol reducing 
agent is cysteine. Of these, cysteine and dithiothreitol are most 
preferred. 
One of the essential conditions of the process of this invention is that it 
be carried out in an environment that provides a source of oxygen. This 
condition can be met merely by permitting the reaction mixture to be open 
to the air. Although a more direct contact method may be employed, such 
as, for example, by bubbling air or oxygen into and through the reaction 
medium, such is not necessary. 
In general, therefore, the process of this invention is carried out by 
preparing a mixture of the A-chain S-sulfonate, the B-chain S-sulfonate, 
and the thiol reducing agent at desired concentrations in an aqueous 
medium at a pH of from about 8 to about 12. The mixture, open to air 
contact, is gently agitated for a period sufficient to allow chain 
combination to occur, generally at least about 30 minutes. During this 
period of agitation, the mixture generally is maintained at a temperature 
of from about 0.degree. C. to about 25.degree. C.; preferably, however, 
the mixture is subjected to moderate cooling to maintain the temperature 
at the lower end of this range, generally from about 2.degree. C. to about 
10.degree. C. 
Once the reaction period has been completed, the insulin or insulin analog 
product can be isolated by any of a wide variety of methods, all of which 
are recognized in the field of insulin technology. The most commonly 
employed techniques for insulin purification are chromatographic 
techniques. These are readily applicable in recovering insulin from the 
process of this invention. These can include gel filtration and 
ion-exchange chromatography. 
Moreover, the product can be assayed for purity and activity by recognized 
methods such as polyacrylamide gel electrophoresis, amino acid analysis, 
insulin radioreceptorassay, insulin radioimmunoassay, high performance 
liquid chromatography (HPLC), ultraviolet spectrum, dansylation, rabbit 
blood glucose assay, and the like. 
The insulins which are available from the process of this invention include 
hybrids comprising the insulin A-chain of one species and the insulin 
B-chain of another species. The thrust of the process of this invention is 
directed to a proper combining of A- and B-chain S-sulfonates, and the 
particular structure of these chains, as long as they truly represent 
insulin or insulin analog A- and B-chains, is immaterial to the process of 
this invention. 
Although an insulin analog or an insulin hybrid, i.e., an A-chain of one 
species and a B-chain of another species, can be prepared by the process 
of this invention, it, of course, is preferred to produce an insulin which 
is structurally identical to that of a naturally occurring insulin by 
using an A-chain S-sulfonate and a B-chain S-sulfonate, each of which has 
the amino acid sequence represented by such insulin. Moreover, it is 
highly preferred to use the process of this invention to produce porcine, 
bovine, or human insulin, and most preferably, to produce human insulin. 
The insulin or insulin analog A- and B-chains, as already indicated, are 
available by recombinant DNA methodology. They also can be prepared from 
natural insulins and by classical peptide synthesis methodology, including 
either solution or solid phase techniques. 
The A- and B-chains are maintained in stable form as S-sulfonates. The 
S-sulfonate starting materials are available by oxidative sulfitolysis, a 
treatment by which the A- and B-chains are reacted with sodium sulfite in 
the presence of a mild oxidizing agent, such as sodium tetrathionate. 
As illustrative of the process of this invention, the following examples 
are provided. These examples are provided for illustrative purposes only, 
and they are not intended to be limiting upon the scope of this invention.

EXAMPLE 1 
Porcine A-chain S-sulfonate (360 mg.) was dissolved in 36 ml. of 0.1 M 
glycine buffer (pH 10.5), and the pH of the mixture was adjusted to 10.5 
with 5 N NaOH. Porcine B-chain S-sulfonate (300 mg.) was dissolved in 30 
ml. of 0.1 M glycine buffer (pH 10.5), and the pH of the mixture was 
adjusted to 10.5 with 5 N NaOH. Dithiothreitol (DTT) (123.4 mg.) was 
dissolved in 4 ml. of the 0.1 M glycine buffer (pH 10.5), and the pH of 
the mixture was adjusted to 10.5 with 0.2 ml. of 5 N NaOH. 
