Biopolymers to prevent cancer |

Background

Colorectal cancer is the third most common cancer in men and women worldwide. Almost 55% of cases occur in the more developed regions of the world, and recent scientific research suggests that the presence of excess iron in the large intestine can irritate and inflame the bowel, eventually leading to the onset of cancer. It is important to note that iron is essential for our diet; many of the processes carried out by our cells are catalysed by iron, including red blood cell production for carrying oxygen around the body. However, some of the iron that we consume through our diet, especially in diets rich in red meat, is not absorbed in the small intestine, and consequently this makes its way through to the large intestine.

Chemists and cancer scientists working with Chris Tselepis at the University of Birmingham are looking at ways to ‘mop up’ excess iron in the large intestine. The aim is to develop a drug that will bind iron in the large intestine, making it safe and non-cancerous, to prevent the onset of cancer. This is called ‘prophylactic‘ treatment. In order to achieve this, researchers are currently looking at a biopolymer, called alginate. This biopolymer binds iron, and also meets many of the requirements needed for a drug to be active in the large intestine.

journal-pone-0138240-g006 — Schematic illustrating the binding of iron to alginate, from the publication by R. Horniblow et al.

Exercise 1

Criteria required for drug design for colorectal cancer prevention

digestion_drugcriteria

A pdf version of this exercise is available here.

What fundamental characteristic are we looking for in order to create a drug to prevent colorectal cancer?
Thinking about the digestive system, and the different chemical environments that a drug will experience before it gets to the large intestine, what sort of things are important for the chemistry of the drug?
What other factors do you think are important when searching for a new drug, to be able to get the drug into clinical testing quite quickly?

Alginates

Alginates are an example of a structural biopolymer; structural biopolymers are polymers that are produced by living organisms, and provide plants with their strong stems, crustaceans with their hard shells, and our bodies with flexibility and movement. One of the most interesting things about biopolymers is that many of them form gels or viscous solutions in water. Gelatin is perhaps the most familiar structural biopolymer used in the food industry, which gives the chewy texture in many sweets, but alginates are also used as thickening agents to improve the texture of food. Alginate, in fact, is the most commn food additive – E400.

Alginates are polymers made from several hundred sugar (saccharides) monomers, and are produced by seaweeds (algae). The polymer is made up of two different sugar units, β-D-mannuronate (M) and α-L-guluronate (G). The segments are not random, but contain blocks of identical or alternating algstructure residues (MMMMMM, GGGGGG or MGMGMG). The strength of the polymer depends on this composition, which varies greatly between different species of seaweed and growth conditions as well as within different parts of the plant.

Alginates are potential cancer prevention drugs, because they can bind iron. However, they also bind calcium really well, and there is a lot of calcium in the body. Where there are sections of the alginate polymer with lots of ‘G’segments, they bind calcium ions particularly strongly, forming crosslinked ‘egg-box’ regions, forming a hard gel-type structure. This effect is used in the food industry, but in order to bind the iron in the large intestine, we do not want the alginate to fill all its binding sites with calcium ions. Therefore researchers at the University of Birmingham are interested in testing alginates that have lots of ‘M’ segments for preferentially binding iron.

Exercise 2

The structure of MMMMM and GGGGG alginate polymers

To download the pdf template for the alginate structures click here.

In order to demonstrate the difference in the structures of ‘MMMMM’ and ‘GGGGG’ regions of alginate polymers, print the following out onto paper, and cut along the black dashed lines. Fold along the red dashed lines in a concertina, cut the end of the strip so that it folds up to the end of the paper, and cut along the green dashed lines.

Open out the chains to see that the ‘GGGGG’ polymer sections can form ‘egg-box’ structures when linked together with calcium ions, where as the ‘MMMMM’ polymer sections are more linear in structure:

Alginate polymer structures, ‘egg-boxes’ between ‘GGGGG’ (orange) sections bound with calcium ions (top), and the more linear ‘MMMMM’ sections (blue, bottom)

The experiment

A pdf version of this experiment is available here. Please ensure you refer to the safety card. Details for teachers or technicians can be found here.

AIM

To observe the binding of 2 different alginates with solutions containing Ca(II) and Fe(III) ions, to determine which is most suited for an anti-cancer drug.

YOU WILL NEED

6 small beakers
plastic pipettes
solution ‘High G’ alginate (Manugel GHB) (aq) (plus food colouring)
solution of ‘High M’ alginate (ProtaSea AFH, or Manucol LD ) (aq) (plus food colouring)
0.1% w/v CaCl₂ (aq) solution
0.1% w/v FeCl₃ (aq) solution

PROCEDURE

Comparing the binding of calcium ions: Pour 25 mL of CaCl₂ solution into two beakers, and label them G and M. Using a plastic pipette, suck up 1-2 mL of the viscous ‘High G alginate’ solution, and pipette it dropwise into the CaCl₂ solution, in the beaker marked G. Repeat this with the ‘High M alginate’, in beaker M, and observe the difference – swirl the beakers for a few seconds. You can tip the gels into your hand, over a sink, to see the differences.

Comparing the binding of iron ions: Pour 25 mL of FeCl₃ solution into two beakers, and label them G and M. Do not get this solution on your hands as it is acidic and corrosive. Using a plastic pipette, suck up 1-2 mL of the viscous ‘High G alginate’ solution, and pipette it dropwise into the FeCl₃ solution, in the beaker marked G. Repeat this with the ‘High M alginate’, in beaker M. Gently swirl the solutions for a few seconds -what happens in each case?

Control experiment: Pour 25 mL of deionised water into two beakers, and label them G and M. Using a plastic pipette, suck up 1-2 mL of the viscous ‘High G alginate’ solution, and pipette it dropwise into the beaker marked G. Repeat this with the ‘High M alginate’, in beaker M. Gently swirl the solutions for a few seconds – what do these two solutions look like?

If you wish to conduct this experiment just to see the gelling of the ‘High G’-type alginates, it can be adapted using Sodium Alginate purchased from a chemical supplier, such as sigma aldrich, or a specialist food supplier; http://www.souschef.co.uk/sodium-alginate.html. Make the alignate solution up, adding solid to stirring water, until a viscous, but pourable consistency is reached. (The amount of alginate needed to be added will vary, in the range 0.5-3 g per 100 mL of water). More information can be found here.

QUESTIONS

What do the two alginates look like when they interact with calcium ions? Can you pick up the gel balls?
Which alginate binds more strongly with calcium ions?
What do the two alginates look like when they interact with iron ions?
Look at the control solutions, where you have just mixed the alginates with water – what happens? What does this confirm?
Which of the two alginates do you think will act best as a drug to bind iron, in a calcium-rich environment?
From left to right: High G alginate in water, Fe(III) solution, Ca(II) solution, blank Fe(III) solution, blank Ca(II) solution, High M alginate in water, Fe(III) solution, Ca(II) solution.

In the research lab

In the School of Cancer Sciences at the University of Birmingham, researchers, including Dr Richard Horniblow, working in the group of Dr Chris Tselepis, are currently testing a ‘high M‘ alginates in clinical trials. Read about this work in the Daily Mail article, and find out more about the group’s research here.

Publications relating to this work

plosone