Insulin Therapy - Properties And Products W3 Flashcards
Chemical structre of insulin
- 2x peptide chains
- chain A = 21 amino.a.residues
- chain B = 30 amino.a.residues
- chains linked by disulphide bridges between A7+B7 and A20+B19
- chain A is also internally bridges between A6+A11
Bovine insulin
derived from cows and has three amino acid differences from human insulin, making it more likely to cause immune reactions when administered.
Porcine insulin
Animal derived - pork
No longer avaliabe as causes allergic reactions
Secondary structure of insulin
- alpha helix
- three segments
- intra-molecular H-bonding between amide groups
Tertiary structure of insulin
- disulphide bridges
Quaternary structure of insulin - dimer
- in solution, exists as dimers
- anti-parallel b sheet formed between B23 and B30, with B28 proline which is important in hydrophobic interactions
- stabilised by H-bonding involving B24 and B26
Quaternary structure of insulin - hexamer
- two zinc ions and three insulin dimers
- globular
- interior is mainly non-polar amino acid side chains
- exterior is mainly polar amino acid side chains
- T (extended B1 to B8) and R (a-helix B1 to B8) confirmations
- R state found in presence of phenol or cresol
- R state more stable
Physical properties of insulin
• White to almost white powder
• Can be amorphous or crystalline
• Practically insoluble in water, in ethanol and in ether
• Dissolves in dilute mineral acids
• Dissolves with decomposition in dilute solutions of alkali hydroxides
Source and manufacture of insulin - porcine and bovine
• Sometimes called “natural insulins”
• Pig or cow pancreas from abattoir
- animals must be suitable for human consumption
- free from transmissable spongiform encephalopathies (TSE)
• Long and involved purification process
- extraction
- purification
- Analysis
Source and manufacture of insulin - semi synthetic human
• Chemically identical to human insulin
• Enzymatically modified porcine insulin
- generate purified porcine insulin as normal
- B30 residue changed from alanine to threonine by action of enzyme
• Label must state “emp”
Source and Manufacture of Insulin - biosynthetic human insulin
• Chemically identical to human insulin
• Recombinant DNA technology
• Simplified procedure:
- identify gene that codes for insulin in humans
- insert this gene into the plasmid of a bacterial or fungal vector (eg Escherichia coli or saccheromyces cerevisiae)
- grow the modified bacterium or fungus
- harvest and purify the human insulin
• Label must state the method of preparation
- “prb” or “crb” = purified recombinant bacterial
- “pyr” = purified yeast recombinant
Expression of strength
• Use “International Units” (IU)
• One unit = 0.0345 mg porcine insulin
= 0.0342 mg bovine insulin
= 0.0347 mg human insulin
General points of injectable insulin
• Insulin is a protein
- unstable in GI tract
• Generally given by sc injection
- occasionally im
- iv in emergencies
• Injection sites
- thighs, upper arms, buttocks, abdominal wall
- need to rotate sites
• Absorption faster from abdominal wall
• Exercise and heat will increase rate of absorption
Insulin Injectable Formulations - soluble
Also called “neutral insulin”
- insulin is all in solution in the dimer form
Insulin Injectable Formulations - zinc susp
- Crystalline / Amorphous / Mixed (70:30 C:A)
- insulin is all as “solid” particles
Insulin Injectable Formulations - isophane
- insulin is complexed with protamine
- protamine sulphate = the sulphates of the basic peptides extracted
from the sperm or roe of fish, usually salmon or herring
Insulin Injectable Formulations - protamine zinc
- insulin is complexed with both protamine and zinc
Insulin Injectable Formulations - biphasic isophane
- mix of protamine insulin and soluble insulin
Biological importance of physico-chemical properties
• Insulin molecules need to be in solution and in the monomer form
for biological action
• Physical state will affect dissolution from solid into solution
- amorphous form will act faster than the crystalline form
- size of the crystals will affect the dissolution rate and onset of action
• Dissociation from the hexamer and the dimer into monomer affects
the rate of onset of action
• Presence of additives affect the dissolution rate
- zinc reduces the dissolution rate of the insulin complex
- protamine reduces the dissolution rate of the insulin complex
Onset of action, peak action and duration of action - soluble
30-60mins
2-4hrs
8hrs
Onset of action, peak action and duration of action - zinc susp crystalline
4-6hrs
8-24hrs
28-36hrs
Onset of action, peak action and duration of action - zinc sup amorphous
1-2hrs
2-4hrs
12-16hrs
Onset of action, peak action and duration of action - zinc susp mixed
1-3hrs
6-12hrs
18-24hrs
Onset of action, peak action and duration of action - isophane protamine
1-2hrs
4-12hrs
24hrs
Onset of action, peak action and duration of action - protamine zinc
4-6hrs
10-20hrs
24-36hrs
New human insulin analogues
Insulin Aspart (NovoRapid® Novo Nordisk Ltd) - similar 2 insulin apart form the B28
• Produced using recombinant yeast (Saccheromyces cerevisiae)
- labelled as “recombinant human insulin analogue”
• B28 proline in human insulin is replaced by aspartic acid
New Human Insulin Analogues - insulin aspart
• Aspartic acid is negatively charged at physiological pH whereas
proline is non-ionisable
- increased charge repulsion between neighbouring molecules
- self aggregation reduced
- molecules act more independently
- molecules diffuse more rapidly to give quicker clinical effect
• Receptor interaction unchanged cf human insulin
• SC PK
- approx. 10 to 20 minutes onset of action
- peak action between approx. 1 and 3 hours
- duration of action approx. 3 to 5 hours
New Human Insulin Analogues - Insulin Glulisine (Apidra® Sanoif)
• Produced using recombinant bacteria (Escherichia coli)
- labelled as “recombinant human insulin analogue”
• B3 asparagine in human insulin is replaced by lysine
• B29 lysine in human insulin is replaced by glutamic acid
New Human Insulin Analogues - Insulin Glulisine
• Ionisation potential (pKa) changed cf native insulin
- increased charge repulsion between neighbouring molecules
- self aggregation reduced
- molecules act more independently
- molecules diffuse more rapidly to give quicker clinical effect
• Receptor interaction unchanged cf human insulin
• SC PK
- approx. 5 to 10 minutes onset of action
- peak action between approx. 1 hour
- duration of action approx. 2 to 4 hours
