Clxxx. Further Observations on the System Ascorbic Acid-glutathione- Ascorbic Acid-oxidase by Eric Mitchell Crook and Frederick Gowland Hopkins

IN a paper by Hopkins & Morgan [1936] it was shown that in aqueous solutions containing Szent Gyorgyi's oxidase together with ascorbic acid and glutathione, the former is protected by the latter from oxidation by the enzyme, while the glutathione is itself oxidized vicariously at a rate corresponding to that at which the ascorbic acid is oxidized when alone with the enzyme in similar concentrations. It was also shown that under the influence of similar enzyme preparations the reduction of dehydroascorbic acid by GSH is much faster than the oxidation of ascorbic acid, and also much faster than the reduction when uncatalysed. Kertesz has since stated [1938] that he was unable to reproduce these experimental results. He found that GSH added to solutions of ascorbic acid containing the oxidase had no effect on the oxidation of the ascorbic acid at pH 6. At pH 7 4, while the GSH inhibited to some extent the oxidation of ascorbic acid, both substances were oxidized simultaneously. He found further that the reduction of dehydroascorbic acid by GSH was not catalysed by the enzyme preparation used by him. It became clearly desirable that the experiments should be repeated, and the present paper deals with such repetitions together with some extensions. They have involved the use of many different enzyme preparations, and they show that the results published by Hopkins and Morgan are invariably reproducible. The enzyme preparations used by these authors were all derived from cabbages or from cauliflower florets. Kertesz employed cauliflowers, but also used cucumber juice. With no preparation did he obtain results comparable with those of the earlier workers. In the following sections experiments with cauliflower florets will be first described; certain others obtained with cucumber juice will afterwards receive special reference. Unless an explanation which we will later venture to put forward is justified, we seem to be faced with an inexplicable difference in experimental experience.