The A- and B-chain solutions were combined in a 100 ml. vial at room 
temperature (.about.25.degree. C.), and 1.91 ml. of the DTT solution then 
were added to provide an --SH to --SSO.sub.3.sup.- ratio of 1.04. The 
resulting solution was mixed gently in an open beaker with magnetic 
stirring at 4.degree.-8.degree. C. for 20 hours. Analysis by high 
performance liquid chromatography (HPLC) indicated an insulin yield of 
193.8 mg., or 29% of the total protein. 
Forty ml. of this final solution were adjusted to pH 3.15 with acetic acid. 
The mixture was gel filtered on a 5.times.200 cm. column of Sephadex G-50 
(Superfine) equilibrated and eluted with 1 M acetic acid at 
4.degree.-8.degree. C. The insulin peak (elution volume, about 2450-2700 
ml.) was pooled and lyophilized with a recovery of 95 mg. of insulin, or 
25% of the total protein. The porcine insulin was judged to be quite pure 
by polyacrylamide gel electrophoresis, amino acid analysis, insulin 
radioreceptor assay, HPLC, and the rabbit blood glucose reduction test. 
EXAMPLE 2 
Solutions of bovine A- and B-chain S-sulfonates having a concentration of 5 
mg./ml. in 0.01 M glycine buffer (pH 10.5) were prepared. The pH of each 
was adjusted to 10.5 with 5 N NaOH. DTT (61.7 mg.) was dissolved in 4.0 
ml. of 0.1 M glycine buffer (pH 10.5), and the pH was adjusted to 10.5 
with 0.15 ml. of 5 N NaOH. To 0.5 ml. of the B-chain solution were added 
0.8 ml. of the A-chain solution and 0.035 ml. of the DTT solution at room 
temperature (.about.25.degree. C.) to provide an --SH to --SSO.sub.3.sup.- 
ratio of 0.91. The resulting solution was stirred at 4.degree.-8.degree. 
C. for 20 hours in an open 3-ml. vial. HPLC analysis of the mixture 
indicated a bovine insulin yield of 1.96 mg., or 30% of the total protein. 
EXAMPLE 3 
Solutions of porcine A- and B-chain S-sulfonates having a concentration of 
10 mg./ml. in 0.1 M glycine buffer (pH 10.5) were prepared. The pH of each 
was adjusted to 10.5 with 5 N NaOH. DTT (61.7 mg.) was dissolved in 2.0 
ml. of glass-distilled water. To 0.5 ml. of the B-chain solution were 
added 0.6 ml. of the A-chain solution and 29.25 .mu.l. of the DTT solution 
at room temperature (.about.25.degree. C.) to provide an --SH to 
--SSO.sub.3.sup.-ratio of 1.00. The resulting solution was stirred at 
4.degree.-8.degree. C. in an open 3-ml. vial for 20 hours. HPLC analysis 
of the mixture indicated a porcine insulin yield of 3.81 mg., or 35% of 
the total protein. 
EXAMPLE 4 
Human (pancreatic) B-chain S-sulfonate and several human (pancreatic and E. 
