Tannins as hydrogen carriers in biological oxidation

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Abstract
Tea tannin functions as a H-carrier in the oxidative breakdown of carbohydrates in bruised tea-leaf. As coenzymes are almost completely inactivated by extensive damage to the leaf (cf. Deb and Roberts, preceding abstract) tea tannin is probably the only link between cytochrome and dehy-drogenases in the damaged tea-leaf. The o-quinone of the tannin probably acts as a direct H-acceptor, in contradistinction to o-quinone itself which requires cozymase (Dixon and Zerfas, Biochem. Jour., 34, 371). A correlation exists between the rate of O2 uptake in a fine mince and the tannin content of the leaf, over a wide range of spp. Tannin-free leaves show only negligible rates of O2 uptake in a mince. The R.Q. of vegetable tissues is reduced on mincing. This is attributed to the possible condensation of the o-quinone of tea tannin before it has functioned as a H-acceptor. The R.Q. is thus a measure of the proportion of o-quinone molecules which are reduced again before having time to condense. Tea tannin will not function as a H-carrier for minced Tropaeolum or Hibiscus. The oxi-dases of tea-leaf and of Eugenia balsamea are each fairly specific for their own tannins and foreign tannins are oxidized more slowly. This specificity of an oxidase system towards its own tannins is considered to be due to the immunity of plant enzymes towards the inactivating effects of their own tannins, as tannin oxidation is a non-enzymic process brought about by oxidised cytochrome. Inhibition of tea oxidase by E. balsamea tannin is reversible as fresh tea tannin is readily oxidised in the presence of this inhibitor. Strong tannins, such as those from Camellia drupifera, inhibit even their own enzymes.

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