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HomemieyecareTear Film Proteins Balance

Tear Film Proteins Balance

Tear proteins are essential to protect the eye against bacteria and maintain comfort; however contact lenses can cause these proteins to denature over time. Scientists are reflecting on the natural eye environment to inspire new ways to care for lenses in vivo.

For decades, the effects of tear proteins adhering to the surfaces and also the internal structure of contact lenses have been explored. Proteins from the tear film can deposit on a soft contact lens within a matter of hours.1 The clinical impact of this is reduced acuity,2 comfort3,4 and wettability5 and increased inflammatory complications such as papillary conjunctivitis.6,7 and acute red eye.8 Given that the role of tear proteins includes defending the eye against bacteria (e.g. Streptococcus and Staphylococcus) and also contribution to the surface tension of the tears which impacts on the natural wettability of the ocular surface, scientists are looking for ways to maintain the natural proteins in the ocular environment, removing only the unwanted ‘changed’ or ‘denatured’ proteins attached to the contact lens.

Hundreds of Proteins

To date, more than 400 proteins have been identified in human tears. Of these, four key proteins are found in significant concentrations: Lysozyme; Lipocalin; Lactoferrin; and Secretory IgA

These proteins are all produced in the lacrimal gland and mixed into the tear film. Research has shown that bacteria attach differently to worn and unworn lenses, with worn lenses reducing the numbers of viable bacteria.9

When proteins attach to the hydrophobic surface of a contact lens, the organisation of the protein begins to… denature

Given the antimicrobial characteristics of the tear film’s naturally produced proteins, it could be considered that the presence of these on or within a contact lens may be desirable.

The Main Proteins

Lysozyme

Lysozyme chemically attacks the external structures of bacteria cells, causing the bacteria to die. Lysozyme is a relatively small protein which carries a high positive charge, and this is the reason why it is more readily deposited on certain contact lens materials that have a negative charge (namely ionic lenses whose materials contain methacrylic acid also known as Group IV materials).

Lipocalin

Lipocalin enhances the performance of lysozyme and contributes to the surface tension of the tears.10 Tears contain fatty acids which can deactivate lysozyme, and lipocalin readily binds with fatty acids to help maintain the antimicrobial activity of lysozyme.11

Lactoferrin

Bacteria need iron to be able to grow in numbers, and lactoferrin binds to free iron in the tear film to reduce its availability to bacteria, thereby inhibiting growth. Alone, lactoferrin binds to the cell membranes of certain species of bacteria, e.g. Streptococcus, Staphylococcus and Pseudomonas, thereby inhibiting their growth. In conjunction with lysozyme, lactoferrin has been shown to have activity against Staphylococcus Epidermis12 indicating synergy between these two proteins.

Secretory IgA

Where lysozyme, lipocalin and lactoferrin production are linked to the production of water by the lacrimal gland, the mechanism of secretory IgA release is different. As the watery component of tear volume reduces overnight, the other proteins reduce in concentration where secretory IgA continues to be produced. Secretory IgA protects the eye by preventing bacteria sticking to the ocular surface, as well as ‘coating’ the offending bacteria in molecules causing attraction of, and destruction by, polymorphonuclear white blood cells in the tear film.

Proteins are 3D

Proteins are long chains of amino acid molecules, made up from the basic elements of carbon, hydrogen, nitrogen, oxygen and sulphur. Their individuality is dictated not only by the order of the molecules in a line, but also how these chains are then bound together and arranged into a 3D structure. The 3D structure is defined by the environment the protein resides in. In ‘water’ the hydrophobic portions of the protein turn inward and the hydrophilic portions are on the outside of the 3D structure.

When proteins attach to the hydrophobic surface of a contact lens, the organisation of the protein begins to change, that is, denature. Other factors also influence their natural state such as pH, heat and the presence of salts.

Keeping Tears ‘Natural’

The natural biology of the tears produces 3D proteins such as lysozyme which serve to protect the eye against micro-organisms. Denaturation of lysozyme, affecting its natural 3D shape, has been shown to reduce its bactericidal action.13 Given that lysozyme constitutes about 20 to 40 per cent of the total protein found in tears, steps to limit the denaturation process could be considered desirable if it is to maintain its protective behaviour. The complex molecular structure of proteins must be sustained if they are to continue to retain their optimal antibacterial activity.