coli) and porcine (pancreatic) A-chain S-sulfonate solutions having a 
concentration of 5 mg./ml. in 0.1 M glycine buffer (pH 10.5) were 
prepared. The pH of each was adjusted to 10.5 with 5 N NaOH. DTT (61.7 
mg.) was dissolved in 4.0 ml. of 0.1 M glycine buffer (pH 10.5), and the 
pH was adjusted to 10.5 with 0.16 ml. of 5 N NaOH. To 1.0 ml of each of 
the A-chain S-sulfonate solutions were added 0.5 ml. of the B-chain 
S-sulfonate solution and 0.05 ml. of the DTT solution at room temperature 
(25.degree. C.) to provide an --SH to --SSO.sub.3.sup.- ratio of 1.09. All 
solutions were stirred in a chill room (4.degree.-8.degree. C.) in an open 
vial for 20-22 hours. They then were analyzed by HPLC using a pancreatic 
human insulin standard for the yield calculations. The results are in the 
Table following: 
TABLE 1 
______________________________________ 
% Yield 
A-Chain Human Insulin 
Relative to 
Source Yield, mg. Total Protein 
______________________________________ 
Porcine (Pancreatic) 
2.00 26.7 
Porcine (Pancreatic) 
2.11 28.1 
Human (Pancreatic) 
1.95 26.0 
Human (Pancreatic) 
2.03 27.1 
Human (E. coli) 
1.99 26.5 
______________________________________ 
EXAMPLE 5 
A solution of each of human A- and B-chain S-sulfonates was prepared at a 
concentration of 5 mg./ml. in a 0.1 M glycine buffer (pH 10.5). The pH of 
each was adjusted to 10.5 with 5 N NaOH. DTT (61.7 mg.) was dissolved in 
4.0 ml. of 0.1 M glycine buffer (pH 10.5), and the pH was adjusted to 10.5 
with 0.16 ml. of 5 N NaOH. To 0.5 ml. of the B-chain solution was added at 
room temperature 1.0 ml. of the A-chain solution followed by 50 .mu.l. of 
the DTT solution to provide an --SH to --SSO.sub.3.sup.- ratio of 1.09. 
The resulting solution was stirred at 4.degree.-8.degree. C. in an open 
3-ml. vial for 22 hours after which HPLC analysis indicated a human 
insulin yield of 2.58 mg., or 34% of the total protein. 
EXAMPLE 6 
Human A-chain S-sulfonate (328 mg.) was dissolved in 65.6 ml. of 0.1 M 
glycine buffer (pH 10.5), and the pH of the mixture was adjusted to 10.5 
with 75 .mu.l. of 5 N NaOH. Human B-chain S-sulfonate (164 mg.) was 
dissolved in 32.8 ml. of 0.1 M glycine buffer (pH 10.5), and the pH of the 
mixture was adjusted to 10.5 with 15 .mu.l. of 5 N NaOH. DTT (61.7 mg.) 
was dissolved in 4.0 ml. of the 0.1 M glycine buffer (pH 10.5), and the pH 
of the solution was adjusted to 10.5 with 160 .mu.l. of 5 N NaOH. 
The A- and B-chain solutions were combined in a 150 ml. glass beaker at 
room temperature (.about.25.degree. C.), and 3.28 ml. of the DTT solution 
were added to provide an --SH to --SSO.sub.3.sup.- ratio of 1.09. The open 
beaker was placed in an ice-water bath in the chill room and stirred 
briskly for 30 minutes. The solution was stirred an additional 24 hours in 
the chill room (4.degree.-8.degree. C.). HPLC analysis at this time 
indicated a human insulin yield of 148 mg., or 30% of the total protein. 
To 100 ml. of this solution were added 25 ml. of glacial acetic acid to a 
final pH of 3.15. The entire sample was gel filtered on a 5.times.200 cm. 
column of Sephadex G-50 (Superfine) equilibrated and eluted with 1 M 
acetic acid at 4.degree.-8.degree. C. All of the eluted protein was 
lyophilized. The insulin peak (elution volume 2465-2781 ml.) weighed 125 
mg. and represented 29.4% of the recovered protein. 
A portion of the above insulin peak (95.5 mg.) was dissolved in about 9 ml. 
of 0.01 M tris-0.001 M EDTA-7.5 M urea-0.03 M NaCl buffer (pH 8.5 at 
4.degree. C.). The mixture was chromatographed through a 2.5.times.90 cm. 
DEAE (diethylaminoethyl)-cellulose ion-exchange column equilibrated in the 
same buffer. The protein was eluted at 4.degree.-8.degree. C. with a 
gradient of 1 liter each of 0.03 M and 0.09 M NaCl in the same buffer 
followed by 1 liter of buffer containing 1 M NaCl. Each of the peaks was 
desalted on Sephadex G-25 (course) columns in 2% acetic acid and 
lyophilized. The insulin peak (elution volume 878-1008 ml.) weighed 55.73 
mg. and represented 84% of the protein recovered. 
