STUDIES ON BIOLOGICAL OXIDATIONS
E. S. Guzmán Barrón, R.H. DeMeio, Friedrich W. Klemperer · Journal of Biological Chemistry · 1936
SUMMARY Ascorbic acid is not autoxidizable in acid and neutral solutions up to pH 7.0. It is autoxidizable in alkaline solutions, the rate of autoxidation increasing considerably as the pH increases. Among the metallic salts tested (Mn, Ni, Fe, Co, Ca, Cu) copper is the only catalyst for the oxidation of ascorbic acid, its catalytic action being noticed in concentrations as small as 46 micrograms of copper per liter. This catalytic effect is completely inhibited by 0.001 M KCN, and to the extent of 80 per cent by mixtures of CO : 02 of 95 : 5. When this oxidation is performed in acid and neutral solutions, there is an uptake of oxygen of 1 atom per molecule of the substrate. The oxidized form can be completely reduced by H&S up to pH 5.0. Above this pH the amount of ascorbic acid recovered gradually decreases until at pH 7.6 only 4 per cent of the oxidized form can be recovered as ascorbic acid. Hydrogen in the presence of colloidal palladium does not reduce oxidized ascorbic acid. The temperature coefficient for the oxidation of ascorbic acid by oxygen with copper as catalyst is 1.65. Electromotively active oxidation-reduction systems of suitable potential can also act as oxidizing catalysts. The hemochromo- gens of nicotine, pyridine, and pilocarpine, taken as representative examples, were found to act as catalysts. The mechanism of the oxidation of ascorbic acid by atmospheric oxygen in the presence of catalysts has been discussed. After this paper was sent in for publication there came to the authors’ attention two recently published communications bearing a relation to our experiments. Mawson (25) and Kellie and Zilva (26) have reported on the stability of ascorbic acid in the absence of catalysts, and on the catalytic power of copper. Kellie and Zilva report also that “the oxidation catalysed by Cu was very much slower when NaCl (0.1 M) was present.” We found no such inhibiting effect in our experiments. In fact, 0.02 mM of ascorbic acid in phosphate buffer + 0.1 M NaCl, with 0.0002 mM of CuCh as catalyst (final pH, 6.00), was half oxidized in 16.2 minutes. In Table II it can be seen that at the same pH value, without NaCl, ascorbic acid was half oxidized in 15 minutes. On the other hand, the addition of amino acids, through the