Trolox, gallic acid, chlorogenic acid, caffeic acid, catechin, epigallocatechin gallate, and ascorbic acid are antioxidants used as standards for reaction with chromogenic radicals, 2,2-diphenyl-1-picrylhydrazyl (DPPH^⋅) and 2,2′-azino-bis-3-ethylbenzotiazolin-6-sulfonic acid (ABTS^⋅+), and Folin–Ciocalteu (FC) reagent. The number of exchanged electrons has been analyzed as function of method and solvent. A majority of compounds exchange more electrons in FC assay than in ABTS and DPPH assays. In reaction with chromogenic radicals, the largest number of electrons was exchanged in buffer (pH 7.4) and the lowest reactivity was in methanol (DPPH) and water (ABTS). At physiological pH, the number of exchanged electrons of polyphenols exceeded the number of OH groups, pointing to the important contribution of partially oxidized antioxidants, formed in the course of reaction, to the antioxidant potential. For Trolox, small impact on the number of exchanged electrons was observed, confirming that it is more suitable as a standard compound than the other antioxidants.

Cathepsin C, a tetrameric lysosomal dipeptidyl‐peptide hydrolase, is activated by chloride ion. The activation is shown here to be specific and pH‐dependent, dissociation constants for chloride being lower at low pH. Bound chloride decreases the K_m for the hydrolysis of chromophore labelled substrates without any significant change in V_max, confirming its involvement in substrate binding. Determination of the kinetic parameters of chloride activation, using unlabelled substrates, has enabled its site of action to be located. The lower K_m for the hydrolysis of simple amide substrates in the presence of Cl⁻ shows that the S sites are involved. Possible involvement of the S′ sites is excluded by the finding that the K_m for the nucleophile in the transferase reaction is unaffected by chloride. The rates of inhibition by E‐64 and iodoacetate are both chloride‐dependent and, from the structure of the papain–E‐64 complex, it is concluded that chloride binds close to the S2 site. The binding of guanidinium ion, a positively charged inhibitor, to the S site is dependent on chloride. Based on these results, a model is proposed to explain the chloride activation of cathepsin C. The possible physiological role of chloride in the regulation of proteolysis in the lysosome is discussed.