Zinc insulin crystals were made by dissolving 11.90 mg. of the insulin 
(DEAE) peak sample in 240 .mu.l. of 0.1 N HCl followed quickly by 2.16 ml. 
of a 0.04% ZnCl.sub.2 -0.05 M sodium citrate-15% acetone solution in a 
glass centrifuge tube. Crystallization proceeded for 72 hours at room 
temperature (.about.25.degree. C.) after which the supernatant was 
removed, and the crystals were washed twice with cold pH 6.1 water with 
centrifugation at 2000 rpm at 3.degree. C. between washes. The crystals 
were redissolved in 0.01 N HCl for analysis. 
The resulting human insulin preparation was judged to be quite pure by 
polyacrylamide gel electrophoresis (a single band), amino acid analysis, 
insulin radioreceptorassay, insulin radioimmunoassay, HPLC, dansylation 
and UV spectrum. The USP rabbit assay (144 rabbits) gave a potency of 
26.3.+-.1.8 units per mg. (anhydrous). 
EXAMPLE 7 
Human (E. coli) [N-formyl-Gly.sup.1 ] A-chain S-sulfonate and human 
(pancreatic) B-chain S-sulfonate solutions having a concentration of 5 
mg./ml. in 0.1 M glycine buffer (pH 10.5) were prepared. The pH of each 
was adjusted to 10.5 with 5 N NaOH. DTT (61.7 mg) was dissolved in 4.0 ml. 
of 0.1 M glycine buffer (pH 10.5), and the pH was adjusted to 10.5 with 
0.16 ml. of 5 N NaOH. To 0.5 ml. of the B-chain S-sulfonate solution were 
added 1.0 ml. of the [N-formyl-Gly.sup.1 ] A-chain S-sulfonate solution 
and 0.05 ml. of the DTT solution at room temperature (25.degree. C.) to 
provide an --SH to --SSO.sub.3.sup.- ratio of 1.10. The solution was 
stirred in a chill room (4.degree.-8.degree. C.) in an open 3-ml. vial for 
23 hours after which HPLC analysis indicated a [N-formyl-Gly.sup.1 -A] 
human insulin yield of 1.46 mg., or 19.5% of the total protein. 
After acidification to pH 3.15 with glacial acetic acid, a portion of this 
solution was gel filtered on a 1.5.times.90 cm. column of Sephadex G-50 
(Superfine) equilibrated and eluted with 1 M acetic acid at 
4.degree.-8.degree. C. The [N-formyl-Gly.sup.1 A] human insulin peak 
(elution volume 87-95 ml.) was pooled, aliquotted and lyophilized. This 
protein was judged to be quite clean by HPLC and amino acid analysis. The 
bioactivity of [N-formyl-Gly.sup.1 -A] human insulin evaluated by 
radioreceptor assay was 17% relative to a human insulin standard. 
EXAMPLE 8 
A solution of each of pork A-chain S-sulfonate and human (E. coli) B-chain 
S-sulfonate was prepared at a concentration of 10 mg./ml. in a 0.1 M 
glycine buffer (pH 10.5). An A-B pool was made using 2 ml. of the A-chain 
solution for each 1 ml. of the B-chain solution. The A-B pool was adjusted 
to pH 10.5 with 5 N NaOH. Cysteine (121.2 mg.) was dissolved in 3.0 ml. of 
0.1 M glycine buffer (pH 10.5), and the pH was adjusted to 10.5 with 0.35 
ml. of 5 N NaOH. To 1.4 ml. of the A-B pool was added at room temperature 
52 .mu.l of the cysteine solution to provide an --SH to --SSO.sub.3.sup.- 
ratio of 0.95. The resulting solution was stirred at 4.degree.-8.degree. 
C. in an open 3 ml. vial for 20 hours after which HPLC analysis indicated 
a human insulin yield of 3.25 mg., or 23.2% of the total protein.