ZINC-MEDIATED URATE BINDING TO HUMAN ALBUMIN: 63

Abstract

Published data both support and dispute the existence of urate binding to serum proteins. The issue remains important since such binding could be relevant to both renal transport and tissue deposition of urate. Using equilibrium dialysis over 18 hours, we studied the effects of selected cations on ¹⁴C-urate binding to human albumin (5 g/dl) in TRIS NaCl buffer at 300 mOsm, 4°C and pH 7.4. We calculated the binding ratio (BR) by dividing the ¹⁴C (cpm/ml) inside the dialysis membrane by that in the bath. The mean coefficient of variation of the method was 4%. We observed minimal binding in the absence of added metal ions (BR 1.04). Added at 5mM, Zn⁺⁺ resulted in strong binding (BR 9.53), Cd⁺⁺ was less effective (BR 2.18),and Cu⁺⁺ was least (BR 1.22). When added in the presence of Zn⁺⁺, Cu⁺⁺ inhibited binding (BR 1.16), but Cd⁺⁺ did not (BR 9.54). Ionic Zn led to an even higher degree of ¹⁴C-hypoxanthine binding to albumin (BR 14.69) but did not affect the lack of albumin binding of ¹⁴C-urea (BR 0.95) nor the avid binding of ¹⁴C-salicylate (BR 10.98). Purine competition for the binding site(s) was suggested when saturation of the buffer with hypoxanthine, allopurinol, oxipurinol and adenine reduced the BR of ¹⁴C-urate (1.50, 3.20, 4.40, 5.28, respectively). Histidine (5mM), a Zn⁺⁺ chelator, strongly inhibited urate binding (BR 1.91), but salicylate (3.5mM) produced only a minimal change (BR 7.65). These findings indicate that zinc-purine complexes form readily in vitro and bind avidly to human serum albumin.

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