The thiol-disulphide system: Complexes of thiol-acids with iron

Structural requirements for human inducible nitric oxide synthase substrates and substrate analogue inhibitors

Inducible nitric oxide synthase (iNOS; EC 1.14.13.39) catalyzes the NADPH-dependent oxidation of one of the free guanidino nitrogens of L-Arg to form nitric oxide and L-citrulline. Analogues of L-Arg and the inhibitor, L-N6-(1-iminoethyl)lysine, were used to define structural elements required for the binding and catalysis of compounds. L-Arg analogues with sequentially shorter methylene spacing between the guanidino group and the amino acid portion of the molecule were not iNOS substrates but were reversible inhibitors. L-Arg analogues such as agmatine with a hydroxyl substitution at the 2-amino position were substrates. Desaminoarginine was not a substrate but a reversible inhibitor. Desaminoarginine, agmatine, and argininic acid bound to the enzyme to give type I difference spectra similar to that of L-Arg. The amidino compounds L-N6-(1-iminoethyl)lysine, L-N5-(1-iminoethyl)ornithine, and N5-(1-iminoethyl)cadaverdine, but not N6-(1-iminoethyl)-6-aminocaproic acid, were NADPH-dependent, irreversible inactivators of iNOS. For both the L-Arg and L-N6-(1-iminoethyl)lysine analogues, the 2-amino group appeared to play an important role in catalytic events leading to either substrate turnover or mechanism-based inactivation. Inactivation of iNOS by L-N6-(1-iminoethyl)lysine was NADPH- and dioxygen-dependent, but low incorporation of radiolabel with DL--4, 5-3H]-N6-(1-iminoethyl)lysine indicates that the mechanism of enzyme inactivation is not covalent modification of the protein.

The use of bis(mercaptoacetato-S,O)hydroxoiron(III) complex for the determination of hydroxamates

A rapid, indirect, spectrophotometric procedure for the determination of hydroxamates, based on the competition for ferric ions of the bis(mercaptoacetato-S,O)hydroxoiron(III) complex, has been developed. The assay is remarkably free of interferences by common ions, thus rendering it useful in the quantitative determination of hydroxamates in culture fluids and crude preparations.

THE REACTION OF POTASSIUM PERSULPHATE WITH THIOGLYCOLIC ACID IN AQUEOUS SOLUTION

The oxidation of thioglycolic acid to dithiodiglycolic acid by potassium persulphate in aqueous solution in the absence of iron did not yield reproducible results, but satisfactory data were obtained when ferrous or ferric ions were added to the system. The catalyst cycle appears to be that ferrous ion is oxidized by persulphate to ferric ion, and the ferric ion is reduced back to ferrous ion by thioglycolic acid. In the absence of persulphate, the reaction of ferric ion with thioglycolic acid in acid solution (pH < 3) was second order in both ferric ion and thioglycolic acid concentrations and was markedly inhibited by hydrogen ion.