Adsorption of thiocyanate to the plasma proteins; with special reference to the distribution of this ion between aqueous humour and plasma.

KINSEY AND PALM (1955) have described the kinetics of penetration of two ions, sodium and thiocyanate, into the aqueous humour, and have concluded that there is a fundamental difference in their modes of penetration; the thiocyanate ion, contrary to an original claim (Kinsey and Grant, 1942), penetrating by " diffusion ", whilst sodium is said to penetrate by " secretion". An inspection of their results shows that this difference really depends largely on the different steady-state distributions between plasma and aqueous humour that are apparently reached by the two ions, sodium giving a ratio: Concentration in Aqueous Humour/Concentration in Plasma of approximately unity, whilst thiocyanate gave a ratio of 0.75. The low steady-state ratio shown by thiocyanate would, in the past, have been attributed to the circumstance that this ion is partly adsorbed to the plasma proteins, so that only 75 per cent. would be freely diffusible; the distribution-ratio of the diffusible fraction would, therefore, have been considered as approximately unity, like that of sodium (Kinsey and Grant, 1942, Davson, DukeElder, Maurice, Ross, and Woodin, 1949; Davson and Matchett, 1953). However, Kinsey and Palm now state that this adsorption of thiocyanate to the plasma proteins is only apparent, resulting from the presence of some other material, with chemical reactions similar to those of thiocyanate, that is adsorbed or chemically united to the plasma proteins. This claim encounters serious theoretical difficulties in the realm of colloid chemistry. The thiocyanate ion lies at the extreme end of the lyotropic series of anions, which means that its power of hydrating colloids is greater than that of iodide, bromide, and chloride, in that order. To increase the hydration of a colloid, it is generally considered necessary that the ion doing so should be adsorbed to the colloidal particles, thereby carrying its own shell of orientated water-dipoles with it. Thus, if Kinsey and Palm's claim could be substantiated, this would demand a complete re-interpretation of lyotropic phenomena; it, on the other hand, it is incorrect, we should expect that, on dialysing plasma against solutions of sodium chloride, bromide, iodide, and thiocyanate, the distribution-ratios: Concentration in Dialysate/Concentration in Plasma would decrease in this order. The distributions of chloride, bromide, and iodide have already been determined (Davson, 1955) by