El aminoácido taurina está presente en grandes cantidades en lágrima humana

Purpose: Amino acids in micromolar range are present in free form in most of the extracellular fluid including blood, cerebrospinal fluid, and urine. Moreover, it is well known that they are also present in liquid secretions from exocrine glands such as milk, saliva, and semen with different functional roles. However, as far as we know there is a scarce knowledge about the presence of these compounds on human tears. Our purpose is to assess the presence of free amino acids on the product secretion of the human lacrimal gland, the tear, to better understand the function of these compounds on corneal and conjunctival physiology. Methods: Tear fluid was taken on filter paper without stimulation during a regular Schirmer`s test from the eyes of 36 healthy people which were aged between 18-50 years, and weighted. Filter papers were immediately dropped on tubes containing a sulfosalicylic acid solution (15% w/v, pH below 1) to extract soluble compounds including the amino acids, and to remove the proteins. Samples were maintained for 20 min at room temperature and then stored at -20oC. Aliquots of these samples were analyzed for amino acid content by reversed-phase HPLC (High Performance Liquid Chromatography) technique using a pre-column fluorescent method with o- phthalaldeyde (OPA) derivatization. Results: We have detected and measured eleven amino acid including: aspartate, glutamate, serine, glutamine, histidine, glycine, threonine, arginine, taurine , alanine and tyrosine. Glutamine, which is the main amino acid in several extracellular fluids is only present in micromolar amounts on tears, in the range of low micromolar (0.06-0.40mM). Surprisingly, however, the tear levels of taurine, an amino acid, not present in proteins; without any metabolic role and seldom exists in extracellular fluid, raises to the millimolar amounts (0.14-5.7mM). The quantities of other amino acids resemble the plasma content. Conclusions: Because of the well known properties of taurine as an antioxidant and a membrane protecting agent, a physiological role in protecting the epithelial cells of the cornea and the conjunctiva has been proposed due to the high amount of taurine found in human tears.