The binding of a number of structurally related xanthine compounds by bovine serum albumin was investigated in an attempt t o elucidate some of the structural specificities of the interaction. The study included caffeine, theophylline, 8-nitrotheophylline, 8-chlorotheophylline, theophylline-7-acetic acid, 8-methoxycaffeine, 8-ethoxycaffeine, 8-chlorocaffeine, 8-methylcaffeine, theobromine, S-nitrotheobromine, uric acid, 1,3,7-trimethyluric acid, uracil, and 1,3-dimethyluracil. Under the conditions employed, interactions between the protein and uric acid, uracil, and dimethyluracil could not be detected. T h e remaining compounds were bound to varying degrees and evidence was obtained to indicate that a single site on the protein was responsible for the major portion of the binding. Anionic species were generally found to possess much stronger interactive tendencies than those which had no formal charge.
HE ABILITY of serum proteins, particularly Talbumin, to reversibly bind numerous small molecules has been recognized for many years (I). The majority of earlier investigations were of a qualitative nature. Only in recent years have attempts been made to quantitate the results and to explain the nature of the equilibria and forces involved in the interactions (2, 3). Concomitant with advances in the theoretical aspects of these interactions, there has been an increase in the study of the associations of drug molecules with proteins. Steroid hormones (4), barbiturates (5), sulfonamides (6, 7), penicillins (8), salicylates (9), and other therapeutic agents have been shown to interact with serum albumins.
Goldstein (l), and Brodie and Hogben (10) have discussed the significance of such behavior in the transport, deposition, and elimination of drugs in the body. In addition, i t appears that the pharmacologic activity of many medicinal agents is intimately related to their abilities to combine with specialized functional proteins. Elucidation of the factors involved in such interactions may thus contribute to the understanding of drug action.
The interaction of xanthine compounds with components in blood serum had been noted in the early investigations of Pak (11) and Aiello (12). More recently, Schack and Waxler (13) demonstrated that theophylline can b e bound by the plasma proteins of rabbits. In the present