An understanding of the interactions of multipurpose lens care solution (MPS) ingredients with tear film components is important because the ingredients of MPS contact the eye during the insertion of contact lenses that have been cleaned, disinfected, and stored in the solutions. MPS may affect the tear film components by impacting the protein denaturisation process and subsequent deposition on contact lenses

A recent in vitro study investigated the ability of a novel Bausch & Lomb investigative multi-purpose (MPS) solution and four commercially available MPS to stabilise a protein (lysozyme) from denaturation.14 The results revealed that there were significant differences in the ability of MPS to help maintain the tear protein lysozyme in its native state. It is apparent that the composition and physical properties of MPS can affect their interaction with lysozyme.

Test Solutions: Five MPS (one novel, and four commercially available) were investigated.

MPS A: B+L novel MPS* (borate/poloxamine)

MPS B: borate/citrate/poloxamine

MPS C: borate/citrate/poloxamine

MPS D: tromethamine/phosphate/poloxamer

MPS E: phosphate/poloxamer

% of Lysozyme Stabilized by multipurpose solutions

How Are Proteins Cleaned Away?

Once proteins have denatured on the surface of a lens, their associated deleterious effects on vision and comfort deems their removal desirable. However, consideration is now being made to the ways in which their positive natural antibacterial state can be maintained. Select ingredients have been developed which can remove denatured proteins and help maintain the balance of the natural proteins on the lens. This protein removal chemistry utilises both ionic charges of the molecules and Van der Waals forces to lift off loosely attached denatured proteins like a gentle magnet.

Impact on Comfort and Wearer Satisfaction

Comfort is the significant goal of contact lens wear, and it is one of the most significants factor in long-term wearer success. Reusable lenses are cared for to prolong their life and on-eye wearability over their recommended interval of use, and contact lens care products strive to maintain a ‘fresh lens feeling’ over this time. Reflecting on the natural eye environment is inspiring new ways to care for lenses in vivo.

Dr. Susan E. Burke received a PhD in Physical Chemistry from McGill University in Canada. She is an inventor of several patent applications and has co-authored more than 20 peer-reviewed journal articles. She is currently Principle Scientist, Global Vision Care Research Development with Bausch & Lomb in the United States.

References

1. Eones, L. et al., An In Vivo Comparison of the Kinetics of Protein and Lipid Deposition on Group II and Group IV Frequent-Replacement Contact lLenses. Optometry and Vision Science: Official Publication of the American Academy of Optometry 2000; 77: 503-510.

2. Gellatly K.W., Brennan N.A., Efron N. Visual Decrement with Deposit Accumulation of HEMA Contact Lenses. Am J Optom Physiol Opt 1988; 65:937 41.

3. Nilsson S.E., Andersson L., ‘Contact Lens Wear in Dry Environments’. Acta Ophthalmol (Copenh) 1986; 64:221-5.

4. Pritchard N., Fonn D., Weed K., ‘Ocular and Subjective Responses to Frequent Replacement of Daily Wear Soft Contact Lenses’. CLAO J 1996; 22:53-9.

5. Bleshoy H., Guillon M., Shah D., Influence of Contact Lens Material Surface Characteristics on Replacement Frequency. ICLC 1994;21: 82-93.

6. Grant T. et al, Contact Lens Related Papillary Conjunctivitis (C.L.P.C): Influence of Protein Accumulation and Replacement Rrequency. Invest Ophthalmol Vis Sci 1989; 30(suppl.):166.

7. Porazinski A.D., Donshik P.C., Giant Papillary Conjunctivitis in frequent Replacement Contact Lens Wearers: A Retrospective Study. CLAO J 1999;25:142-7.

8. Kotow M., Holden B.A., Grant T., The Value of Regular Replacement of Low Water Content Contact Lenses for Extended Wear. J Am Optom Assoc 1987;58:461-4.

9. Williams, T.J., Schneider, R.P. & Willcox, M.D.P., The Effect of Protein-Coated Contact Lenses on the Adhesion and Viability of Gram Negative Bacteria. Curr Eye Res 2003; 27: 227-235.

