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Proinsulin is a biosynthetic precursor of insulin, it was discovered by Steiner in 1967.
Insulin is synthesized in the cells of the islets of Langerhans as a single chain precursor. Proinsulin, has a molecular weight of 9082. It contains 84 aminoacid residues the sequence of which is known. Studies using cell free systems indicate that the immediate translation product of proinsulin, messenger RNA is a larger peptide of 11,500 dalton containing additional 23 amino-acid sequences at the amino- terminal. This precursor has been designated preproinsulin. It is converted to proinsulin by microsomal protease's within minutes of its synthesis. The intracellular sites where pre-proinsulin is synthesized and rapidly cleared to proinsulin are the polysomes of the rough ER. Proinsulin is then transferred by an energy dependent process to the Golgi apparatus. Secretory granules are formed in the Golgi complex. Beginning within the Golgi complex & continuing in the secretory granules, membrane bound specific protease's cleave proinsulin into equimolar amounts of insulin & C peptide. The insulin along with zinc accumulates within the central core of the maturing secretory granule which becomes progressively more electrodense, the C peptide is localized in the peripheral clear space of the secretory granule. Although proinsulin cross reacts to a small degree with antibodies to insulin it has only 3-7% of the biological effectiveness of native issulin. The conversion of proinsulin to insulin begins in the Golgi complex, continues in the secretory granules & is nearly complete at the time of secretion. Thus equimolar amounts of C peptide & insulin are released into the circulation. The C peptide has no known biological function but it can serve as useful index of insulin secretion. Small quantities of proinsulin are also released from Beta cells. This presumably reflects either exocytosis of granules in which the conversion of proinsulin to insulin is not complete or secretion by another pathway.
Two distinct Ca2+ dependent endopeptidases which are found in islet cell granules and in
other neuroendocrine cells are responsible for the conversion of proinsulin to insulin. These endoproteases PC2 & PCS have catalytic domains related to that of subtilisin and cleave at Lysine-Arginine or Arginine-Arginine sequences. PC2 selectively cleaves at the C peptide - A chain junction. PC3 preferentially cleaves at the C peptide-B chain junction but has some action at A chain junction members of the family of endoproteases. PCI & furin, PC2 &L PC3 appear to be the enzymes responsible for processing proirisulin to insulin.
The half life of insulin in plasma is 5 -6 minutes in normal subjects. The half life of
proinsulin is longer than insulin about 17 minutes and this protein accounts for about 10% of "immunoreactive" insulin in plasma. In patients with insulinoma, the percentage of proinsulin in the circulation usually is increased & may be as much as 80% of immunoreactive insulin in plasma. Since proinsulin, is only about 3-7% as potent as insulin the biologically effective concentration of insulin is somewhat lower than estimated by immunoassay. In familial hyper proinsulinemia an asymptomatic genetic defect with autosomal dominant inheritance. 65 - 90% of total plasma insulin imrnunoreactivity is accounted for by promsulin. This defect probably represents an abnormality in an intermediate formed on cleavage of proinsulin. A mutation affecting the dibasic aminoacids arginine & lysine which link the C peptide to the A chain of insulin impairs the conversion of proinsulin to insulin.
Increased plasma levels of proinsulin are also seen in
CRF
Hyperthyoidism
Hypokalemia
Cirrhosis Acromegaly
Obesity
Cross Reactivity with C-peptide
Because the C-peptide sequence is a part of the proinsulin molecule antibodies against
C- peptide cross react with proinsulin that may be present in a sample. Under normal circumstances the contribution of proinsulin to the estimated C- peptide reactivity is to the nine of 3% which is often ignored. However, in situations where the amount of proinsulin in the blood is increased one would need to take this aspect into consideration. The proinsulin in these cases can be removed from the sample with the use of antiinsulin antibodies coupled to sepharose beads.
Estimation of C-peptide is complicated when patients receive exogenous insulin and
therefore may have antiinsulin antibodies. These are known to bind to proinsulin thus prolonging its half life and markedly increasing the contribution of the estimated C- peptide reactivity. Under these circumstances it would be necessary to precipitate the insulin antibodies (eg. With the use of polyethylene glycol) which would also remove the bound proinsulin leaving the C peptide free in the supernatant.
Therapeutic role of Proinsulin
Initial trials with HPI were encouraging. Animal studies using Porcine proinsulin indicated
that the compound was a soluble intermediate acting insulin that had a suppressive effect on hepatic glucose production. This profile of action appeared favourable for clinical use in diabetic subjects. Since unrestrained hepatic glucose production is a hallmark of the disease & the hepatospecific insulin would tend to reduce peripheral hyperinsulinemia & the attendant risk of hypoglycemia. Early studies with HPI in human beings confirmed its relatively hepatospecific action and demonstrated that its duration of action was similar to that of NPH insulin. Although all clinical studies were soon suspended because of high incidence of MI in HPI treated patients. The artificial human proinsulin is now available from recombinant DNA technique. It has several unique features like -
It has circulation half life 4-6 times longer than that of human insulin.
Unlike NPH and Lente formulation of human insulin in that it provides prolonged insulin
action without need of fish protamine additives or larger quantities of zinc acetate to precipitate insulin.
Some reports indicate that HPI is less immunogenic and has more reproducible absorption
characteristic than precipitated formulation of human insulin.
Current consensus, suggest a controversial role of HPI in DM management.
Secretory products from pancreatic Beta cells rather than insulin could be responsible for
association of atherosclerosis in patients with insulin resistance, obesity & diabetes. Proinsulin has structural similarities to circulating growth factors & has been suspected of increasing risk for coronary artery diseases when administered in therapeutic doses that produce very high circulating levels. At present there is no hard evidence to incriminate the elevated levels of insulin found in many obese NIDDM patients as contributing to the prevalence of atherosclerosis in syndrome X. However, if excessive secretion of proinsulin is detrimental, its raises the possibility that exogenous insulin therapy could conceivably join this list of drugs that lower blood glucose without increasing beta cells secretion. Exogenous, insulin therapy has been shown to suppress hyperproinsulinemia in patients with NIDDM. Further, studies are needed to determine whether "insulin sparing" is a desirable goal for hypoglycemic therapy in patients with syndrome X and if it turns out to be beneficial whether insulin itself was deleterious or possibly some other products of hypersecreting beta cells such as proinsulin is harmful.
REFERENCES
1. Stephen N Davis & Darryl K Granner : Insulin, OHA & the pharmacology of the
Endocrine pancreas in Goodman & Gilman's. The pharmacological basis of
therapeutics. Ninth edition Me. Graw Hill 1996 : 1487 - 1518.
2. Eleazer Shafrir, Michael Bergman, Philip Felig : The endocrine pancreas Diabetes
Mellitus in endocrinology &: metabolism. 2nd Edition Me. Graw Hill 1043 - 1178.
3. John H. Karan Type II Diabetes & Syndrome X Endocrinology & metabolism clinics
of North America. Jume 1992, 329 - 350.
4. John H. Karam : Pancreatic hormones & antidiabetic Drugs in Basic & clinical
pharmacology Appleton Lange, 6th Edition 1995, 637 - 654.
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