M.Prasad NaiduMSc Medical Biochemistry,

Ph.D.Research Scholar

PANCREASBoth endocrine and exocrineEndocrine function is localized in the

islets of Langerhans, epithelial cells that are dispersed throughout the entire organ.

3 distinct types of cells namely AlphaGlucagonBeta InsulinGama not knownDelta secretes & stores somatostatin

Insulin -- IntroductionPlays imp role in metabolism causing Increased Carbohydrate MetabolismGlycogen storageFatty acid synthesisAmino acid uptake and Protein synthesisThus imp anabolic hormone which acts on

variety of tissues including liver, fat and muscle.

chemistryProtein hormone produced from the islets of

Langerhans which make up1% of the pancreas.Crystallization requires Zn , which is a constituent

of stored insulin since normal islets are relatively rich in this element.

Insulin is made up of 51 AAs arranged in 2 PP chains(A&B) having a mol.wt of 5734.

2 inter chain and 1 intra chain –S-S- bondsInsulin can not be administered orallyThe pork pancreas is the most similar to human

insulin. The 2 insulins diff only in the terminal AA no of B chain(30)

This terminal AA in porcine insulin is Ala and Thr in Hum.

ChemistryThis terminal AA can be removed easily with CP.

(retains the biologic activity)Thus it is possible to convert porcine insulin into a

Human ( 1 AA shorter)Insulins from the pig, whale and dog are structurally

identical.Those from the sheep, horse and cow differ form

porcine insulin only in 3 AAs under the –S-S brige in the A chain.

Other species may differ in as much as 29 out of the 51 AAs.

Two structurally diff insulins – isolated – a single rat pancreas, diff by a single AA(Lys/Met) in the A chain.

The pancreas of certain fishes – more than one insulin

Structure of InsulinDespite the wide variation in 10 structure , the bio.

Activity per unit wt is remarkably constant for all insulins.

It exists in Zn crystals as hexamers and dimers. When diluted in the circulation, insulin exists as a

monomer.The m-RNA for preproinsulin has been isolated

from islets, the complementary DNA synthesized and active insulin has been sucessfully cloned in bacteria.

Therefore insulin is the first high mol.wt hormone to be made by the process of molecular engineering.

Structure of InsulinThe 20&30 structures of Bovine insulin have

been determined by X-ray crystallography.These studies indicate that the A chain

portion of the molecule is the more exposed, including the 6-11-S-S- bond possibly involved in hormonal activity.

The B chain in in the internal portion of the molecule.

Non covalent binding b/n B chain is respible for the formation of insulin dimer & higher polymers.

Structure of InsulinRemoval of the terminal octapeptide on B

chain / removal of Asn on A chain inactivates insulin.

Removal of terminal carboxy AA of the B chain and some AAs in the N-ter portion of the molecule does not affect the activity.

The –OH groups of Ser, Thr can be modified by sulfation with little loss of potency.

Destruction of His by photo-oxidation results in complete inactivation.

Increasing iodination of Tyr causes progressive inactivation.

Structure of InsulinSulfated bovine insulin, porcine insulin and

fish insulins are sometimes used in the resistant diabetics because of their reduced antigenicity and cross-reactivity with circulation antibody.

Other modifications reduce the absorption of insulin from the injected sites, resulting in prolonging the axn of insulin.

Structure of InsulinProtamine Zn insulin is a combination of

insulin with protamine which is absorbed more slowly than the ordinary insulin.

1 injection of protamine Zn insulin may lower the blood glucose for more than 24hrs, whereas 2or3 injections of regular (crystalline) insulin might be required for the same effect.

Structure of InsulinGlobin insulin , combination with a

protein has an effect of 12-15 hr duration of action

Ultralente insulin is a slow-acting insulin prepared by controlled crystallization in the presence of high conc of Zn and acetate in order to produce large crystals which are therefore slowly absorbed.

Lente insulin is a 7:3 mixture of ultralente and regular insulin which has a duration of effect between the two.