Interactions of hypochlorous acid with pyrimidine nucleotides, and secondary reactions of chlorinated pyrimidines with GSH, NADH, and other substrates

Abstract

HOCl-induced chlorination of pyrimidine nucleotides, PyNH, strikingly depends on the nature of the available chlorine acceptor group. For CMP, with an -NH2 group as acceptor, the reaction is slow and involves predominantly the acid [k(CMP + HOCl) approximately 100 M-1 s-1 at pH 6]; apparent rate constants of the reaction decrease around the pK alpha (HOCl), to 0 in alkaline solution. For TMP and UMP, with a heterocyclic > NH group (at 3N) as acceptor, the reaction is faster and involves mainly the conjugated CIO- anion [e.g., k(UMP + ClO-) approximately 3 x 10(4) M-1 s-1 and k(UMP + HOCl) approximately 200 M-1 s-1]. The 3-N-methylthymidine derivative is inert toward HOCl. Reactions of ClO- with TMP, UMP, and poly(U) are shown to be reversible, PyNH + ClO- = PyNCl + OH-; an increase in pH due to this reaction was confirmed, and equilibrium constants have been estimated. The chlorinated derivatives of TMP and UMP are very reactive toward GSH, disulfide, aliphatic amines, and NADH. In contrast, the PyNCl derivative of CMP is unreactive, except with GSH. Rate constants of reactions of PyNCl species with various substrates are presented. Oxidation of NADH, by both HOCl and PyNCl derivatives, leads to a stable product (not NAD+) which is irreversibly degraded by reaction with excess HOCl, but inert toward acsorbate, GSH, and H2O2. Thiols (GSH) and disulfides (DTPA) were previously found capable of scavenging up to four HOCl molecules (Prütz, W. A., Arch. Biochem. Biophys. 332, 110-120, 1996). In the present study it was established that reactions of GSH or DTPA with excess HOCl give rise to a rapid drop in the pH by release of up to four HCl molecules per GSH or DTPA, as expected for a sequence of consecutive sulfoxidations. Reactions of GSH and DTPA with PyNCl efficiently regenerate PyNH, namely up to four molecules per GSH or DTPA in the case of TMP and UMP, but only one molecule per GSH in the case of CMP. The PyNCl derivatives of TMP and UMP transfer chlorine slowly but completely to CMP or AMP. Such chlorine transfer between nucleic acid bases is likely to occur also in DNA; it is shown that HOCl in fact induces a complex series of reactions on interaction with native DNA.