The degradation of some Bz-substituted tryptophans by Escherichia coli tryptophanase.

The present paper is the second in a series designed to correlate substrate structure with the kinetic constants of enzyme-catalysed reactions and thus throw light on the nature of the attachment of the substrate to the enzyme. Since the preparation of the first paper (Nath & Rydon, 1954), which describes the influence of structure on the hydrolysis of substituted phenylf-D-glucosides by emulsin, investigations of a similar nature have been reported by Gawron, Grelecki & Duggan (1953) on the hydrolysis of substituted phenyl acetates by wheat-germ lipase and by Dodgson, Spencer & Williams (1956) on the hydrolysis of substituted phenyl sulphates by the arylsulphatase of Alcaligenes metalcaligene8. Selection of the tryptophanase system for the current study stems from the observation of Anderson (1945) that 5-methyltryptophan is inhibitory to the growth of E8cherichia coli and from the demonstration by Fildes & Rydon (1947) that the four Bz-methyltryptophans are decreasingly effective as inhibitors of Salmonella typhi in the order 4-methyl-, 5-methyl-, 6-methyland 7methyl-tryptophan and are also competitive with tryptophan. The growth-inhibitory properties of the methyltryptophans have been examined rather more recently by other workers (Marshall & Woods, 1952; Akiba & Arai, 1951; Beerstecher, 1954; Trudinger & Cohen, 1956). The formation of 5-methylindole from 5-methyltryptophan in the presence of a suspension ofviable cells of E. coli was studied by Beerstecher & Edmonds (1951). According to these workers 5methyltryptophan is degraded only in the presence of small amounts of tryptophan or indole. The reaction was considered, therefore, to be autocatalytic in the presence of tryptophan. While the present work was in progress Trudinger & Cohen (1956) demonstrated the formation of 4-methylindole from 4-methyltryptophan by tryptophanasecontaining extracts of E. coli cells and cited evidence in favour of the view that the decompositions of tryptophan and of 4-methyltryptophan are catalysed by the same enzyme system. Mention should also be made of preliminary observations on the action of tryptophanase on substituted methyltryptophans by Dr D. Herbert (see Fildes & Rydon,