Insights into the Passive Partitioning of Amino Acids and Codeine into Human Hair

Abstract

Background: Cells that form hair are among the body's most rapidly dividing cells, with each daughter cell becoming part of the hair shaft as it differentiates and lengthens. It follows that hair might contain concentrations of xenobiotics (environmental chemicals, therapeutic drugs, illicit substances, etc.) reflective of the concentration in blood at the time of division and onset of differentiation of cells that give rise to hair. Objective: This study was designed to gain insight into the passive incorporation of xenobiotics into hair. Methods: This study focused on the parameters of the passive partitioning of three agents, two water-soluble amino acids (cysteine and glutamine) and codeine into hair. Undamaged black hair from various ethnic groups was incubated in solutions containing the three agents as a function of time and concentration. Hair was washed extensively; time and dose to steady state as well as partitioning coefficients were determined for each agent. Codeine washing with a pH gradient also was carried out. Results: Data show that the partitioning coefficient of the agents is linear but not stoichiometric. At low doses passive partitioning into hair is efficient, a 1 minute exposure leading to significant retention. Washing extensively with deionized water, the fluid used for passive partitioning of the test agent, will not reversibly extract all of the agent, regardless of race. Conclusion: Hair can effectively absorb and retain agents of the environment, and this retention occurs via mechanisms other than those inherent to an ion exchange column.