A study on binding of PAS with serum albumin by BCG metachromasy.

Changes in the visible absorption spectra of dyes due to binding with polyelectrolytes of high molecular weight are known as metachromasy and have been extensively studied. Recent advances in metachromasy have been summarized in several reviews (1-3). It is also known that metachromasy due to binding with proteins is generally weak but oc curs with both acidic and basic dyes. Methyl orange (MO) and bromocresol green (BCG) as acidic dyes and toluidine blue and acridine orange as basic dyes have often been used in studies of metachromasy. As pointed out by Klotz (4), the addition of a suitable organic acid to a solution of an MO or azosulfathiazole-protein complex reversed the effect of protein on the spectrum. This reversal of metachromasy is considered to be due to the fact that the organic acid anion displaces the dye anion from the binding sites on the protein molecule. Thus it seems likely that the competitive ability of an organic acid against an acidic dye should be a measure of the strength of its binding with the protein molecule and in fact this has been confirmed by equilibrium dialysis binding studies. In the present work the metachromasy of BCG with bovine serum albumin was chosen for study, since BCG was tightly bound to albumin and a relatively great spectral change was produced, as has been shown by Rodkey (5) and others (6). p-Aminosalicylic acid (PAS) is an important antituberculous agent but its activity is known to be greatly decreased in the presence of serum albumin. This indicates a high affinity of PAS for serum albumin. In this work the competitive ability of PAS was compared with those of benzoic acid (BA), salicylic acid (SA) and p-aminobenzoic acid (PABA) by spectro