TOPIC

JTCM |www. journaltcm. com April 15, 2014 |Volume 34 | Issue 2 |

Online Submissions: http://www.journaltcm.com J Tradit Chin Med 2014 April 15; 34(2): 234-237info@journaltcm.com ISSN 0255-2922

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REVIEW

Insulin sources and types: a review of insulin in terms of its modeon diabetes mellitus

Kafeel Ahmadaa

Kafeel Ahmad, Centre of Biotechnology and Microbiology,University of Peshawar, Peshawar 45320, PakistanCorrespondence to: Assistant Prof. Kafeel Ahmad, Centreof Biotechnology and Microbiology, University of Peshawar,Peshawar 45320, Pakistan. kafeelpbg@gmail.comTelephone: +92-3429143809Accepted: November 20, 2013

AbstractInsulin is involved in regulation of glucose utiliza-tion in the body. Inability of the body to synthesizeinsulin or human cells resistance to insulin leads toa condition called Diabetes mellitus which is char-acterized by chronic hyperglycaemia. There are twotypes of diabetes; type 1 and type 2. Exogenoussupply of insulin is needed consistently for type 1diabetes treatment and type 2 diabetes also needsto be cured by the exogenous supply of insulin inadvance stages of the disease. These sources havebeen proved very useful to meet the needs of thepatients. However, these insulin types are expen-sive for the large population of patients in the de-veloping countries. Furthermore, the incidence ofdiabetes is advancing at an alarming rate. Henceproduction systems with even higher capabilitiesof production are desired. Therefore, currentlyplants are being investigated as alternative produc-tion systems. Based on the mode of action of insu-lin various formulations of insulin have been devel-oped that have different onset of action, peak ef-fect and duration of action according to the needsof the patients.

© 2014 JTCM. All rights reserved.

Key words: Insulin; Diabetes mellitus; Insulin ana-logs

INTRODUCTIONInsulin is an important polypeptide hormone that regu-lates carbohydrate metabolism. Insulin is derived fromthe Latin word insula meaning "island" because thehormone is produced in the islets of langerhans. It wasdiscovered by Banting and Best in 1921-1922 at theUniversity of Toronto. It helps transport blood glucoseinto the body cells where the glucose is metabolized toproduce energy. It maintains glucose concentration inthe blood. When glucose concentration in the blood isincreased, insulin lowers it by increasing glucose up-take by muscle, liver and fat cells. Excess glucose is con-verted to glycogen by these tissues. When glucose con-centration is reduced in the blood, glycogen is convert-ed back to glucose and released in the blood. It is in-volved in regulating amino acid uptake by increasingDNA replication and protein synthesis. Insulin facili-tates fatty acid synthesis through the uptake of lipidfrom blood by fat cells. It also decreases proteinolysis,lipolysis and gluconeogenesis.Insulin is synthesized by β-cells of the pancreas in theform of a single chain of three peptides B, C and A inthe order: B chain-C peptide-A chain.1,2 This proinsu-lin is converted to mature insulin after the removal ofthe central C-peptide by the action of proteolytic en-zymes known as prohormone convertases PCl/PC3and PC2.3 The mature insulin consists of B-chain (30amino acids) and A- chain (21 amino acids) linked bytwo inter-chain and one intra-chain disulphide bridge.Its structure has been highly conserved among verte-brates.4-7

When the body is unable to produce insulin or thebody develops resistance against insulin, a metabolicdisorder called diabetes mellitus arises which leads to

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abnormal carbohydrate metabolism. Diabetes mellitusis treated by the exogenous supply of insulin. For thispurpose, various types of insulin have been developedwith different action times according to the needs ofpatients. In this review, insulin sources and types weresummarized in terms of its action mode.

Treatment of diabetes mellitusThe condition refers to a metabolic disorder character-ized by chronic hyperglycaemia in which the patient ex-periences disturbances in metabolism of carbohydrate,fat and protein as a result of deficiency in insulin secre-tion, insulin action or both.8 Diabetes is the fifth-lead-ing causes of mortality in most developed countriesand there are thoughts to be about 246 million peoplesuffering from diabetes.9 It has been estimated that by2025 there will be nearly 380 million people sufferingfrom diabetes if not controlled.9

