Seaweed brominating species

On the basis of kinetic stndies the presence of a peroxo-intermediate was postdated and spectroscopic evidence for such an intermediate has been obtained. Brs", Br2, or HOBr appear to be the primary reaction prodncts of the enzyme-mediated peroxidation of bromide. The vanadium enzyme also nses phenylperacetic acid, m-chloroperoxybenzoic acid, and jo-nitroperoxybenzoic acid as oxidants, but alkyl peroxides such as ethylhydroperoxide, tert-butyl hydroperoxide, and cuminyl hydroperoxide are not substrates for the enzyme in the oxidation of bromide. The enzyme from the brown seaweed Ascophyllum nodosum does not display any specificity with regard to bromination of various organic nucleophilic acceptors which suggests a mechanism in which the oxidized bromine species are released into solution by the enzyme. Figure 1 gives a simple model for the reaction mechanism of the enzyme. 

On the other hand, since the absorption cross section of tribromomethane (bromoform, CHBt3) released from seaweed is small in the spectrum region of actinic flux (GiUotay et al. 1989), it is not a very important source of photoactive bromine species in the troposphere. 

Surprisingly, the bromoperoxidases do not (like other peroxidases) oxidize organic electron donors il9,31) such as o-dianisidine or guaia-col, which are normally used to detect peroxidase activity, and thus enzymic activity easily escapes detection. When bromide or iodide are present in the assay medium HOBr and HOI are formed these reactive species will oxidize, brominate, or iodinate the organic dyes, thus allowing detection. Early in this century similar organic dyes were used to detect peroxidase activity in seaweed tissues and extracts. However, the results were not clearly reproducible. This is understandable since in seaweed variable amounts of iodide and bromide are present. On purification of the bromoperoxidase the halides are removed, resulting in an apparent decrease in or lack of activity. 

In contrast, the fungal heme-containing chloroperoxidase from Cal-dariomyces fumago is rapidly inactivated by HOBr (31). As early as 1926 Sauvageau (30) concluded that certain species of seaweeds are able to produce free bromine in solution. Scheme I gives the tentative reaction mechanism of bromoperoxidase with its substrates (32,34). 

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