10. Nagyova, B., Tiffany, J.M., Components Responsible for the Surface Tension of Human Tears. Curr Eye Research 1999, Vol. 19, No. 1, 4-11.

11. Gasymov, O.K., et al., Interaction of Tear Lipocalin with Lysozyme and Lactoferrin. Biochemical and Biophysical Research Communications 1999; 265: 322-325.

12. Flanagan, J.L. & Willcox, M.D.P., Role of Lactoferrin in the Tear Film. Biochimie 2009; 91: 35-43.

13. Masschalck, B. et al., Inactivation of Gram-Negative Bacteria by Lysozyme, Denatured Lysozyme, and Lysozyme Derived Peptides Under High Hydrostatic Pressure. Appl Environ Microbiol 2001; 67: 339-344.

14. Barniak,V., Burke, S., Venkatesh, S., Presented at; Annual Meeting of the American Academy of Optometry, November 11-14, 2009: Orlando, FL.

Hyaluronan: Properties and Ophthalmic Uses

By Dr. Marjorie J. Rah, O.D., Ph.D.

The eye, as a self-maintaining, self-contained organ, performs essential functions and relies on a host of self-produced elements to maintain itself. Hyaluronan (H.A.) is one such element, with its main function being to keep the eye lubricated.

Hyaluronan is a naturally occurring glycosaminoglycan (a mucopolysaccharide),1 shown to have anti-inflammatory properties,2 play a role in wound healing,3 and have a protective effect against oxidative damage.4

Lubricating Properties

H.A. has unique water-retention properties and viscoelasticity thanks to its random coil structure, which allows each H.A. molecule to hold up to 1000 times its weight in water.5

Changes in temperature, pH, and shear rate can have an effect on viscoelasticity.1,6,7 H.A. has two distinct roles, one when the eye is open, and one when a blink occurs. When the eye is open, it is more viscous and coats the surface of the eye without draining, resulting in an improvement in tear break-up time.6,8 During a blink, its viscosity is reduced resulting in the spread of H.A. across the eye as the eye lids retreat to their original positions.6

H.A. keeps corneal epithelium hydrated and stabilises preocular tear film.3

In addition to being found in some artificial tear products and contact lens re-wetting drops,8 H.A. is currently used in medical treatments, especially surgeries.

Conclusions

H.A. is produced by the body and can be an inspiration for developing artificial tears, contact lens rewetting agents, and as a wetting agent incorporated into contact lens care products or contact lens materials, all of which can help to combat ocular dryness and discomfort. As advancements continue to be made in contact lens care and design, eye care professionals will be able to offer their contact lens patients improved comfort, which will hopefully lead to a reduction in the number of patients who discontinue wearing contact lenses and an increase in the wearing time of their lenses.

Dr. Marjorie Rah is an optometrist who specialises in medically necessary and other advanced contact lens designs. She is currently the Manager of Global Medical Affairs with Bausch & Lomb.

References

1. Lapcik L. Jr., et al. Hyaluronan: Preparation, Structure, Properties and Applications. Chemical Reviews. 1998;98:2663-2684.

2. Pauloin T., et al., Mol Vis. 2009;15:577.

3. Lerner L. et al., Exp Eye Res. 1998;67:481.

4. Presti D., Scott J.E., Cell Biochem Func. 1994;12:281.

5. Rosenbaum D., et al., Hyaluronan in Radiation-induced Lung Disease in Rat. Radiat Res. 1997;147:585-591.

6. Szczotka-Flynn L.B., Chemical Properties of Contact Lens Rewetter. Contact Lens Spectrum. 2006(4).

7. Scott J.E., et al., Secondary and Tertiary Structures of Hyaluronan in Aqueous Aolution, Investigated by Rotary Shadowing-Electron Microscopy and Computer Simulation. Biochem J. 1991;274:699-705.

8. Johnson M.E., Murphy P.J., Boulton M., Effectiveness of Sodium Hyaluronate Eyedrops in the Treatment of Dry Eye. Graefes Arch Clin Exp Ophthalmol. 2006; 244:109-112.