There are two types of diabetes mellitus i.e. type 1 andtype 2 diabetes mellitus. Type 1 diabetes is caused bythe destruction of β-cells of pancreas that are involvedin insulin synthesis. The destruction is caused by an au-to-immune reaction where these cells are destroyed bythe body's own defence system. As a result, very littleor no insulin is produced and hence carbohydrate, espe-cially glucose, metabolism is disturbed leading to thecharacteristic symptoms. Type 1 diabetes is also knownas insulin dependent, juvenile or immune- mediated di-abetes. The causes of type 1 diabetes are still largely un-known; however, both genetic and environmental fac-tors have been implicated. Children of parents withtype 1 diabetes have an increased risk of developing thedisease. Important environmental factors include obesi-ty, diet, physical inactivity and viral infection. Type 1diabetes mostly affects children and young adults. Pa-tients suffering from type 1 diabetes consistently re-quire insulin treatment (via injection or oral adminis-tration) for their continued survival.Type 2 diabetes occurs as a result of insulin resistanceand relative insulin deficiency. Insulin resistance refersto the condition where the insulin is not very effectivein reducing the blood glucose levels. Type 2 diabetescan appear at any stage of life; however, it is mostly di-agnosed in adults at the age of 40 or above. Symptomsof type 2 diabetes involve frequent urination, constantthirst, increased hunger, weight loss and tiredness. Liketype 1 diabetes both genetic and environmental factorscontribute to the development of type 2 diabetes. Ge-netic factors leading to type 2 diabetes are not well un-derstood, while potential environmental factors in-clude physical inactivity, obesity, diet and aging. Pa-tients with type 2 diabetes generally do not need to beadministered exogenous insulin; however, in extremecases of hyperglycemia administration of exogenous in-sulin might be required. Treatment generally involvescontrolling blood glucose, blood pressure, lipids, chang-ing lifestyle through exercise and medication.Treatment of diabetes involves regular monitoring of

glucose levels in the blood so as to provide insulinwhen required to prevent hyperglycaemia. Necessarychanges in lifestyle and diet are also required to keep di-abetes in control. Insulin is primarily given in the formof injections; however, recent advances in drug deliveryhave included pulmonary, oral and nasal administra-tion routes which have been developed to overcomethe inconveniences associated with regular insulin injec-tions.10,11

INSULIN SOURCES: PLANTS ASALTERNATIVE FOR MASS SCALEPRODUCTIONSince the 1920s, insulin has been used for treatment ofdiabetes. The initial sources of insulin were from bo-vine, porcine, and equine. Methods to convert porcineinsulin into the human insulin equivalent were devel-oped in the 1970s and early 1980s.12,13 With the devel-opment of genetic engineering and modern biotechnol-ogy, efforts were shifted towards the production of insu-lin through recombinant DNA technology. Human in-sulin was one of the first pharmaceutical proteins thatwere manufactured through recombinant DNA tech-nology in the late 1970s, which has been marketedsince 1982.Since 1980s, recombinant insulin from bacteria14 andyeast.15 has been the main source of commercial insu-lin. These systems have been helping to meet the de-mand of this important pharmaceutical since this time.However, the incidence of diabetes is increasing andthis demand for insulin cannot be provided by thesesources in the near future due to the scale and costs in-volved. It has been estimated that the demand for insu-lin will double in the next 10 years.16 Developing coun-tries are facing further challenges as patients in thesecountries are not able to afford the cost of this expen-sive drug. Furthermore, alternate delivery systems suchas oral, inhalable and buccal forms which aim to pro-vide relief to patients from the painful insulin injec-tions require a higher amount of insulin to be effectiveas a treatment as these methods are not very efficient.16

In view of the increasing demand for insulin, currentlyplants are being investigated as factories for the produc-tion of insulin which have the potential for a highyielding and cost effective method of production forthis important pharmaceutical.17-19 In an attempt toachieve this goal, Arakawa et al 17 produced a choleratoxin B subunit-insulin fusion protein in transgenic po-tato tubers as an autoantigen against insulin-dependentdiabetes mellitus. The fusion protein accumulated to0.1% of total soluble protein. The transgenic tobaccotuber tissues were fed to diabetic mice which helped inthe reduction of insulitis and delayed the onset of dia-betes. Nykiforuk et al 18 expressed the desB30 form ofhuman insulin (lacking threonine at position B30) asan oleosin-insulin fusion protein in Arabidopsis thali-

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ana. The fusion protein could be subsequently purifiedby the oleosin fusion technology to reduce the cost ofproduction. They replaced the central c-peptide of hu-man insulin with a trypsin cleavable mini c-peptide(AAK). The purified desB30 insulin could be enzymati-cally matured through trypsin treatment to remove thecentral mini c-peptide. Mature insulin accumulated up-to 0.13% of total seed protein. Biological activity ofthe isolated insulin was demonstrated by an insulin tol-erance test in mice and phosphorylation assay in amammalian cell culture system. Boothe et al 16 utilizeda similar strategy to produce the oleosin-insulin fusionprotein in safflower seeds for industrial exploitation ofthe system. The safflower produced insulin is currentlyunder clinical trials with promising initial results.16

Ruhlman et al 19 produced cholera toxin B subunit-hu-man proinsulin (CTB-Pins) fusion protein in chloro-plasts of transgenic tobacco and lettuce with a view ofdeveloping an oral delivery system for insulin to targetinsulitis. Eight milligrams of leaf powder from trans-genic tobacco expressing the fusion protein were fed tonon-obese diabetic mice for seven weeks. After thestudy, normal blood and urine glucose level were ob-served in CTB-Pins treated mice with reduced insulitisand reduced damage of β-cells of pancreas. Research iscurrently being conducted to optimize plant produc-tion systems for achieving the goal of high level produc-tion capability and much cheaper availability of insu-lin. Hopefully, in the near future we will see plantbased insulin easily available in the market to meet thedemand of the masses.

INSULIN TYPES ACCORDING TOMODE OF ACTIONHealthy humans produce insulin at a basal level. Thesecretion of insulin increases to a peak level 1 h aftereating and dropping back to normal level after a fur-ther 2 h. To achieve a normal 24 h insulin profile andavoid nocturnal hypoglycaemia in diabetic patients, asingle insulin formulation with a specific onset of ac-tion, peak effect time and duration of action cannot beused. Normally, when insulin is injected into the body,the insulin molecules form hexamers. For diffusionthrough interstitial fluid and penetration into the capil-lary walls to enter the bloodstream these hexamersneed to dissociate into dimers and monomers.20 There-fore, different formulations of insulin with different on-set of action, peak effect and duration of action havebeen developed to meet specific needs of patients.These formulations change the rate of dissociation ofhexamers into dimers and monomers and the resultingmovement of free insulin molecules in the bloodstream. These different types of insulin formulation aredescribed in the following sections.

Rapid acting insulinThese insulin analogs have a more rapid onset of ac-

tion (15-30 min) and shorter activity duration (4 to 5 h).Their peak action ranges from 30-90 min post injec-tion. By a single or two amino acid alterations in theinsulin molecule, the ability to associate into hexamersis reduced such that they are readily absorbed, howev-er, these modifications do not change the biologicalproperties of these analogs.21,22 Examples of rapid actinginsulin include Lispro and Aspart. In Insulin Lispro(LysB28, ProB29), the positions of proline at positionB28 and lysine at position B29 in the B chain havebeen reversed.21 In insulin Aspart, the proline at posi-tion 28 has been replaced by aspartic acid.22 These rap-id acting analogs can be used at mealtime to achieveoptimum level of insulin for utilization of glucose re-leased after eating.

Short acting insulinShort acting insulin analogs have an onset of action ofaround 0.5-1 h, peak action of 2-4 h and activity dura-tion of 6-8 h. Examples of these preparations includeActrapid, Humulin, Hypurin and Neutral. These insu-lin analogs should be injected into the body 20-30 minbefore meal so as to get optimum insulin activity forcarbohydrate metabolism.

Intermediate acting insulinIntermediate acting insulin analogs have an onset ofaction around 1-2 h, peak action of 6-10 h and activ-ity duration of 10-16 h. Examples of intermediateacting insulin include NPH (Neutral ProtamineHagedorn) and LENTE (from the Latin "lentus,"meaning slow, or sluggish) insulin. The absorptionrate of NPH insulin is reduced by the addition ofprotamine to the insulin preparation. In insulinLENTE, the same is achieved by the addition of zincto the insulin preparation.

Long acting insulinThese insulin analogs have an onset of action around 2 h,peak action (sometimes no peak action) of 6-20 h andactivity duration of up to 36 h. One way to prolong in-sulin activity is designing analogs with more positivelycharged amino acids so as to raise the isoelectric pointof insulin to near neutral pH.23 This helps in reducingthe solubility of insulin at neutral pH after injection in-to the body and the absorption into the blood streamwill be delayed. Some of the long acting insulin prepa-rations also have protamine or zinc added to them toincrease absorption time. Insulin detemir, also calleddesB30 insulin, is an example of long acting insulin. Ininsulin detemir, the threonine at position B30 in the Bchain is removed and a 14-C fatty acid i.e. myristic ac-id is attached to the lysine at position B29 in the Bchain.15,16 Attachment of myristic acid helps in insulinhexamer formation and increases the binding of insulinto plasma albumin which delays the free insulin releaseand which prolongs the activity of insulin.24

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CONCLUSIONIncidence of diabetes is increasing at an alarming rate.Exogenous supply of insulin is needed consistently fortype 1 diabetes treatment and type 2 diabetes alsoneeds to be cured by the exogenous supply of insulinin advance stages of the disease. The current sources ofinsulin will not be enough in the near future to pro-vide insulin to the vast majority of patients especiallyin the poor countries. Hence, alternative productionsystems like plants need to be investigated to meet thedemands of near future.

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